Friday, 25 November 2011

CBSE Class 10th Science | Chapter 1. Chemical Reactions And Equations | Solved Exercises

Question 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).

In th above given reaction, Lead oxide is getting reduced and Carbon is getting oxidised. Therefor The statements a and b are inccorect hence answer is '(i) (a) and (b)' .
StatementCorrect / Incorrect
(a) Lead is getting reducedIncorrect
(b) Carbon dioxide is getting oxidised.Incorrect
(c) Carbon is getting oxidisedCorrect
(d) Lead oxide is getting reduced.Correct

Question 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.

Question 3. What happens when dilute hydrochloric acid is added to iron fillings? 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.

Question 4. What is a balanced chemical equation? Why should chemical equations be balanced?
Answer. When the numbers of atoms of each type of chemical involved in a chemical reaction are the same on the reactant and product sides of a chemical equation, it is called a balanced chemical equation. Chemical equations should be balanced because for a given chemical reaction, it provides us basic
information about the type of chemicals involved as reactants and products, their respective numbers of atoms or molecules along with physical states symbolically.

Question 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 sulpur 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.
StatementsBalanced Chemical Equation
(a) N2(g) + 3H2(g)2NH3(g)
(b) 2H2S(g) + 3O2(g)2H2O(l) + 2SO2(g)
(c) 3BaCl2(g) + Al2(SO4)33BaSO4 + 2AlCl3
(d) 2K + 2H2O 2KOH + H2(g)

Question 6. Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
Answer.
Given Chemical EquationBalance Chemical Equation
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(c) NaCl + AgNO3 → AgCl + NaNO3(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl(d) BaCl2 + H2SO4 → BaSO4 + 2HCl

Question 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.
Reaction(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Balance Chemical Equation(a) Ca(OH2 + CO2 → CaCO3 + H2
Reaction(b) Zinc + Silver nitrate → Zinc nitrate + Silver
Balance Chemical Equation(b) Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
Reaction(c) Aluminium + Copper chloride → Aluminium chloride + Copper
Balance Chemical Equation(c) 2Al + 3CuCl2 → 2AlCl3 + 3Cu
Reaction(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Balance Chemical Equation(d) BaCl2 → BaSo4 + 2KCl

Question 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.
Reaction(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
Balance Chemical Equation(a) 2KBr(aq) + BaCl2(aq) → 2KI(aq) + BaBr2(aq)
Type of ReactionDouble Displacement Reaction
Reaction(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
Balance Chemical Equation(b) ZnCo3(s) → ZnO(s) + CO2(g)
Type of ReactionDecomposition reaction
Reaction(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
Balance Chemical Equation(c) H2(g) + Cl2(g) → 2HCL (aq)
Type of ReactionCombination reaction
Reaction(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Balance Chemical Equation(d) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2O
Type of ReactionDisplacement Reaction

Question 9. What does one mean by exothermic and endothermic reactions? Give examples.
Answer.
Exothermic reactions: Those reaction in which energy is released are called exothermic reactions.
Examples: (i) All combustion reactions like.
Ch4 + 2O2 → CO2+2H2O + Heat
(ii) Thermite reactions like
2A1 + Fe2O3 → 2Fe + Al2O3 + Heat

Endothermic reactions: Those reactions in which energy is utilised are called endothermic reactions.
CaCO3
  heat
------→
CaO + CO 2
Question 10. Why is respiration considered an exothermic reaction? Explain.
Answer. In respiration, food is broken or oxidised in the presence of oxygen inhaled. In this process, energy is liberated. So respiration is called an exothermic reaction.
C6H12O6 + 6CO2 → 6H2O + Energy

Question 11. Why are decomposition reactions called the opposite of combination reactions?
Write equations for these reactions.
Answer. In a combination reaction two or more substances combine to form a new single substance.
Where as in Decomposition reactions, a single substance decomposes to give two or more substances. That is why decomposition reactions are called the opposite of combination reactions.
For example consider the following chemical reactions.
(i) CaCO3
  heat
------→
CaO + CO 2

(ii) CaO + CO2 → CaCO3
In above examples, both the reactions are same but show opposite directions, so decomposition reaction is called the opposite of combination reaction.

Question 12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer
Decomposition reaction in which Energy is supplied in the form of Heat:



Heat
CaCOO3---------→CaO + CO 2

Decomposition reaction in which Energy is supplied in the form of Light:



Sunlight
2AgCI(s)-------→ 2Ag(s) + C12(g)

Decomposition reaction in which Energy is supplied in the form of Electricity:



Electricity
2H2O----------→2H2 + O2
(Electrolysis of water)

Question 13. What is the difference between displacement and double displacement reactions?
Write equations for these reactions.
Answer. In displacement reaction, one element from its salt is displaced by a more reactive element. For example in following reaction, Cu is displaced by Zn from CuSO4 because Zn is more reactive.
CuSO4 + Zn → ZnSO4 + Cu
In double displacement reaction, exchange of two different atoms or groups of atoms (ions)  takes place between two reactants to forms new products.
Na2SO4(aq) + BaCI4(aq) → BaSO4(s)+ 2NaCI(aq)

Question 14. In the refining of silver, the recovery of silver from silver nitrate solution involved
displacement by copper metal. Write down the reaction involved.
Answer. When copper is mixed in silver nitrate solution, it displaces the silver because copper is more reactive than silver.
2AgNC + Cu → Cu(NO3)2 + Ag.

Question 15. What do you mean by a precipitation reaction? Explain by giving examples.
Answer. The reaction in which a precipitate in the form of an insoluble salts is produced is called precipitation reaction.
Examples:
1. Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCI(aq)
2. Pb(NO3)2) + 2KI → 2KNO3 + PbI2

Question 16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction
Answer.
(a) Oxidation:The reaction in which gain of oxygen or loss of hydrogen takes place are called Oxidation.
Examples:
(i) 2Cu + O2 → 2CuO
(ii) 2H2 + O2 → 2H2O

(b) Reduction: The reactions in which loss of oxygen or gain of hydrogen takes place are called reduction.
Examples:
(i) Zno + C → Zn + CO
(i) CuO + H2 → Cu + H2O

Question 17. A shiny brown coloured element ‘X’ on heating in air becomes black in colour.
Name the element ‘X’ and the black coloured compound formed.
Answer. The shiny brown coloured element is copper. When it is heated in air, it becomes black due to the formation of Copper Oxide layer.




heat
2Cu + O2-------→2CuO
BrownBlack

Question 18. Why do we apply paint on iron articles?
Answer. We apply paint on iron articles to prevent them from corrosion. The coating of Paint forms a thin layer on the surface of metal like iron, which isolates the iron surface from air or water and protects its surface

from reacting with air of water in the form of corrosion.

Question 19. Oil and fat containing food items are flushed with nitrogen. Why?
Answer. Oil and fat containing food items are flushed with nitrogen to prevent them from damage caused by oxidation. When such items come in contact of air, they get oxidised and becomes rancid. Their smell and taste change.

Question 20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Answer. (a) Corrosion: The process of rusting of the surface of some metals when they are left for some time in moist air is called corrosion.
Conditions of corrosion:
(i) Presence of moisture (water)
(ii) Presence of air.

Example: Rusting of iron is a common example of corrosion.

Rancidity: When fats and oils are oxidised, they become rancid and their smell and taste change.These phenomenon is called rancidity.

Example: Fat and oil containing substances are damaged and their taste or smell is changed.
 CBSE Class X (10th) SCIENCE | Chapter 1. Chemical Reactions And Equations | Solved Exercises

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