Chemical Reactions and Equations: Class 10 Science Chapter 1

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In Class 10 Science Chapter 1: Chemical Reactions and Equations (NCERT, NBSE, SEBA, TBSE etc.), you will be introduced to the fundamental principles of chemistry. You will learn about the different types of chemical reactions that occur in nature, including synthesis reactions, decomposition reactions, displacement reactions, and redox reactions. You will also learn how to write balanced chemical equations and how to identify the reactants and products in a chemical reaction. In addition, this chapter covers the concepts of oxidation and reduction, which are reactions that involve the gain or loss of oxygen atoms. By the end of this chapter, you should have a good understanding of the basic principles of chemical reactions and be able to apply this knowledge to a variety of real-world situations.

Chemical Reactions and Equations

Below are questions, answer, MCQs, textbook solutions of the chapter. If you are a registered user, you can also download it in PDF by clicking on the PDF icon.

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1. Why should a magnesium ribbon be cleaned before it is burnt in the air?

Answer: Magnesium is a very reactive metal. When stored, it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sandpaper for removing this layer so that the underlying metal can be exposed to air.

2. Write the balanced equation for the following chemical reactions.

(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Answer: (i) H2 (g) + Cl2 (g) → 2HCl (g)
(ii) 3BaCl2 (s) + Al2(SO4)3 (s) → 3BaSO4 (s) + 2AlCl3 (s)
(iii) 2Na(s) + 2H2O (l) → 2NaOH (aq) + H2 (g)

3. Write a balanced chemical equation with state symbols for the following reactions.

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer: (i) BaCl2 (aq) + Na2SO4 (aq) → BaSO4 (s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl (aq) → NaCL (aq) + H2O (l)

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1. A solution of a substance ‘X’ is used for whitewashing.

(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Answer: (i) The substance ‘X’ is calcium oxide. Its chemical formula is CaO.

(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).
CaO (s) + H2O (l) → Ca(OH)2 (aq)
Calcium Oxide (Quick Lime) + Water → Calcium Hydroxide (Slaked Lime)

2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Answer: Water contains two parts of hydrogen and one part oxygen. Therefore, during the electrolysis of water, the amount of hydrogen gas collected in one of the test tubes is double than that of the oxygen produced and collected in the other test tube.

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1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer: When an iron nail is dipped in the copper sulphate solution, iron displaces copper from the copper sulphate because iron is more reactive than copper. Therefore, the colour of the copper sulphate solution changes.

The reaction involved here is: Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)

2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer: 2KBr (aq) + BaI2 (aq) → 2KI (aq) + BaBr2 (aq)

3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.

(i) 4Na (s) + O2 (g) → 2Na2O (s)

Answer: Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.

(ii) CuO (s) + H2 (g) → Cu (s) + H2O (l)

Answer: Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H2) gets oxidised to water (H2O).

Exercise questions and answers

1. Which of the statements about the reaction below are incorrect?
2PbO (s) + C (s) → 2Pb (s) + CO2 (g)

(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.

(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all

Answer: (i) (a) and (b)

2. Fe2O3 + 2Al → Al2O3 + 2Fe

The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.

Answer: (d) displacement reaction.

3. What happens when dilute hydrochloric acid is added to iron filings? Tick the correct Answer:

(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.

Answer: (a) Hydrogen gas and iron chloride are produced.

4. What is a balanced chemical equation? Why should chemical equations be balanced?

Answer: A balanced chemical equation is a chemical equation in which the number of atoms of each element is equal on both the reactant and product side of the equation. Chemical equations should be balanced because the law of conservation of mass states that matter cannot be created or destroyed, so the total mass of the reactants must be equal to the total mass of the products.

5. Translate the following statements into chemical equations and then balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer: (a) H2 (g) + N2 (g) → 2NH3 (g)
(b) 2H2S (g) + 3O2 (g) → 2H2O (l) + 2SO2 (g)
(c) 3BaCl2 (aq) + 2Al2(SO4)3 (aq) → 6BaSO4 (s) + 2AlCl3 (aq)
(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)

6. Balance the following chemical equations.

(i) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(ii) NaOH + H2SO4 → Na2SO4 + H2O
(iii) NaCl + AgNO3 → AgCl + NaNO3
(iv) BaCl2 + H2SO4 → BaSO4 + HCl

Answer: (i) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(ii) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(iii) NaCl + AgNO3 → AgCl + NaNO3
(iv) BaCl2 + H2SO4 → BaSO4 + 2HCl

7. Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Answer: (a) Ca(OH)2 + CO2 → CaCO3 + H2O
(b) Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
(c) 2Al + 3CuCl2 → 2AlCl3 + 3Cu
(d) BaCl2 + K2SO4 → BaSO4 + 2KCl

8. Write the balanced chemical equation for the following and identify the type of reaction in each case.

(a)Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium bromide(s)
(b) Zinc carbonate (s) → Zinc oxide (s) + Carbon dioxide (g)
(c) Hydrogen (g) + Chlorine (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)

Answer: (a) 2KBr (aq) + BaI2 (aq) → 2KI (aq) + BaBr2 (s): Double displacement reaction
(b) ZnCO3 (s) → ZnO (s) + CO2 (g): Decomposition reaction
(c) H2 (g) + Cl2 (g) → 2HCl (g): Combination reaction
(d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g): Displacement reaction

9. What does one mean by exothermic and endothermic reactions? Give examples.

Answer: Exothermic reactions are chemical reactions that release energy in the form of heat, light, or sound. Examples of exothermic reactions include:

C (g) + O2 (g) → CO2 + Heat Energy
2H2 (g) + O2 (g) → 2H2O (l) + Heat Energy

Endothermic reactions are reactions that absorb energy or require energy in order to proceed. Examples of endothermic reactions include:

N2 (g) + O2 (g) → 2NO + Heat Energy
H2O (l) + Heat Energy → H2O (g)

10. Why is a combustion reaction an oxidation reaction?

Answer: A combustion reaction is an oxidation reaction because it involves the combination of a substance with oxygen. In a combustion reaction, a fuel (such as wood, gasoline, or natural gas) reacts with oxygen to produce heat, light, and often, other products such as water or carbon dioxide. The process of adding oxygen to a substance is known as oxidation, so a combustion reaction can be considered an oxidation reaction. The process of removing oxygen from a substance, on the other hand, is known as reduction, and is often associated with the opposite of an oxidation reaction.

11 (i) What happens when copper is heated in air?

Answer: When copper is heated in air, it combines with oxygen to form copper oxide, a black oxide. The chemical equation for this reaction is: 2Cu + O2 → 2CuO

(ii) What happens when the product obtained in the above reaction is heated in hydrogen?

Answer: If hydrogen gas is passed over heated copper oxide, then the black copper oxide is reduced and copper metal is obtained. The chemical equation for this reaction is: CuO (s) + H2 (g) → Cu (s) + H2O (g)

12. An aluminium can was used to store ferrous sulphate solution. It is observed that in lbw days holes appeared in the can. Explain the reason and write the chemical equation to support your answer.

Answer: Aluminium is more reactive than iron, so when the aluminium can is used to store ferrous sulphate solution, the aluminium will displace the iron from the solution, forming holes in the can. The chemical equation for this reaction is: 2FeSO4 + 2Al → Al2(SO4)3 + 3Fe

13. A metal ‘P’ when exposed to moist air for longer period of time, loses its shiny brown surface and gains a green coat. Why this happened? Identify the metal. Write the name and chemical formula of this green coloured compound. List any two ways to prevent this process.

Answer: The metal has corroded because of being exposed to moist air. The green compound formed is basic copper carbonate. The metal is copper. Two ways to prevent this process are painting and oiling.

14. Can oxidation or reduction take place alone? Why or why not? What are such reactions called?

Answer: Oxidation and reduction cannot occur alone, they must always occur together in a reaction. These reactions are called redox reactions.

15. Why is respiration considered an exothermic reaction? Explain.

Answer: Respiration is considered an exothermic reaction because energy is released in this process. During respiration, glucose combines with oxygen present in our cells to form carbon dioxide and water along with energy. The chemical equation for this reaction is: C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + Energy

16. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer: Decomposition reactions are called the opposite of combination reactions because they involve the breakdown of a compound into two or more substances, while combination reactions involve the combination of two or more substances to form a new compound. An example of a decomposition reaction is: CaCO3 → CaO + CO2, and an example of a combination reaction is: CaO + H2O → Ca(OH)2

17. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Answer: (Heat) CaCO3 (s) → CaO (s) + CO2 (g), (Light) 2AgCl (s) → 2Ag (s) + Cl2 (g), (Electricity) 2H20 (I) → 2H2 (g) + 02 (g)

18. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer: In a displacement reaction, a more reactive element replaces a less reactive element in a compound. An example of a displacement reaction is: CuSo4 (aq) + Zn (s) → ZnSO4 (aq) + Cu (s). In a double displacement reaction, atoms or groups of atoms switch places to form new compounds. An example of a double displacement reaction is: Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq)

19. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Answer: The chemical equation for the recovery of silver from silver nitrate solution through displacement by copper metal is: 2AgNO3 (aq) + Cu (s) → Cu(NO3)2 (aq) + 2Ag (s)

20. What do you mean by a precipitation reaction? Explain by giving examples.

Answer: A precipitation reaction is a chemical reaction in which an insoluble solid (precipitate) is formed from two soluble reactants in aqueous solution. An example of a precipitation reaction is the reaction between silver nitrate and sodium chloride solutions, which forms silver chloride precipitate: AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq). Another example is the reaction between potassium chloride and sodium sulfate solutions, which forms potassium sulfate precipitate: KCl (aq) + Na2SO4 (aq) → K2SO4 (s) + 2NaCl (aq)

21. (a) Explain the following in terms of gain or loss of oxygen with two examples each.

(i) Oxidation
(ii) Reduction

Answer: (a) (i) Oxidation is a chemical reaction in which a substance gains oxygen or loses hydrogen. Examples include:

(i) The reaction between magnesium and oxygen to form magnesium oxide: 2Mg (s) + O2 (g) –(Heat)→ 2MgO (s)
(ii) The reaction between copper and oxygen to form copper oxide: 2Cu (s) + O2 (g) –(Heat)→ 2CuO (s)

(a) (ii) Reduction is a chemical reaction in which a substance loses oxygen or gains hydrogen. Examples include:

(i) The reaction between copper oxide and hydrogen to form copper and water: CuO + H2–(Heat)→ Cu + H2O
(ii) The reaction between zinc oxide and carbon to form zinc and carbon monoxide: ZnO + C –(Heat)→ Zn + CO

22. A shiny brown-coloured element ‘X’ on heating in the air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Answer: ‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved in heating copper is given below.

2Cu +O2 –(Heat)→ 2CuO
(Shiny Brown in Colour) (Black in Colour)

23. Why do we apply paint on iron articles?

Answer: Iron articles are painted to prevent them from rusting. After painting iron surface does not come in contact with moisture and air which cause rusting.

24. Oil and fat containing food items are flushed with nitrogen. Why?

Answer: In the presence of oxygen in the air, fats present in fatty food are oxidized to compounds and cause a bad smell. That is why oil and fat containing food items are flushed with nitrogen to cut off the contact of food with oxygen and protect the food from rancidity.

25. Explain the following terms with one example each.

(a) Corrosion
(b) Rancidity

Answer: (a) Corrosion is the deterioration of a material, typically a metal, due to its reaction with its environment. An example of corrosion is the rusting of iron when it is exposed to moisture.

(b) Rancidity is the process of fats or oils becoming spoiled or spoiled-tasting due to oxidation or hydrolysis. An example of rancidity is the change in taste and smell of cooking oil after it has been used and exposed to air for a long period of time.

Tick (✓) the correct option

1. The changes which take place when fats and oils are oxidized:

Answer: (c) They become rancid and their smell and taste change.

2. The following reaction is an example of a:
4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g)

Answer: (c) (i) and (iii)

3. The substances which get oxidized easily can be kept for longer duration by:

Answer: (d) Adding nitrogen and keeping them in airtight containers.

4. What happens when hydrogen gas is passed over the heated copper oxide (CuO)?

Answer: (b) Black coating on the surface turns brown.

5. Which is the oxidizing agent in the following reaction?
CuO(s) + H2(g) -+ Cu(s) + H2O(l)

Answer: (a) CuO(s)

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