Get notes, summary, questions and answers, MCQs, extras, competency-based questions and PDFs of Electrolysis: ICSE Class 10 Chemistry (Concise/Selina). However, the notes should only be treated as references, and changes should be made according to the needs of the students.
Summary
Electrolysis is a process where electricity is used to cause a chemical change in certain substances. Some materials, like metals, are called conductors because they allow electricity to pass through them easily due to the movement of electrons. Other materials, called non-conductors or insulators, do not allow electricity to pass.
There are special compounds called electrolytes. These compounds conduct electricity when they are melted or dissolved in water. When electricity passes through an electrolyte, it breaks down chemically. Electrolytes can be strong, meaning they conduct electricity very well, or weak, meaning they conduct poorly. Substances like sugar solution are non-electrolytes; they do not conduct electricity because they consist of molecules and do not form ions in solution.
The process of electrolysis takes place in an electrolytic cell. This cell contains the electrolyte and two electrodes: an anode and a cathode. The anode is connected to the positive terminal of a power source, and the cathode is connected to the negative terminal. Inside the electrolyte, there are charged particles called ions. Positively charged ions, known as cations, move towards the cathode. Negatively charged ions, known as anions, move towards the anode.
At the electrodes, chemical reactions occur. Oxidation, which is the loss of electrons, happens at the anode. Reduction, which is the gain of electrons, happens at the cathode. The tendency of different ions to be discharged at the electrodes can be predicted using the electrochemical series. Generally, ions that are lower in this series are discharged more easily. However, the concentration of the ions and the material of the electrodes can also influence which ion is discharged. For example, if an electrode is “active,” it might dissolve into the solution instead of an ion from the electrolyte being discharged.
Electrolysis has several practical uses. One common application is electroplating, where a thin layer of one metal is coated onto another. For instance, a spoon can be electroplated with silver to make it look attractive. In this process, the object to be plated is made the cathode. Another use is electrorefining, which is a method to purify metals. Impure copper, for example, can be refined by making it the anode; pure copper then deposits on the cathode, and impurities fall away. Electrometallurgy is the extraction of highly reactive metals, like aluminum, from their molten compounds using electrolysis.
Workbook solutions (Concise/Selina)
Intext Questions and Answers I
1. Fill in the blanks.
(a) Powdered sodium chloride (common salt) does not conduct an electric current, but it does so when _____or when _________.
(b) Molten lead bromide conducts electricity. It is called an ______. It is composed of lead _____and bromide ____. The lead ions are _____charged and are called ____. The bromide ____are _____charged and are called _______.
(c) Substances which conduct electricity in the solid state are generally _______.
(d) The electron releasing tendency of zinc is _____than that of copper.
(e) A solution of HCl gas in water conducts electricity because _______ but a solution of HCl gas in toluene does not conduct an electric current because _____
(f) Pure water consists entirely of ______.
(g) We can expect that pure water ______ normally conduct electricity.
(h) Electrolysis is the passage of _______ through a liquid or a solution accompanied by a ______change.
Answer:
(a) Powdered sodium chloride (common salt) does not conduct an electric current, but it does so when dissolved in water or when melted (fused state).
(b) Molten lead bromide conducts electricity. It is called an electrolyte. It is composed of lead ions and bromide ions. The lead ions are positively charged and are called cations. The bromide ions are negatively charged and are called anions.
(c) Substances which conduct electricity in the solid state are generally metallic conductors (metals and alloys).
(d) The electron releasing tendency of zinc is greater (more) than that of copper.
(e) A solution of HCl gas in water conducts electricity because it ionises in water to form free mobile ions (hydronium ions and chloride ions) but a solution of HCl gas in toluene does not conduct an electric current because it does not ionise in toluene and remains in molecular form (toluene is a non-polar solvent).
(f) Pure water consists entirely of molecules.
(g) We can expect that pure water will not normally conduct electricity.
(h) Electrolysis is the passage of electricity through a liquid or a solution accompanied by a chemical change.
2. Define the following terms:
(a) Electrolysis,
(b) Non-elecrolyte
(c) Cation and anion,
(d) Weal electrolytes.
Answer: (a) Electrolysis: It is the process of decomposition of a chemical compound in aqueous solution or in molten state accompanied by a chemical change by using direct electric current.
(b) Non-electrolyte: It is a compound which neither in solution nor in the molten state allows an electric current to pass through it. It also does not decompose at the electrodes. Non-electrolytes do not have ions even in solution. They contain only molecules.
(c) Cation and anion: Atoms which carry positive charge are called cations and the atoms which carry negative charge are called anions.
(d) Weak electrolytes: Electrolytes which allow small amounts of electricity to flow through them. These are poor conductors of electricity. These are partially dissociated in fused or aqueous solution state. These solutions contain ions as well as molecules. Weak electrolyte allows a bulb to glow dimly.
3. What is the difference between :
(a) Modern explanation and Arrhenius explanation for the theory of electrolysis,
(b) Electrolytic dissociation and ionisation,
(c) A cation and an anion,
(d) Electrolytic dissociation and thermaldissociation.
(e) Strong electrolyte and weak electrolyte.
Answer: (a) Arrhenius considered that water ionises electrolytes but Modern concept considers that electrolytes are ionic even in solid state and their ions are held by strong electrostatic forces which make them immobile. Water renders these ions mobility by breaking the electrostatic forces.
(b) Electrolytic dissociation and ionisation,
(c) A cation and an anion,
(d) Electrolytic dissociation and thermal dissociation.
(e) Strong electrolyte and weak electrolyte.
4. Name:
(a) a salt which is a weak electrolyte,
Answer: Ammonium carbonate or lead acetate.
(b) a base which is a weak electrolyte,
Answer: Ammonium hydroxide or calcium hydroxide.
(c) an inert electrode and an active electrode,
Answer: Inert electrode: Graphite or Platinum.
Active electrode: Copper (Cu), Nickel (Ni), or Silver (Ag).
(d) a positively charged non-metallic ion,
Answer: Hydrogen ion (H⁺), Hydronium ion (H₃O⁺), or Ammonium ion (NH₄⁺).
(e) the electrode at which reduction occurs,
Answer: Cathode.
(f) a non-metallic element which is a conductor of electricity.
Answer: Graphite.
5. Electrolysis is a redox process. Explain.
Answer: The process of electrolysis is a redox reaction. The reaction at the cathode involves reduction of cations as they gain electrons to become neutral atoms while that at anode involves oxidation of anions as they lose electrons to become neutral.
6. Classify the following substances under three headings:
(a) strong electrolytes
(b) weak electrolytes
(c) non-electrolytes
Acetic acid, ammonium chloride, ammonium hydroxide, carbon tetrachloride, dilute hydrochloric acid, sodium acetate, dilute sulphuric acid.
Answer: (a) Strong electrolytes: Ammonium chloride, Dilute hydrochloric acid, Sodium acetate, Dilute sulphuric acid
(b) Weak electrolytes: Acetic acid, Ammonium hydroxide
(c) Non-electrolytes: Carbon tetrachloride
7. Explain why :
(a) Cu, though a good conductor of electricity is a non electrolyte,
Answer: Copper is a metallic conductor and allows an electric current to pass through it due to the flow of electrons, without undergoing any chemical change. Electrolytes are chemical compounds that conduct electric current in molten or aqueous state and are decomposed by the passage of electric current. Since copper does not undergo chemical decomposition, it is a non-electrolyte.
(b) Solid sodium chloride does not allow electricity to pass through?
Answer: In solid state sodium and chloride ions are not free but are held together by an electrostatic force of attraction hence it does not conduct electricity (due to the absence of free ions).
8. Choose A, B, C or D to match the descriptions (i) to (v) below. Some alphabets may be repeated.
A. non-electrolyte
B. strong electrolyte
C. weak electrolyte
D. metallic conductor
(i) Molten ionic compound
(ii) Carbon tetrachloride
(iii) An aluminium wire
(iv) A solution containing solvent molecules, solute molecules and ions formed by the dissociation of solute molecules.
(v) A sugar solution with sugar molecules and water molecules.
Answer: (i) Molten ionic compound—B. strong electrolyte
(ii) Carbon tetrachloride—A. non-electrolyte
(iii) An aluminium wire—D. metallic conductor
(iv) A solution containing solvent molecules, solute molecules and ions formed by the dissociation of solute molecules—C. weak electrolyte
(v) A sugar solution with sugar molecules and water molecules—A. non-electrolyte
9. An electrolyte which completely dissociates into ions is:
A. Alcohol
B. Carbonic acid
C. Sucrose
D. Sodium Hydroxide
Answer: D. Sodium hydroxide
Intext Questions and Answers II
1. Name two substances in each case:
(a) Contain only molecules,
(b) Contain only ions,
(c) Contain ions as well as molecules.
Answer: (a) Substances that contain only molecules are distilled water and alcohol.
(b) Substances that contain only ions are molten sodium chloride (NaCl) and molten lead bromide (PbBr₂).
(c) Substances that contain ions as well as molecules are aqueous acetic acid and aqueous ammonium hydroxide.
2. Select the ion in each case, that would get selectively discharged from the aqueous mixture of the ions listed below:
(a) SO₄²⁻, NO₃⁻ and OH⁻,
(b) Pb²⁺, Ag⁺ and Cu²⁺, the Ag⁺
Answer: (a) From SO₄²⁻, NO₃⁻ and OH⁻, the OH⁻ ion would get selectively discharged because it is lower in the electrochemical series of anions.
(b) From Pb²⁺, Ag⁺ and Cu²⁺, the Ag⁺ ion would get selectively discharged because it is lowest in the electrochemical series of metals (cations).
3. (a) Among Zn and Cu, which would occur more readily in nature as metal and which as ion?
Answer: Among Zn and Cu, Cu would occur more readily in nature as a metal because it is lower in the electrochemical series and less reactive. Zn, being higher in the electrochemical series and more reactive, would occur more readily as an ion.
(b) Why we cannot store AgNO₃ solution in a copper vessel ?
Answer: We cannot store AgNO₃ solution in a copper vessel because copper (Cu) is higher than silver (Ag) in the electrochemical series of metals. Therefore, copper will displace silver from the AgNO₃ solution, causing the copper vessel to react and the silver to precipitate.
(c) Out of Cu and Ag, which is more active ?
Answer: Out of Cu and Ag, Cu is more active because it is higher in the electrochemical series of metals.
4. (a) How would you change a metal like Cu into its ions?
Answer: A metal like Cu can be changed into its ions (Cu²⁺) by oxidation, which involves the loss of electrons. This can occur if copper acts as an active anode during electrolysis, where Cu atoms lose electrons to become Cu²⁺ ions (Cu → Cu²⁺ + 2e⁻).
(b) How would you change Cu²⁺ ion to Cu ?
Answer: A Cu²⁺ ion can be changed to Cu metal by reduction, which involves the gain of electrons. This occurs at the cathode during electrolysis, where Cu²⁺ ions gain electrons to become neutral Cu atoms (Cu²⁺ + 2e⁻ → Cu).
5. A solution of caustic soda (NaOH) in water or when fused, conducts an electric current. What is the similarity in these two cases ?
Answer: The similarity in both cases, whether caustic soda (NaOH) is in water (aqueous solution) or in a fused (molten) state, is that NaOH, being an electrovalent compound, dissociates into free mobile ions (Na⁺ and OH⁻). These free mobile ions are responsible for conducting the electric current in both states.
6. During electrolysis of an aqueous solution of sulphuric acid between platinum electrodes, two types of anions migrate towards the anode but only one of them is discharged:
(a) Name the two anions,
Answer: The two anions that migrate towards the anode are hydroxide ions (OH⁻) and sulphate ions (SO₄²⁻).
(b) Name the main product of the discharge of anion at the anode and write the anode reaction,
Answer: The main product of the discharge of anions at the anode is oxygen gas (O₂). The OH⁻ ion, being lower in the electrochemical series, is discharged preferentially.
The anode reactions are:
OH⁻ → OH + e⁻
OH + OH → H₂O + O
O + O → O₂
(c) Name the product at the cathode and write the reaction.
Answer: The product at the cathode is hydrogen gas (H₂). Hydrogen ions (H⁺) migrate to the cathode and gain electrons.
The cathode reactions are:
H⁺ + e⁻ → H
H + H → H₂
(d) Do you notice any change in colour. State why?
Answer: No change in colour is noticed. The aqueous solution of sulphuric acid is colourless, and the products of electrolysis, hydrogen gas and oxygen gas, are also colourless. The overall reaction involves the decomposition of water, and sulphuric acid acts as a catalyst and is not consumed, so the appearance of the solution does not change.
(e) Why this electrolysis, is considered as an example of catalysis?
Answer: This electrolysis is considered an example of catalysis because dilute sulphuric acid catalyses the ionisation of water. Water in its pure state consists almost entirely of molecules and is a poor conductor. The addition of dilute sulphuric acid increases the concentration of ions, allowing electrolysis to occur more readily, with the acid itself not being consumed in the net reaction.
7. Copper sulphate solution is electrolysed using a platinum cathode and carbon anode. Study the diagram given alongside and answer the following questions.
(a) Give the names of the electrodes A and B.
Answer: In the diagram, electrode A is connected to the positive terminal of the battery, so it is the anode. Electrode B is connected to the negative terminal, so it is the cathode. Given that a carbon anode and platinum cathode are used, electrode A is the carbon anode, and electrode B is the platinum cathode.
(b) Which electrode is the oxidizing electrode ?
Answer: The anode is the oxidizing electrode. Therefore, electrode A (the carbon anode) is the oxidizing electrode because oxidation takes place at the anode.
8. To carry out the so-called “electrolysis of water”. sulphuric acid is added to water. How does the addition of sulphuric acid produce a conducting solution?
Answer: The addition of sulphuric acid produces a conducting solution because pure water is a very weak electrolyte with very few ions. Sulphuric acid is a strong electrolyte which ionises in water to produce a high concentration of mobile ions (H⁺ and SO₄²⁻). It also catalyses the ionisation of water (H₂O ⇌ H⁺ + OH⁻), further increasing the number of free mobile ions in the solution. These free mobile ions are responsible for carrying the electric current, thus making the solution conducting.
9. (a) Choosing only words from the following list, write down the appropriate words to fill in the blanks (i) to (v) below: anions, anode, cathode, cations, electrode, electrolyte, nickel, voltameter
The electroplating of an article with nickel requires (i) ………… which must be a solution containing (ii) ………… ions. The article to be plated is placed as the (iii) ………… of the cell in which the plating is carried out. The (iv) ………… of the cell is made from pure nickel. The ions that are attracted to the negative electrode and discharged are called (v) …………
Answer: The electroplating of an article with nickel requires (i) electrolyte which must be a solution containing (ii) nickel ions. The article to be plated is placed as the (iii) cathode of the cell in which the plating is carried out. The (iv) anode of the cell is made from pure nickel. The ions that are attracted to the negative electrode and discharged are called (v) cations.
(b) When a molten ionic compound is electrolysed, the metal is always formed at ………… and the non-metal is formed at …………
Answer: When a molten ionic compound is electrolysed, the metal is always formed at the cathode and the non-metal is formed at the anode.
(c) Electrolysis of acidulated water is an example of ………… (Reduction/oxidation/redox reaction/synthesis).
Answer: Electrolysis of acidulated water is an example of a redox reaction.
10. Explain the following:
(a) A solution of cane sugar does not conduct electricity, but a solution of sodium chloride is a good conductor,
Answer: A solution of cane sugar does not conduct electricity because cane sugar is a non-electrolyte. In solution, it exists as molecules and does not produce ions. Conduction of electricity in solution requires the presence of free mobile ions. A solution of sodium chloride is a good conductor because sodium chloride is an electrolyte. In aqueous solution, it dissociates into free mobile sodium ions (Na⁺) and chloride ions (Cl⁻), which can carry an electric current.
(b) Hydrochloric acid is a good conductor of electricity,
Answer: Hydrochloric acid (HCl) is a good conductor of electricity when dissolved in water because it is a polar covalent compound that ionises in aqueous solution to produce free mobile hydrogen ions (H⁺, which form H₃O⁺ with water) and chloride ions (Cl⁻). These ions are responsible for conducting electricity.
(c) During the electrolysis of an aqueous solution of NaCl, hydrogen ion is reduced at the cathode and not the sodium ion though both Na⁺ and H⁺ ions are present in the solution.
Answer: During the electrolysis of an aqueous solution of NaCl, both Na⁺ ions (from NaCl) and H⁺ ions (from the dissociation of water) are present and migrate to the cathode. However, H⁺ ions are preferentially reduced at the cathode instead of Na⁺ ions. This is because H⁺ ions are lower than Na⁺ ions in the electrochemical series, meaning H⁺ ions gain electrons more easily than Na⁺ ions.
(d) On electrolysis of dilute copper (II) sulphate solution, copper is deposited at the cathode but no hydrogen gas evolves there. Explain why?
Answer: On electrolysis of dilute copper (II) sulphate solution, both Cu²⁺ ions (from CuSO₄) and H⁺ ions (from water) migrate to the cathode. Copper is deposited at the cathode because Cu²⁺ ions are lower than H⁺ ions in the electrochemical series. This means Cu²⁺ ions have a greater tendency to gain electrons and get discharged in preference to H⁺ ions.
(e) When a dilute aqueous solution of sodium chloride is electrolysed between platinum electrodes, hydrogen gas is evolved at the cathode but metallic sodium is not deposited. Why?
Answer: When a dilute aqueous solution of sodium chloride is electrolysed, H⁺ ions (from the dissociation of water) are preferentially discharged at the cathode over Na⁺ ions (from NaCl). This is because H⁺ is lower than Na⁺ in the electrochemical series, indicating that H⁺ ions accept electrons more readily than Na⁺ ions. Therefore, hydrogen gas is evolved, and metallic sodium is not deposited.
(f) Zinc can produce hydrogen on reacting with acids but copper cannot. Explain.
Answer: Zinc can produce hydrogen on reacting with acids because zinc (Zn) is placed above hydrogen [H] in the electrochemical series of metals. Metals above hydrogen in the series are more reactive and can displace hydrogen from acids. Copper (Cu), however, is placed below hydrogen in the electrochemical series, meaning it is less reactive than hydrogen and cannot displace hydrogen from acids.
Exercise
MCQs
1. During the electrolysis of molten lead bromide, which of the following takes place?
(a) Bromine is released at the cathode,
(b) Lead is deposited at the anode,
(c) Bromine ions gain electrons,
(d) Lead is deposited at the cathode.
Answer: (d) Lead is deposited at the cathode.
2. The aqueous solution of the compound which contains both ions and molecules is :
(a) H₂SO₄
(b) HCl
(c) HNO₃
(d) CH₃COOH
Answer: (d) CH₃COOH
3. A compound which during electrolysis in its molten state, liberates a reddish brown gas at the anode is :
(a) Sodium chloride
(b) Copper [II] oxide
(c) Copper [II] sulphate
(d) Lead [II] bromide
Answer: (d) Lead [II] bromide
4. Identify the weak electrolyte from the following :
(a) Sodium chloride solution,
(b) Dilute hydrochloric acid,
(c) Dilute sulphuric acid,
(d) Aq. acetic acid
Answer: (d) Aq. acetic acid
5. An aqueous electrolyte consists of the following ions. The ion which could be discharged most readily during electrolysis is :
(a) Fe²⁺
(b) Cu²⁺
(c) H⁺
(d) Al³⁺
Answer: (b) Cu²⁺
6. During silver plating of an article using potassium argentocyanide as an electrolyte, the anode material should be :
(a) Cu
(b) Ag
(c) Pt
(d) Fe
Answer: (b) Ag
7. State which of these will act as a non-electrolyte :
(a) Liquid carbon tetrachloride
(b) Acetic acid
(c) Sodium hydroxide aqueous solution
(d) Potassium chloride aqueous solution
Answer: (a) Liquid carbon tetrachloride
8. The observation seen when fused lead bromide is electrolysed is:
(a) A silver grey deposit at anode and a reddish brown deposit at cathode.
(b) A silver grey deposit at cathode and a reddish brown deposit at anode.
(c) A silver grey deposit at cathode and reddish brown fumes at anode.
(d) Silver grey fumes at anode and reddish brown fumes at cathode.
Answer: (c) A silver grey deposit at cathode and reddish brown fumes at anode.
9. During electroplating an article with silver, the electrolyte used is:
(a) Silver nitrate solution
(b) Silver cyanide solution
(c) Sodium argentocyanide solution
(d) Nickel sulphate solution.
Answer: (c) Sodium argentocyanide solution
10. The metallic electrode which does not take part in an electrolytic reaction is :
(a) Cu
(b) Ag,
(c) Pt
(d) Ni
Answer: (c) Pt
11. The ion which is discharged at the anode during the electrolysis of copper sulphate solution using platinum electrodes as anode and cathode is :
(a) Cu²⁺
(b) OH⁻
(c) SO₄²⁻
(d) H⁺
Answer: (b) OH⁻
12. When dilute sodium chloride is electrolysed using graphite electrodes, the cation which is discharged at the cathode most readily is :
(a) Na⁺
(b) OH⁻
(c) H⁺
(d) Cl⁻
Answer: (c) H⁺
13. Cathode is a reducing electrode because :
(a) it has less number of electrons
(b) it has deficiency of electrons
(c) cations gain electrons from cathode
(d) anions lose electrons to cathode
Answer: (c) cations gain electrons from cathode
14. Which statement about conduction of electricity is correct?
(a) Electricity is conducted in an aqueous solution by electrons.
(b) Electricity is conducted in a metal wire by ions.
(c) Electricity is conducted in a molten electrolyte by electrons.
(d) Electricity is conducted in an acidic solution by ions.
Answer: (d) Electricity is conducted in an acidic solution by ions.
15. The oxidation reaction among the following is:
(a) Fe³⁺ + 3e⁻ → Fe
(b) Fe²⁺ – e⁻ → Fe³⁺
(c) Cl₂ + 2e⁻ → 2Cl⁻
(d) Cu²⁺ + 2e⁻ → Cu
Answer: (b) Fe²⁺ – e⁻ → Fe³⁺
16. During electrolysis, what happens to metallic ions and at which electrode do they get discharged ?
P Reduction of metallic ions, cathode
Q Oxidation of metallic ions, anode
R Oxidation of metallic ions, cathode
(a) Only P
(b) Only Q
(c) both P and Q
(d) Both Q and R
Answer: (a) Only P
The following questions are Assertion-Reason based questions. Choose the answer based on the codes given below.
(1) Both A and R are true and R is the correct explanation of A.
(2) Both A and R are true but R is not the correct explanation of A.
(3) A is true but R is false.
(4) A is false but R is true.
17. Assertion (A): Electrolysis establishes a relationship between electrical energy and chemical energy.
Reason (R): Substances like metals, carbon (graphite) are conductors while salts like NaCl, KCl are electrolytes.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (b) (2)
18. Assertion (A) : Benzene acts as non-electrolyte.
Reason (R) : Benzene does not have free ions.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (a) (1)
19. Assertion (A): Metals and alloys conduct electricity with the help of electrons.
Reason (R): Non-metals do not have free electrons.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (b) (2)
20. Assertion (A): Weak electrolytes do not allow any electricity to flow through them.
Reason (R): Substances which are composed of only molecules do not allow any current to flow through them.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (d) (4)
21. Assertion (A) : NaCl is an example of an electrolyte.
Reason (R): NaCl conducts electricity in aqueous or molten state.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (a) (1)
22. Assertion (A): An electrolyte can be acid, base or salt.
Reason (R): An electrolyte is a substance which can conduct electricity in aqueous or molten state.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (b) (2)
23. Assertion (A): Oxidation occurs at the anode.
Reason (R): Electrons are gained at the anode.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (c) (3)
24. Assertion (A): Polar covalent compounds can form ions when dissolved in water.
Reason (R): Electrovalent compounds show dissociation.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (b) (2)
25. Assertion (A) : A block of pure silver metal is used as anode during silver plating.
Reason (R): Silver ions are deposited as metal at the anode.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (c) (3)
Very Short Answer Type
1. (Choose the correct word to fill in the blanks).
Cations are formed by ….. (loss/gain) of electrons and anions are formed by ……….. (loss/gain) of electrons.
Answer: Cations are formed by loss of electrons and anions are formed by gain of electrons. Metal elements lose electrons from the outermost shell of their atoms, forming positive metallic ions, i.e. forming cations. For example, sulphur, a non-metal, gains electrons to form an anion like S²⁻.
2. Match the following in column A with the correct answer from the choices given in column B :
Answer:
3. Name the kind of particles present in:
(a) Sodium hydroxide solution,
(b) Carbonic acid
(c) Sugar solution
Answer: (a) In Sodium hydroxide solution, which is a strong electrolyte, the particles present are (almost) only free mobile ions (specifically, Na⁺ and OH⁻ ions).
(b) In Carbonic acid, which is a weak electrolyte, the particles present are ions (such as H⁺, HCO₃⁻ ions) as well as molecules (H₂CO₃ molecules), as it is partially dissociated.
(c) In Sugar solution, which is a non-electrolyte, the particles present are only molecules, as non-electrolytes do not have ions even in solution.
4. (a) What kind of particles will be found in a liquid compound which is a non- electrolyte?
(b) If HX is a weak acid, what particles will be present in its dilute solution apart from those of water
(c) What ions must be present in a solution used for electroplating a particular metal ?
Answer: (a) In a liquid compound which is a non-electrolyte, the kind of particles found will be only molecules, as non-electrolytes do not have ions.
(b) If HX is a weak acid, its dilute solution, apart from those of water, will contain HX molecules, H⁺ ions, and X⁻ ions, because weak electrolytes are partially dissociated and their solutions contain ions as well as molecules.
(c) For electroplating a particular metal, the electrolyte solution must contain ions of the metal which is to be plated on the article.
5. A strip of copper is placed in four different salt solutions. They are KNO₃, AgNO₃, Zn(NO₃)₂, Ca(NO₃)₂. Which one of the solutions will finally turn blue ?
Answer: The AgNO₃ solution will finally turn blue. Copper is above silver in the electrochemical series, meaning copper can displace silver from its salt solution. When copper displaces silver from AgNO₃, copper ions (Cu²⁺) are formed (Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)). Aqueous solutions containing Cu²⁺ ions are blue. Copper cannot displace K, Zn, or Ca from their salt solutions as these metals are higher than copper in the electrochemical series.
6. Here is an electrode reaction Cu → Cu²⁺ + 2e⁻. At which electrode (anode or cathode) would such a reaction takes place? Is this an example of oxidation or reduction ?
Answer: The reaction Cu → Cu²⁺ + 2e⁻ involves the loss of electrons. Oxidation is defined as a process in which an atom or an ion loses electron(s). Therefore, this reaction is an example of oxidation. Oxidation takes place at the anode. So, this reaction would take place at the anode.
7. Aqueous solution of nickel sulphate contains Ni²⁺ and SO₄²⁻ ions.
(a) Which ion moves towards the cathode ?
(b) What is the product at the anode ?
Answer: (a) Cations, which are positively charged ions, migrate to the cathode. In an aqueous solution of nickel sulphate, Ni²⁺ is the cation. Therefore, the Ni²⁺ ion moves towards the cathode.
(b) In an aqueous solution of nickel sulphate, the anions present are SO₄²⁻ from nickel sulphate and OH⁻ from the dissociation of water. Comparing their positions in the electrochemical series of anions, OH⁻ ions are discharged more easily than SO₄²⁻ ions at an inert anode. The discharge of OH⁻ ions (4OH⁻ → 2H₂O + O₂ + 4e⁻) results in the formation of oxygen gas. Therefore, assuming an inert anode, the product at the anode is oxygen gas.
8. Identify: A gas which does not conduct electricity in the liquid state but conducts electricity when dissolved in water.
Answer: The gas is hydrogen chloride (HCl). In the gaseous state or in the pure liquid state, HCl is unionized and does not conduct an electric current. However, when hydrogen chloride is added to water, it ionises to form H⁺ ions (which combine with water to form hydronium ions, H₃O⁺) and Cl⁻ ions, and this solution conducts electricity.
9. Give one word or phrase for: Electrolytic deposition of a superior metal on a baser metal.
Answer: The phrase is Electroplating. Electroplating is a process in which a thin film of a metal like gold or silver (superior metals) gets deposited on another metallic article (baser metal) with the help of electricity.
Short Answer Type
1. Give reasons for the following :
(a) The blue colour of aqueous copper sulphate fades when it is electrolysed using platinum electrodes.
Answer: When aqueous copper sulphate solution is electrolysed using platinum electrodes, copper ions (Cu²⁺) are discharged at the cathode to form neutral copper atoms (Cu²⁺ + 2e⁻ → Cu). At the anode (platinum, an inert electrode), hydroxide ions are discharged to form oxygen gas, and copper ions are not replenished from the anode. As the electrolysis proceeds, the concentration of Cu²⁺ ions in the solution decreases. Since Cu²⁺ ions are responsible for the blue colour of the solution, the blue colour fades due to this decrease in Cu²⁺ ions, and finally the solution becomes colourless as soon as Cu²⁺ ions are finished.
(b) Lead bromide undergoes electrolytic dissociation in the molten state but is a non-electrolyte in the solid state.
Answer: In the solid state, lead bromide is an electrovalent compound where lead ions (Pb²⁺) and bromide ions (Br⁻) are held together by strong electrostatic forces of attraction. These ions are not free to move, and therefore solid lead bromide does not conduct electricity and acts as a non-electrolyte. When lead bromide is in the molten (fused) state, the high temperature weakens these electrostatic forces, and the ions become free and mobile. These mobile ions can then conduct an electric current, and lead bromide undergoes electrolytic dissociation, making it an electrolyte.
(c) Aluminium is extracted from its oxide by electrolytic reduction and not by conventional reducing agents.
Answer: Aluminium is a metal that is high in the electrochemical series and has a strong affinity for oxygen. Its oxide, alumina (Al₂O₃), is highly stable. Due to this high stability, it is not possible to reduce aluminium oxide to aluminium metal using common reducing agents like carbon monoxide or hydrogen. Therefore, aluminium is extracted from its oxide by electrolytic reduction (electrolysis), a process known as electrometallurgy, which provides a convenient method to extract such reactive metals.
(d) The ratio of hydrogen and oxygen formed at the cathode and anode is 2:1 by volume.
Answer: During the electrolysis of acidified water, hydrogen ions (H⁺) are discharged at the cathode and hydroxide ions (OH⁻) are discharged at the anode.
At the cathode: 4H⁺ + 4e⁻ → 2H₂
At the anode: 4OH⁻ → 4OH + 4e⁻; and 4OH (or 2OH + 2OH) → 2H₂O + O₂
The formation of 1 molecule of oxygen at the anode releases 4 electrons. To maintain electrical neutrality, the reaction at the cathode must take up these 4 electrons, which results in the formation of 2 molecules of hydrogen. This shows that the number of molecules of hydrogen produced is twice that of oxygen molecules. According to Avogadro’s Law, molecules can be substituted by volumes. Hence, the electrolysis of water gives 2 volumes of H₂ and 1 volume of O₂, making the ratio of hydrogen to oxygen 2:1 by volume.
(e) In the electrolysis of acidified water, dilute sulphuric acid is preferred to dilute nitric acid for acidification.
Answer: In the electrolysis of acidified water, dilute sulphuric acid is preferred for acidification because sulphuric acid is non-volatile. Dilute nitric acid or hydrochloric acid are volatile acids, which means they can evaporate along with the gases produced during electrolysis, potentially altering the concentration of the electrolyte or introducing impurities.
(f) Ammonia is unionised in the gaseous state but in the aqueous solution, it is a weak electrolyte.
Answer: Ammonia (NH₃) is a polar covalent compound. In its gaseous state, it exists as molecules and is unionised. When ammonia is dissolved in water (an aqueous solution), it ionises to a small extent to form ammonium ions (NH₄⁺) and hydroxide ions (OH⁻), forming ammonium hydroxide (NH₄OH).
NH₃ + H₂O ⇌ NH₄OH ⇌ NH₄⁺ + OH⁻
Because it only partially dissociates into ions in aqueous solution, it allows only a small amount of electricity to flow through, and is therefore classified as a weak electrolyte.
(g) A direct current instead of alternating current should be used in electrolysis.
Answer: Electrolysis is caused by Direct Current (DC). An alternating current (AC) causes the direction of ion movement and electrode polarity to reverse periodically. This prevents a sustained, unidirectional migration of ions and discharge at specific electrodes, meaning AC does not cause any net chemical change when passed through an electrolyte and therefore does not help electrolysis to occur effectively. Direct current ensures that ions migrate consistently towards the appropriate electrodes, allowing for the continuous chemical reactions of oxidation and reduction that define electrolysis.
(h) Carbon tetrachloride is a liquid but is a non-electrolyte which does not conduct electricity.
Answer: Carbon tetrachloride is a non-electrolyte because it is a covalent compound that does not ionise or dissociate to produce free mobile ions when in liquid state or when dissolved (though it is largely insoluble in water). Non-electrolytes contain only molecules and do not have ions. The conduction of electricity in electrolytes is due to the movement of ions, and since carbon tetrachloride does not furnish ions, it does not conduct electricity.
(i) Potassium is not extracted by electrolysis of its aqueous salt solution.
Answer: Potassium is a highly reactive metal, placed high in the electrochemical series. When an aqueous salt solution of potassium is electrolysed, the solution contains potassium ions (K⁺) from the salt and hydrogen ions (H⁺) from the dissociation of water. During electrolysis, H⁺ ions get discharged at the cathode in preference to K⁺ ions because H⁺ ions are lower in the electrochemical series and gain electrons more easily. Thus, hydrogen gas is formed at the cathode instead of potassium metal.
(j) The electrolysis of acidulated water is considered to be an example of catalysis.
Answer: Pure water consists almost entirely of molecules and is a very poor conductor of electricity because it ionises to a very small extent. When a trace of dilute sulphuric acid is added to water, the acid catalyses the ionisation of water, increasing the concentration of H⁺ and OH⁻ ions. This increased ion concentration allows an electric current to pass through more easily, facilitating electrolysis. As dilute sulphuric acid catalyses this ionisation, the electrolysis of acidified water is considered an example of catalysis.
(k) During electrolysis of molten lead bromide, graphite anode is preferred to other electrodes.
Answer: During the electrolysis of molten lead bromide, bromine is liberated at the anode. Graphite is preferred as the anode material over other inert electrodes like platinum because graphite is unaffected by the reactive bromine vapours produced at the high temperatures of the electrolysis. Platinum, while inert, can be attacked by bromine under these conditions.
(l) Electrolysis of molten lead bromide is considered to be a redox reaction.
Answer: The process of electrolysis is inherently a redox reaction. In the electrolysis of molten lead bromide:
At the cathode, lead ions (Pb²⁺) gain electrons and are reduced to lead metal: Pb²⁺ + 2e⁻ → Pb (Reduction).
At the anode, bromide ions (Br⁻) lose electrons and are oxidised to bromine molecules: 2Br⁻ – 2e⁻ → Br₂ (Oxidation).
Since reduction (gain of electrons) occurs at the cathode and oxidation (loss of electrons) occurs at the anode simultaneously, the overall process is a redox reaction.
2. Explain :
(a) Sodium chloride will conduct electricity only in fused or aqueous solution state.
Answer: In the solid state, sodium chloride (NaCl) is an electrovalent compound consisting of sodium ions (Na⁺) and chloride ions (Cl⁻) held rigidly in a crystal lattice by strong electrostatic forces of attraction. These ions are not free to move, and therefore solid NaCl cannot conduct electricity.
When NaCl is in the fused (molten) state or dissolved in water (aqueous solution), these electrostatic forces are overcome. In the molten state, the high temperature provides enough energy to break the bonds, and in aqueous solution, polar water molecules weaken the forces of attraction. This results in the Na⁺ and Cl⁻ ions becoming free and mobile. These mobile ions are then able to carry an electric current, allowing NaCl to conduct electricity in these states.
(b) In the electroplating of an article with silver, the electrolyte sodium argento-cyanide solution is preferred over silver nitrate solution.
Answer: In the electroplating of an article with silver, sodium argento-cyanide solution (Na[Ag(CN)₂]) is preferred over silver nitrate solution (AgNO₃) as the electrolyte. If silver nitrate solution is used directly, the deposition of silver will be very fast. This rapid deposition results in a coating that is not very smooth and uniform. Sodium argento-cyanide solution provides a slower, more controlled deposition of silver ions, leading to a smoother, more uniform, and adherent coating on the article.
(c) Although copper is a good conductor of electricity, it is a non-electrolyte.
Answer: Copper is a good conductor of electricity because it is a metallic conductor. In metals like copper, electric current is conducted by the flow of free electrons present in the metallic lattice. This conduction occurs without any chemical change or decomposition of the copper itself.
An electrolyte, on the other hand, is a chemical compound that conducts electricity in its molten state or in aqueous solution due to the presence of mobile ions, and it undergoes chemical decomposition during the process. A non-electrolyte is a compound that does not conduct electricity in these states because it does not produce ions.
Since copper conducts electricity via electrons and does not undergo chemical decomposition (it’s an element, not a compound that dissociates into ions for conduction in the electrolytic sense), it does not fit the definition of an electrolyte. Therefore, in the context of electrolytic phenomena, copper is considered a non-electrolyte, even though it is an excellent metallic conductor.
3. Write two applications of electrolysis in which the anode diminishes in mass.
Answer: Two applications of electrolysis in which the anode diminishes in mass are:
(i) Electroplating with metals: In electroplating, the anode is made of the metal that is to be plated onto an article (e.g., a pure silver anode for silver plating). During electrolysis, the metal atoms of the anode lose electrons, dissolve as ions into the electrolyte solution, and are then deposited on the cathode. This causes the anode to continuously diminish in mass and require periodic replacement.
(ii) Electrorefining of metals: In electrorefining, such as the refining of copper, the impure metal is made the anode (e.g., slabs of impure copper). During electrolysis, metal atoms from the impure anode lose electrons and pass into the electrolyte solution as ions, leaving impurities behind. These pure metal ions are then deposited on the pure metal cathode. As a result, the impure anode gradually gets consumed and diminishes in mass.
4. A solution contains magnesium ions (Mg²⁺), iron (II) ions (Fe²⁺) and copper ions (Cu²⁺). On passing an electric current through this solution, which ions will be the first to be discharged at the cathode? Write the equation for the cathode reaction.
Answer: On passing an electric current through the solution containing magnesium ions (Mg²⁺), iron (II) ions (Fe²⁺), and copper ions (Cu²⁺), the ion that will be discharged first at the cathode is determined by their relative positions in the electrochemical series. Elements lower in the series get discharged more easily at the cathode because their cations can more easily gain electrons.
Comparing the positions:
Magnesium (Mg) is high in the series.
Iron (Fe) is below magnesium but above copper.
Copper (Cu) is lower in the series compared to magnesium and iron.
Therefore, copper ions (Cu²⁺) will be the first to be discharged at the cathode.
The equation for the cathode reaction is:
Cu²⁺ + 2e⁻ → Cu
5. During electroplating of an article with nickel –
(a) Name
A. The electrolyte
B. The cathode
C. The anode
Answer: A. The electrolyte: Aqueous solution of nickel sulphate [NiSO₄], often containing a small amount of dilute sulphuric acid.
B. The cathode: The article to be electroplated (e.g., an iron article).
C. The anode: A block of pure nickel metal.
(ii) Give the reaction of the electrolysis at
A. The cathode
B. The anode
Answer: A. Reaction at the cathode: Nickel ions from the electrolyte gain electrons and are deposited as neutral nickel atoms on the article.
Ni²⁺ + 2e⁻ → Ni [deposited]
B. Reaction at the anode: The pure nickel anode loses electrons and nickel atoms enter the solution as nickel ions, replenishing the ions discharged at the cathode.
Ni – 2e⁻ → Ni²⁺
6. A, B and C are three electrolytic cells, connected in different circuits. Cell ‘A’ contains NaCl solution, and the bulb in the circuit glows brightly, when the circuit is completed. Cell ‘B’ contains acetic acid and the bulb glows dimly. Cell ‘C’ contains sugar solution, and the bulb does not glow. Give reason for each observation.
Answer:
- Cell ‘A’ (NaCl solution, bulb glows brightly): Sodium chloride (NaCl) solution is a strong electrolyte. Strong electrolytes dissociate almost completely into ions (Na⁺ and Cl⁻) in aqueous solution. The high concentration of mobile ions allows a large amount of electric current to flow through the solution, causing the bulb to glow brightly.
- Cell ‘B’ (Acetic acid, bulb glows dimly): Acetic acid (CH₃COOH) is a weak electrolyte. Weak electrolytes dissociate only partially into ions (CH₃COO⁻ and H⁺) in aqueous solution. The lower concentration of mobile ions means that only a small amount of electric current can flow through the solution, causing the bulb to glow dimly.
- Cell ‘C’ (Sugar solution, bulb does not glow): Sugar solution is a non-electrolyte. Non-electrolytes are substances that do not dissociate into ions when dissolved in water; they exist as molecules in solution. Since there are no free mobile ions to carry the electric current, the sugar solution does not conduct electricity, and therefore the bulb does not glow.
7. Differentiate between electrical conductivity of copper sulphate solution and of copper metal.
Answer: The differences between the electrical conductivity of copper sulphate solution (an electrolytic conductor) and copper metal (a metallic conductor) are as follows:
(Metallic Conductor)
(Electrolytic Conductor)
8. State one appropriate observation for: Electricity is passed through molten lead bromide.
Answer: When electricity is passed through molten lead bromide, one appropriate observation is that silvery gray metal lead is formed on the cathode. (Alternatively: Dark reddish brown fumes of bromine evolve at the anode.)
9. State your observation: At the cathode when acidified aqueous copper sulphate solution is electrolysed with copper electrodes.
Answer: When acidified aqueous copper sulphate solution is electrolysed with copper electrodes, the observation at the cathode is that pink or reddish brown copper is deposited.
10. State observation at the anode when aqueous copper sulphate solution is electrolysed using copper electrodes.
Answer: When aqueous copper sulphate solution is electrolysed using copper electrodes, the observation at the anode is that the copper anode keeps dissolving and becomes thinner.
11. State which electrode anode or cathode is the oxidising electrode. Give a reason for the same.
Answer: The anode is the oxidising electrode.
The reason is that oxidation takes place at the anode. Oxidation, in the electronic concept, is defined as a process in which an atom or an ion loses electron(s).
12. M₂O is the oxide of a metal ‘M’ which is above hydrogen in the activity series. M₂O when dissolved in water forms the corresponding hydroxide which is a good conductor of electricity.
(a) State the reaction taking place at the cathode
Answer: The solution contains M⁺ ions from the dissolved hydroxide MOH, and H⁺ ions from the dissociation of water. Since metal ‘M’ is above hydrogen in the activity series, H⁺ ions will be preferentially discharged at the cathode.
The reaction taking place at the cathode is:
H⁺ + e⁻ → H
H + H → H₂
(or 2H⁺ + 2e⁻ → H₂)
Hydrogen gas is evolved at the cathode.
(b) Name the product at the anode.
Answer: The anions present in the solution are OH⁻ ions (from the dissolved hydroxide MOH and from the dissociation of water). These OH⁻ ions migrate to the anode and are discharged.
The reactions leading to the product are:
OH⁻ – e⁻ → OH
OH + OH → H₂O + O
O + O → O₂
(or 4OH⁻ – 4e⁻ → 2H₂O + O₂)
The product at the anode is oxygen gas.
Long Answer Type
1. Element X is a metal with a valency 2. Element Y is a non-metal with a valency 3.
(a) Write equations to show how X and Y form ions ?
Answer: The equations to show how X and Y form ions are:
For metal X with valency 2, it loses 2 electrons to form a positive ion:
X → X²⁺ + 2e⁻
For non-metal Y with valency 3, it gains 3 electrons to form a negative ion:
Y + 3e⁻ → Y³⁻
(b) If Y is a diatomic gas, write the equation for the direct combination of X and Y to form a compound.
Answer: If Y is a diatomic gas (Y₂), and X has a valency of 2 and Y has a valency of 3, the compound formed is X₃Y₂. The equation for the direct combination is:
3X + Y₂ → X₃Y₂
(c) If the compound formed between X and Y is melted and an electric current passed through the molten compound, the element X will be obtained at the ……………… and Y at the ………………. of the electrolytic cell. (Provide the missing words).
Answer: If the compound formed between X and Y is melted and an electric current passed through the molten compound, the element X, being a metal and forming positive ions (cations), will be obtained at the cathode. Element Y, being a non-metal and forming negative ions (anions), will be obtained at the anode of the electrolytic cell. This is because only hydrogen gas and metals are liberated at the cathode, and only non-metals are liberated at the anode.
2. (a) Draw a labelled diagram to show how iron is electroplated with copper.
Answer: To show how iron is electroplated with copper:
The setup would involve an electrolytic cell. The anode would be a plate or rod of pure copper. The cathode would be the iron article to be electroplated. The electrolyte would be a solution of copper sulphate, possibly acidified with a small amount of dilute sulphuric acid. A battery would be connected such that the copper plate is the anode (connected to the positive terminal) and the iron article is the cathode (connected to the negative terminal).
(b) Which solution is preferred as electrolyte, CuSO₄ or FeSO₄ ?
Answer: For electroplating iron with copper, the electrolyte must contain ions of the metal which is to be plated on the article. Therefore, copper sulphate (CuSO₄) solution is preferred as the electrolyte because it provides copper ions (Cu²⁺) for deposition.
(c) Describe what happens to the iron object and the copper rod.
Answer: During the electroplating of iron with copper:
The iron object, which is the cathode, will have copper ions (Cu²⁺) from the copper sulphate solution migrating towards it. These copper ions gain electrons at the cathode and are deposited as neutral metallic copper atoms on the surface of the iron object. Thus, the iron object gets a coating of copper and its thickness increases over time.
The copper rod, which is the anode, continuously dissolves as copper atoms lose electrons and go into the solution as copper ions (Cu²⁺). This replenishes the copper ions in the electrolyte. Consequently, the copper rod will become thinner over time and may need to be periodically replaced.
3. A metal article is to be electroplated with silver. The electrolyte selected is sodium argentocyanide.
(a) What kind of salt is sodium argentocyanide ?
Answer: Sodium argentocyanide, Na[Ag(CN)₂], is the electrolyte used for silver plating. It is a salt of silver, specifically a cyanide complex, prepared by adding sodium cyanide to a solution of silver nitrate until the precipitate of silver cyanide formed just dissolves.
(b) Why is it preferred to silver nitrate as an electrolyte?
Answer: Sodium argentocyanide is preferred over silver nitrate as an electrolyte for silver plating because if silver nitrate solution is used directly, the deposition of silver will be very fast and hence not very smooth and uniform. Sodium argentocyanide allows for a slower, smoother, and more uniform deposition of silver.
(c) State one condition to ensure that the deposit is smooth, firm and long lasting.
Answer: One condition to ensure that the deposit is smooth, firm, and long lasting is that a low current for a longer time should be used. Higher current causes uneven deposition of the metal, while longer time and low current initiates a thicker uniform deposition.
(d) Write the reaction taking place at the cathode.
Answer: The reaction taking place at the cathode, where the article to be electroplated is placed, is the reduction of silver ions from the electrolyte:
Ag⁺ (from electrolyte) + e⁻ → Ag (deposited on article)
(e) Write the reaction taking place at the anode.
Answer: The reaction taking place at the anode, which is a plate of pure clean silver, is the oxidation of silver metal:
Ag (from anode) – e⁻ → Ag⁺ (goes into electrolyte)
4. During the electrolysis of copper [II] sulphate solution using platinum as cathode and carbon as anode
(a) State what you observe at the cathode and at the anode.
Answer: During the electrolysis of copper (II) sulphate solution using a platinum cathode and a carbon (inert) anode:
At the cathode (platinum): Pink or reddish brown copper is deposited.
At the anode (carbon): The OH⁻ ions, being lower in the electrochemical series as compared to SO₄²⁻ ions, discharge to form neutral (OH) radical. The neutral (OH) radicals reunite to form water and oxygen. Thus, bubbles of oxygen gas are evolved at the anode.
(b) State the change noticed in the electrolyte.
Answer: The blue colour of Cu²⁺ ions in the electrolyte solution fades due to the decrease in Cu²⁺ ions as they are deposited at the cathode. Finally, the solution becomes colourless as soon as Cu²⁺ ions are finished. Also, as Cu²⁺ ions are removed and OH⁻ ions are discharged, the concentration of H⁺ ions (from water dissociation) and SO₄²⁻ ions increases, making the solution acidic.
(c) Write the reactions at the cathode and at the anode.
Answer: The reactions are:
At cathode (platinum): Copper ions are discharged.
Cu²⁺ + 2e⁻ → Cu
At anode (carbon): Hydroxide ions are discharged.
OH⁻ – e⁻ → OH
OH + OH → H₂O + O
O + O → O₂
Alternatively, the overall anode reaction for oxygen evolution from hydroxide ions can be written based on the discharge of 4OH⁻:
4OH⁻ – 4e⁻ → 4OH
Then, 2OH + 2OH → 2H₂O + O₂ (or 4OH → 2H₂O + O₂)
5. (a) Copy and complete the following table which refers to two practical applications of electrolysis
Answer:
(b) Write equations of reactions taking place at anode for Q. 2(a).
Answer: For Q. 2(a), which is the electroplating of iron with copper, the anode is made of copper. The reaction taking place at the copper anode is:
Cu → Cu²⁺ + 2e⁻
6. The following sketch represents the electroplating of an Iron cup with Nickel metal. Study the diagram and answer the following questions:
(a) During electroplating the iron cup is placed at the cathode. Why?
Answer: During electroplating, the iron cup is placed at the cathode because, during the electrolytic reaction, the metal (nickel in this case) is always deposited at the cathode by the gain of electrons.
(b) Name the ion that must be present in the electrolyte.
Answer: The electrolyte must contain ions of the metal which is to be plated on the article. For electroplating with nickel, the electrolyte is an aqueous solution of nickel sulphate [NiSO₄], so nickel ions (Ni²⁺) must be present.
(c) State one condition that is necessary to ensure that the deposit is smooth, firm and even.
Answer: A low current for a longer time should be used. This is because a higher current causes uneven deposition of the metal, while a longer time and low current initiate a thicker, uniform deposition.
(d) Write the reaction taking place at the cathode.
Answer: The reaction taking place at the cathode is:
Ni²⁺ + 2e⁻ → Ni [deposited]
(e) What change would you observe at the anode?
Answer: The nickel anode loses electrons to give Ni²⁺ ions in solution. The metal anode continuously dissolves as ions in solution, so it will become thinner and will need to be periodically replaced.
7. Copper sulphate soln. is electrolysed using copper electrodes.
(a) Which electrode to your left or right is known as the oxidising electrode and why?
Answer: In the provided diagram, the electrode to the left is the anode. The anode is the oxidising electrode because oxidation takes place at the anode.
(b) Write the equation representing the reaction that occurs.
Answer: The reactions that occur are:
At the anode: Cu – 2e⁻ → Cu²⁺ (Copper anode dissolves)
At the cathode: Cu²⁺ + 2e⁻ → Cu (Copper is deposited)
(c) State two appropriate observations for the above electrolysis reaction.
Answer: Two appropriate observations for this electrolysis reaction are:
(i) The copper anode keeps dissolving during the reaction and becomes thinner.
(ii) Pink or reddish brown copper is deposited on the cathode, causing the cathode to become thicker.
(iii) The blue colour of the aqueous copper (II) sulphate solution remains unchanged during its electrolysis due to the copper electrodes.
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