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

Ans. Decomposition reactions are those in which a compound breaks down to form two or more substances.

$$ 2H_2O(l)  \space\xrightarrow{Electrolysis}\space 2H_2(g) + O_2(g) $$

These reactions require a source of energy to proceed. In case of combination reactions, two or more substances combine to give a new substance with the release of energy. Thus, both the reactions are opposite.

$$ 2H_2(s) + O_2(g)  \space \longrightarrow \space 2H_2O(l) + Energy $$

Q. Why do we apply paint on iron articles?

Ans. Iron articles are painted to prevent them from rusting. When iron articles are painted, the contact of iron articles from moisture and air is cut off. Therefore, rusting is prevented.

Q. Why should a magnesium ribbon be cleaned before burning in air?

Ans. Magnesium is a very reactive metal, it reacts with oxygen to form a layer of magnesium oxide on its surface when stored. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. Hence, this layer is removed by cleaning magnesium ribbon by sand paper so that the underlying metal can be exposed into air.

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

Ans. In human beings , energy is obtained from the food we eat. During digestion of food, large molecules of food like carbohydrates, proteins etc. are broken down into simpler substances such as glucose, amino acids etc. Glucose combines with oxygen in the cells and provides energy. The special name of this combustion reaction is respiration. Since energy is released in the whole process, it is an exothermic process.

Q. In electrolysis of water, why is the volume of gas collected over one electrode double that of gas collected over the other electrode ?

Ans. In electrolysis, water is decomposed in the presence of electricity to its components. The reaction is shown as below:

$$ 2H_2O(I) \longrightarrow \space 2H_2(g) + O_2(g) $$

As you can see that water (H2O) contains two parts hydrogen and one part oxygen. Therefore, the amount of hydrogen and oxygen produced during electrolysis of water is in a 2 : 1 ratio. Since, the number of molecules of hydrogen released is double the number of molecules of oxygen released, Volume occupied by hydrogen gas is double the volume occupied by oxygen gas.

Hence, electrode at which hydrogen gas is collected (Cathode) shows double the volume than the electrode at which oxygen gas is collected (Anode).

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

Ans. When an iron nail is placed in a copper sulphate solution, iron being more reactive displaces copper from copper sulphate solution and forms iron sulphate, which is green in colour. Therefore, the blue colour of copper sulphate solution fades and green colour appears.

$$ \underset{\text{Copper Sulphate Blue Colour}}{CuSo_4} + \underset{\text{Iron}}{Fe} \longrightarrow \underset{\text{Iron Sulphate Green Colour}}{FeSo_4} + \underset{\text{Copper}}{Cu} $$

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


Explain why, food products containing fats and oils (like potato chips) are packaged in nitrogen.

Ans. Oil and fat containing food items are flushed with nitrogen because it is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is avoided.

Q. Name and state the Law which is kept in mind while we balance a chemical reaction?


Why should a chemical equation be balanced?

Ans. The balancing of chemical equation is based on the ‘Law of Conservation of Mass’. This law states that, “Mass can neither be created nor be destroyed during a chemical reaction”. Thus, the chemical equation should always be balanced.

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

Ans. Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions. Combination reactions are exothermic. Example: Sodium oxide dissolves in water to form sodium hydroxide and releases large amount of energy.

$$Na_2O + H_2O \longrightarrow 2NaOH + Energy $$

Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.
Example: Combination of nitrogen and oxygen to form nitric oxide.

$$ N_2 + O_2 + Heat \longrightarrow 2NO $$

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

Ans. A reaction in which an insoluble solid (called precipitate) is formed is called a precipi-tation reaction.

For example :

$$ \underset{\text{Sodium carbonate}}{Na_2CO_3(s)} + \underset{\text{Calcium chloride}}{CaCl_2(s)} \longrightarrow \underset{\text{Calcium Carbonate}}{CaCO_3(s)\downarrow} + \underset{\text{Sodium chloride}}{2NaCl(aq)} $$

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.

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

Ans. 1. Thermal decomposition: Energy in the form of heat.

$$ \underset{\text{Sodium carbonate}}{2FeSo_4(s)} \xrightarrow{\Delta} \underset{\text{Ferric oxide}}{Fe_2O_3(s)} + \underset{\text{Sulphur dioxide}}{SO_2(s)} + \underset{\text{Sulphur trioxide}}{SO_3(s)} $$

2. Decomposition by light: It is known as photolysis.

$$ \underset{\text{Silver Chloride}}{2AgCl(s)} \xrightarrow{Light} \underset{\text{Silver}}{2Ag(s)}+ \underset{\text{Chlorine}}{Cl_2(s)} $$

3. Decomposition by electricity: It is also known as electrolysis. Electrolysis of lead bromide:

$$ PbBr_2 \longrightarrow Pb + Br_2 $$

Q. Explain the following in terms of gain or loss of oxygen with two examples each.

  • (i) Oxidation
  • (ii) Reduction

Ans (i) 4Na(s) + O2(g) → 4Na2O(s)
2Cu + O2 →2CuO Oxidation is the gain of oxygen.

In the above examples, both Na and Cu gains oxygen and get oxidised.

(ii) Reduction is loss of oxygen.

$$1. \space CO_2 + H_2 \longrightarrow CO + H_2O \space \space \space \space \space \space \space \space \space \space \space \space \space \space \space \space 2.\space CuO(s) + H_2(g) \longrightarrow Cu(s) + H_2O(I) $$

In the above reactions, carbon dioxide and CuO lose oxygen.

Q. Metallic oxides of zinc, magnesium and copper were heated with the following metals. In which case, will you find displacement reactions taking place?

Metal Zinc Magnesium Copper
Zinc Oxide
Magnesium Oxide
Copper Oxide

Ans. Magnesium is the most reactive among these three metals and Zinc is more reactive than Copper. So, Magnesium will displace Zinc oxide and Copper oxide whereas Zinc will displace Copper oxide only.

Metal Zinc Magnesium Copper
Zinc Oxide No Reaction Displacement Reaction No Reaction
Magnesium Oxide No Reaction No Reaction No Reaction
Copper Oxide Displacement Reaction Displacement Reaction No Reaction

Q. Answer the following questions:

(i) Define a balanced chemical equation. Why should an equation be balanced?

(ii) Write the balanced chemical equation for the following reaction:

  • (a) Phosphorus burns in presence of chlorine to form phosphorus penta chloride.
  • (b) Burning of natural gas.
  • (c) The process of respiration.

Ans. (i) Balanced chemical equation is one in which there are equal number of atoms of different elements in the reactants and products. According to law of conservation of mass, matter can neither be created nor be destroyed in a chemical reaction. Therefore, number of atoms for each element on reactants and products formed after the reaction should be equal.

(ii) (a) P4(s) + 10Cl2(g) → 4PCl5(Ss)
(b) CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + Heat energy
(c) C6H12O6 (s) + 6O2(g) + 6H2O → 6CO2(aq) + 12H2O (l) + Energy