Chapter 4. CHEMICAL REACTION AND EQUATIONS
Chapter 4. Chemical Reaction and Equations Class 10 Science and Technology (212) NIOS Textbook Solutions
Chapter4. CHEMICAL REACTION AND EQUATIONS
INTEXT QUESTION 4.1
1. Write a chemical equation for each of the following reactions:
(i) Zinc metal reacts with aqueous hydrochloric acid to produce a solution of zinc chloride and hydrogen gas.
(ii) When solid mercury(II) oxide is heated, liquid mercury and oxygen gas are produced.
Answer: The balanced chemical equations are:
(i) Zinc metal reacts with aqueous hydrochloric acid: 
(ii) On heating mercury(II) oxide: 
2. Balance the following chemical equations:
(i) 
(ii) 
Answer: The Balance chemical equations are :
(i) 
(ii) 
3. What is a balanced chemical equation? Why should a chemical equation be balanced?
Answer: A balanced chemical equation is a chemical equation in which the number of atoms of each type involved in the chemical reaction is equal on the reactants and products sides of the equation.
A chemical equation should be balanced because of the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total number of atoms of each element must remain the same before and after the reaction.
INTEXT QUESTIONS 4.2
1. Examine the following reaction(s) and identify which of them are not example(s) of a redox reaction?
(i) 
(ii) 
(iii) 
Answer: (i) AgNO₃ (aq) + HCl (aq) → AgCl (s) + HNO₃ (aq)
No oxygen is gained or lost.
No hydrogen is gained or lost.
NOT a redox reaction.
(ii) MnO₂ (s) + 4HCl (aq) → MnCl₂ (aq) + 2H₂O (l) + Cl₂ (g)
In MnO₂, oxygen is lost (reduction of MnO₂).
In HCl, hydrogen is lost (oxidation of HCl to Cl₂).
It is a redox reaction.
(iii) 4Na (s) + O₂ (g) → 2Na₂O (s)
Sodium (Na) gains oxygen → Oxidation.
Oxygen (O₂) loses oxygen (it is being taken by Na) → Reduction.
It is a redox reaction.
2. Identify the substances which are oxidized and the substances that are reduced in the following reactions:
(i) 
(ii) 
(iii) 
Answer: (i)
is oxidized and
is reduced.
(ii)
is oxidized and CuO is reduced.
(iii)
Zn is oxidized and 
(in 
) is reduced.
TERMINAL EXERCISE
1. A. Write chemical equations of the following and balance them:
(a) Carbon + oxygen → Carbon dioxide
(b) Hydrogen + Chlorine → Hydrogen chloride
(c) Barium chloride + Sodium sulphate → Barium sulphate + sodium chloride.
Answer: The balanced chemical equations are:
(a) Carbon + Oxygen → Carbon dioxide
(b) Hydrogen + Chlorine → Hydrogen chloride
(c) Barium chloride + Sodium sulphate → Barium sulphate + Sodium chloride
B. Write balanced chemical equations with physical state symbols and necessary conditions, if any:
(a) Nitrogen reacts with hydrogen in the presence of iron as a catalyst at 200 atmospheric pressure and 600°C temperature, and the product obtained is ammonia.
(b) Aqueous solution of sodium hydroxide reacts with hydrochloric acid and produces sodium chloride and water.
(c) Phosphorus burns in chlorine gas to form phosphorous pentachloride.
Answer: The balanced chemical equations with physical state symbols and necessary conditions are:
(a) Nitrogen reacts with hydrogen to form ammonia:
(b) Aqueous sodium hydroxide reacts with hydrochloric acid:
(c) Phosphorus burns in chlorine gas to form phosphorus pentachloride:
C. Balance the following chemical reactions:
(a) 
(b) 
(c) 
(d) 
(e) 
(f) 
(g) Calcium hydroxide + carbon dioxide → Calcium carbonate + water
(h) Aluminium + Copper (II) chloride → Aluminium chloride + copper
(i) Calcium carbonate + hydrochloric acid → Calcium chloride + water + carbon dioxid
Answer: The balanced chemical equations are:
(a) 
(b) 
(c)
(d) 
(e) 
(f) 
(g) Calcium hydroxide + carbon dioxide → Calcium carbonate + water
(h) Aluminium + Copper (II) chloride → Aluminium chloride + copper
(i) Calcium carbonate + hydrochloric acid → Calcium chloride + water + carbon dioxide
2. What is a balanced chemical equation? Write 3 characteristics of a balanced chemical equations?
Answer: A balanced chemical equation is a chemical equation in which the number of atoms of each type involved in the chemical reaction is equal on the reactants and products sides of the equation.
Three characteristics of a balanced chemical Equation:
(i) The number of atoms of each element is the same on both sides of the equation.
(ii) It obeys the law of conservation of mass (mass of reactants = mass of products).
(iii) The chemical formulae of reactants and products remain unchanged; only the coefficients are adjusted to balance the equation.
3. In what way is a displacement reaction different from a double-displacement reaction? Explain with two suitable examples.
Answer: The difference between displacement reaction and double-displacement reaction:
|
Displacement Reaction |
Double-displacement Reaction |
|
(i) In this reaction, one element replaces another element from a compound. |
(i) In this reaction, two compounds exchange their ions to form new compounds. |
|
(ii) It involves one compound and one element. |
(ii) It involves two compounds. |
Examples of displacement Reaction: (i) 
Zinc displaces copper from copper sulphate.
Iron displaces copper.
Examples of double-displacement Reaction:
(i) 
Barium and sodium exchange ions.
(ii) 
Silver and sodium exchange ions to form new compounds.
4. What happens when dilute hydrochloric acid is added to iron filings? Mark (√) at the correct answer from the following:
(a) Hydrogen gas and iron chloride are produced and is classified as a displacement reaction.
(b) Iron chloride and chlorine gas are produced and is classified as a decomposion reaction.
(c) Iron hydroxide and water are produced and is classified as a combination reaction.
(d) No reaction takes place but is classified as a double displacement reaction.
Answer: (a) Hydrogen gas and iron chloride are produced and it is classified as a displacement reaction.
[ When dilute hydrochloric acid (HCl) is added to iron filings (Fe), iron displaces hydrogen from hydrochloric acid and forms iron chloride and hydrogen gas.
The chemical equation is : 
]
7. What do you mean by an exothermic reaction? Give a suitable example.
Answer: A reaction in which heat is released along with the formation of the products is called an exothermic reaction.
For example : (i) Burning of natural gas (
) used for cooking.
8. Classify each of the following reactions as combination, decomposition, displacement or double displacement reactions:
(a) 
(b) 
(c) 
(d) 
(e) 
Answer: The reactions are classified :
(a) Zn (s) + 2AgNO₃ (aq) → Zn(NO₃)₂ + 2Ag (s) → Displacement reaction
Zinc displaces silver from silver nitrate.
(b) 
→ Decomposition reaction
Potassium nitrate breaks down into simpler substances on heating.
(c) Ni(NO₃)₂ (aq) + 2NaOH (aq) → Ni(OH)₂ (s) + 2NaNO₃ (aq) → Double displacement reaction
Exchange of ions takes place between the reactants.
(d) → Decomposition reaction
Potassium chlorate decomposes on heating.
(e) MgO (s) + C (s) → CO (g) + Mg (s) → Displacement reaction
Carbon removes oxygen from magnesium oxide and displaces magnesium.
9. What is the difference between a combination and a decomposition reaction? Illustrate with suitable examples.
Answer: The difference between combination and decomposition Reaction:
|
Combination Reaction |
Decomposition Reaction |
|
A reaction in which a single product is formed from two or more reactants is known as a combination reaction. |
A decomposition reaction is the one in which a compound decomposes into two or more substances (elements or compounds). |
|
General form: A + B → AB |
General form: AB → A + B |
Example of Combination Reaction: CaO + H₂O → Ca(OH)₂
(Quick lime combines with water to form slaked lime.)
Example of Decomposition Reaction: 
(Potassium chlorate decomposes on heating to form potassium chloride and oxygen.)
10. Is there any oxidation without reduction? Justify your answer.
Answer: No, there is no oxidation without reduction and there is no reduction without oxidation.
This is because oxidation and reduction always occur together in a reaction. Such reactions are called redox reactions. This aspect of redox reactions can be explained broadly in terms of electron loss and electron gain.
Example: CuO + H₂ → Cu + H₂O
In this reaction: CuO is reduced to Cu (oxygen is removed).
H₂ is oxidised to H₂O (oxygen is added).
11. ‘Both combination reaction and displacement reaction fall in the category of redox reactions’. Do you agree? If so discuss this aspect with suitable examples.
Answer: Yes, I agree. Both combination reaction and displacement reaction fall in the category of redox reactions because oxidation and reduction occur simultaneously in these reactions.
Combination reaction:
Example: 2Mg + O₂ → 2MgO
In this reaction, Magnesium gains oxygen and gets oxidised.
Oxygen gets reduced.
Therefore, combination reaction is a redox reaction.
Displacement reaction:
Example: Zn + CuSO₄ → ZnSO₄ + Cu
In this reaction, Zinc loses electrons and gets oxidised.
Copper ions gain electrons and get reduced.
Hence, displacement reactions are also redox reactions because oxidation and reduction occur together.
12. Give two examples from everyday life situation where redox reaction takes place. How will you prove it?
Answer: Two examples from everyday life where redox reaction takes place are:
Iron reacts with steam, the reaction is 3Fe + 4H₂O (steam) → Fe₃O₄ + 4H₂
In this reaction, Iron is oxidised to form iron oxide (Fe₃O₄) and Water is reduced to form hydrogen gas.
Therefore, it is a redox reaction.
Respiration : During respiration, glucose reacts with oxygen to produce carbon dioxide, water and energy, the reaction:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
This is a redox reaction because glucose gets oxidised and oxygen gets reduced.
13. In the following reactions name the substances which are oxidized and reduced and also mention the oxidizing and reducing agents:
(a) 
(b) 
(c) 
Answer: (a) 
Oxidised substance: Calcium (Ca)
Reduced substance: Chlorine (Cl₂)
Oxidising agent: Chlorine (Cl₂)
Reducing agent: Calcium (Ca)
(b)
Oxidised substance: Aluminium (Al)
Reduced substance: Manganese dioxide (MnO₂)
Oxidising agent: Manganese dioxide (MnO₂)
(c)
Oxidised substance: Carbon monoxide (CO)
Reduced substance: Iron(III) oxide (Fe₂O₃)
Oxidising agent: Fe₂O₃
Reducing agent: CO
14. Explain the following in terms of electron transfer: (a) Oxidation (b) Reduction
Answer: (a) A reaction in which a species loses electrons is called an oxidation reaction.
Example: Na → Na⁺ + e⁻
Sodium loses one electron, so it is oxidised.
(b) A reaction in which a species gains electrons is called a reduction reaction.
Example: Cl + e⁻ → Cl⁻
Here, chlorine gains one electron, so it is reduced.
15. What is the law of definite proportion by volume? Explain.
Answer: Law of definite proportion by volume: The volume of reactants and products in gaseous state are related to each other by small integers, provided the volumes are measured at the same temperature and pressure.
This law states that when gases react, the volumes of reacting gases and gaseous products bear a simple whole number ratio if measured under the same conditions of temperature and pressure.
Example: 2H₂ + O₂ → 2H₂O
2 volumes of hydrogen + 1 volume of oxygen → 2 volumes of water vapour
Thus, the ratio of volumes is 2 : 1 : 2 .
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