CBSE Class 10 Science Chapter 1 Chemical Reactions and Equations Exercise Questions Answer

 CBSE Class 10 Chapter 1 Chemical Reactions and Equations  Exercise Questions Answer

 

Q1	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
Ans.	(i) (a) and (b) Explanation: (a) Oxygen is eliminated in this process. (b) The oxygen removed from lead is transferred to the elemental carbon.

  

Q2

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.

Ans.

(d) displacement reaction. Explanation: The oxygen from ferrous oxide is being displaced by the aluminum metal, resulting in the formation of aluminum oxide. In this reaction, aluminum exhibits higher reactivity compared to iron (Fe). Consequently, aluminum displaces iron from its oxide. This type of chemical reaction is known as a displacement reaction, where one element displaces another. In this case, a less reactive metal is replaced by a more reactive metal. As this displacement occurs only once, it is referred to as a single displacement reaction.

 

Q3	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.
Ans.	(a) Hydrogen gas and iron chloride are produced. Explanation: The chlorine in hydrogen chloride is substituted by iron fillings in the following reaction. 2HCl + Fe → FeCl2 + H2

 

Q4	What is a balanced chemical equation? Why should chemical equations be balanced?
Ans.	A balanced equation is achieved when the count of various atoms on both the reactant and product sides is equal. Balancing chemical equations is crucial to ensure that the reaction adheres to the Law of Conservation of Mass. The process of balancing chemical equations doesn't follow a specific method; rather, it involves a trial-and-error approach.

 

Q5	Translate the following statements into chemical equations and then balance them.
(a)	Hydrogen gas combines with nitrogen to form ammonia.
Ans.	Unbalanced equation: H2 + N2 → NH3 Balanced equation: 3H2 + N2 → 2NH3
(b)	Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
Ans.	Unbalanced equation: H2S + O2 → H2O + SO2 Balanced equation: 2H2S + 3O2 → 2H2O + 2SO2
(c)	Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
Ans.	Unbalanced equation: BaCl2 + Al2(SO4)3 → AlCl3 + BaSO4 Balanced equation: 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
(d)	Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Ans.	Unbalanced equation: K + H2O → KOH + H2 Balanced equation:  2K + 2H2O → 2KOH + H2

 

Q6	Balance the following chemical equations.
(a)	HNO3+Ca(OH)2 → Ca(NO3)2 + H2O
Ans.	2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b)	NaOH + H2SO4 → Na2SO4 + H2O
Ans.	2NaOH + H2SO4 → Na2SO4 + 2H2O
(c)	NaCl + AgNO3 → AgCl + NaNO3
Ans.	NaCl + AgNO3 → AgCl + NaNO3
(d)	BaCl2 + H2SO4 → BaSO4 + HCl
Ans.	BaCl2 + H2SO4 → BaSO4 + 2HCl

 

Q7	Write the balanced chemical equations for the following reactions.
(a)	Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Ans.	2Ca(OH)2 + 2CO2 → 2CaCO3 + 2H2O
(b)	Zinc + Silver nitrate → Zinc nitrate + Silver
Ans.	Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
(c)	Aluminium + Copper chloride → Aluminium chloride + Copper
Ans.	2Al + 3CuCl2 → 2AlCl3 + 3Cu
(d)	Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Ans.	BaCl2 + K2SO4 → BaSO4 + 2KCl

 

Q8	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)
Ans.	It is a Double Displacement Reaction Unbalanced equation: KBr + BaI2 → KI + BaBr2 Balanced equation: 2KBr + BaI2 → 2KI + BaBr2
(b)	Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide (g)
Ans.	It is a Decomposition Reaction. Unbalanced equation: ZnCO3 → ZnO + CO2 Balanced equation: ZnCO3 → ZnO + CO2
(c)	Hydrogen (g) + Chlorine(g) → Hydrogen chloride(g)
Ans.	It is a Combination Reaction. Unbalanced equation: H2 + Cl → HCl Balanced equation: H2 + Cl → 2HCl
(d)	Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Ans.	It is a Displacement Reaction Unbalanced equation: Mg + HCl → MgCl2 + H2 Balanced equation: Mg + 2HCl → MgCl2 + H2

    

Q9	What does one mean by exothermic and endothermic reactions? Give examples.
Ans.	An endothermic reaction happens when heat energy is taken in from the surroundings (like in photosynthesis, melting ice, or evaporation). In contrast, an exothermic reaction is when energy is given off from the system to the surroundings (like in explosions, concrete setting, nuclear fission, and fusion).

 

Q10

Why is respiration considered an exothermic reaction? Explain.

Ans.

To sustain life, our bodies require energy, which we derive from the food we consume. Through the process of digestion, complex food molecules are broken down into simpler ones like glucose. These substances then interact with the oxygen within our body cells, resulting in the formation of carbon dioxide and water, accompanied by the release of a certain amount of heat energy (a part of the respiration process). As this energy is emitted in the form of heat, contributing to the maintenance of our body temperature, respiration is categorized as an exothermic reaction. The reaction occurring can be represented as follows: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

 

Q11

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

Ans.

A combination reaction involves the merging of two or more molecules to form a larger molecule. In contrast, a decomposition reaction is the process of breaking down larger molecules into two or more smaller ones, essentially making it the opposite of a combination reaction. Typically, decomposition reactions are endothermic because they require heat from the surroundings or an external source to break the bonds within the larger molecule. Here are a few examples of decomposition reactions: ZnCO3 → ZnO + CO2 CaCO3 + Energy → CaO + CO2 2HgO → 2Hg + O2

 

Q12

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

Ans.

(i) Thermal decomposition reaction (Thermolysis): If we heat up potassium chlorate quite a bit, it breaks down into two things: potassium chloride and oxygen gas. This is a way to make oxygen. 2KClO3 + Heat → 2KCl + 3O2 (ii) Electrolytic decomposition reaction (Electrolysis): When we pass electricity through melted sodium chloride (that's a special kind of salt), it splits into two things: sodium and chlorine. (iii) Photo-decomposition reaction (Photolysis): In the presence of light, hydrogen peroxide breaks apart into water and oxygen.

  

Q13

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

Ans.

In a displacement reaction, a highly reactive substance can replace a less reactive one in a salt solution. However, in a double displacement reaction, there's an exchange of ions between two compounds. In a displacement reaction, there's only one replacement, while in a double displacement reaction, as the name suggests, there are two replacements between the molecules. For Examples: Displacement reaction: Mg + 2HCl → MgCl2 + H2 Double displacement reaction: 2KBr + BaI2 → 2KI + BaBr2

  

Q14	In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Ans.	Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)

 

Q15

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

Ans.

When we mix two solutions that contain soluble salts, a double displacement reaction occurs where ions swap places between the compounds. If one of the resulting compounds is a solid that doesn't dissolve in water, it sinks to the bottom of the container. This solid is called a "precipitate" and the whole process is known as a "precipitation reaction".Examples are: CdSO4(aq) + K2S(aq) → CdS(s) + K2SO4(aq) 2NaOH(aq) + MgCl2(aq) → 2NaCl(aq) + Mg(OH)2(s)

 

Q16	Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation                     (b) Reduction
Ans.	(a) Oxidation: In a chemical reaction, when an element combines with oxygen to create its specific oxide, we say the element is undergoing oxidation. For example: 4Na(s) + O2(g) → 2Na2O(s)  H2S + O2 → H2O + SO2 (b) Reduction: In a chemical reaction, if a compound loses oxygen, we describe it as undergoing reduction. For example: CuO(s) + H2(g) → Cu(s) + H2O(l) 2HgO → 2Hg + O2

 

Q17	A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Ans.	The shiny brown element is copper metal (Cu). When this metal is heated in the presence of air, it reacts with the oxygen in the air to create copper oxide. This is why the black-colored substance is copper oxide. 2Cu(s) + O2(g) → 2CuO(s)

 

Q18

Why do we apply paint on iron articles?

Ans.

Iron articles are coated with paint to shield them from rusting. Without paint, the metal surface reacts with atmospheric oxygen and moisture, forming Iron (III) oxide, which is rust. This process called corrosion. Paint protects Iron from moisture and air and improves their appearance, increase durability, provide a smooth surface and allow for customization and also effectively prevent rust formation.

  

Q19	Oil and fat containing food items are flushed with nitrogen. Why?
Ans.	The primary reason for flushing nitrogen into food packages containing oil and fat products is to oppose rancidity. Rancidity happens when the oil or fat reacts with oxygen, resulting in an unpleasant smell and taste. Therefore, by introducing nitrogen, an inert environment is established for effectively prevent rancidity.

 

Q20

Explain the following terms with one example each. (a) Corrosion             (b) Rancidity

Ans.

(a) Corrosion: Corrosion is a natural process where clean metal reacts with the air to create more stable compounds, like rust. Over time, this causes the metal to deteriorate. Rusting, like when iron turns into iron oxide, is a common example of corrosion. We spend a lot of money every year to protect things like bridges and monuments from rusting. (b) Rancidity: Rancidity is the bad taste and smell caused by the air's effect on the oil and fat in food. Storing food in the refrigerator slows down rancidity because the cold temperature doesn't encourage the bad reactions.