Cls 10 Sci Chp 1 slun

Class 10 Science

Chapter 1 Chemical Reactions and Equations

Questions and Answers 

Page Number: 6

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

Ans: Because Magnesium metal reacts with the atmospheric oxygen and forms Magnesium Oxide (MgO) layer and it prevent further reactions with Oxygen, therefore necessary to clean the ribbon to remove the layer of MgO.

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

i) Hydrogen + Chloride -> Hydrogen chloride

ii) Barium chloride + Aluminium sulphate   -> Barium sulphate + Aluminium chloride

iii) Sodium + Water -> Sodium hydroxide + Hydrogen

Ans:

i) H2 + Cl2 → 2HCl

ii) 3BaCl2 + Al2(SO4)3 →3BaSO4 + 2AlCl3 

iii) 2Na + 2H2O → 2NaOH + H2

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 solution of Sodium chloride.

ii) Sodium hydroxide solution in water reacts with hydrochloric acid solution to produce Sodium chloride solution and water.

Ans:

i) BaCl2 + Na2SO4 → BaSO4 + 2NaCl

ii) NaOH + HCl → NaCl + H2O

Page number – 10

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.

Ans:

i) The substance ‘X’ which is used in whitewashing is quick lime or Calcium Oxide and its formula is CaO.

ii) CaO + H2O → Ca(OH)2

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

Ans:

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  oxygen.

Page number – 13

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

Ans:

When an iron nail 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 to the formation of light green solution of iron sulphate. The reaction is:

Fe + CuSO4 → FeSO4 + Cu

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

Ans:

Sodium hydroxide and hydrochloric acid react to form sodium chloride and water.

NaOH + HCl -> NaCl + H2O

3. Identify the substances that are oxidized and that are reduced in the following equation.

i) 4Na(s) + O2(g) → 2Na2O(s)

ii) CuO(s) + H2(g) → Cu(s) + H2O(l)

Ans:

i) Oxidized substance : Na, 

  Reduced substance: O2

ii) Oxidized substance : H2

    Reduced substance: CuO

Exercise Questions 

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

Ans: (i) (a) and (b)

2. Fe2O3 + 2Al → Al2O3 + 2Fe

The above reaction is an example of a

1. Combination reaction.
2. Double displacement reaction.
3. Decomposition reaction.
4. Displacement reaction.

Ans: 4. Displacement reaction.

3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

1. Hydrogen gas and Iron chloride are produced.
2. Chlorine gas and Iron hydroxide are produced.
3. No reaction takes place.
4. Iron salt and water are produced.

Ans: 1. Hydrogen gas and Iron chloride are produced.

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

Ans: A balanced equation is the number of different atoms in reactant and product sides are equal. Balancing chemical equation is necessary for follow the Law of Conservation of mass.

5. Translate the following statements into chemical equations and 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.

Ans:

(a) 3H2 (g) + N2 (g) → 2NH3(g)

(b) H2S (g) + 3O2 (g) → SO2 (g) + 2H2O(l)

(c) 3BaCl2 (aq) + Al2(SO4)3 (aq) → 2AlCl3 (aq) + 3BaSO4 ↓(s)

(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)

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

Ans:

(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O

(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O

(c) NaCl + AgNO3 → AgCl + NaNO3

(d) 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

Ans:

(a) Ca (OH)2 + CO2 → CaCO3 + H2O

(b) Zn + 2AgNO3 → Zn(NO3)2 + 2 Ag

(c) 2Al + 3 CuCl2 → 2AlCl3 + 3 Cu

(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

(b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g) bromide(s)

(c) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)

(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)

Ans:

(a) 2KBr (aq) + Bal2(aq) → 2Kl(aq) + BaBr2(s)

Type : Double displacement reaction

(b) ZnCO3 (s) → ZnO (s) + CO2 (g)

Type : Decomposition reaction

(c) H2 (g) + Cl2 (g) → 2HCl(g)

Type : Combination reaction

(d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)

Type : Displacement reaction

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

Ans:

Exothermic reactions : The reactions in which heat is evolved are known as exothermic reactions.

Ex:

(i) C (s) + O2 (g) → CO2 (g) + Heat

(ii) N2 (g) + 3H2 (g) → 2NH3 (g) + Heat

Endothermic reactions : The reaction in which heat is absorbed are known as endothermic reactions.

Ex:

(i) C (s) + 2S (s) → CS2 (l) – Heat

(ii) N2 (g) + O2 (g) → 2NO(g) – Heat

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

Ans:

Because in respiration oxidation of glucose takes place which produces large amount of heat energy.

C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + Energy

11. Why are decomposition reactions called the opposite of Combination reactions? Write equations for decomposition reactions.

Ans:

In a decomposition reaction, a single compound breaks down to produce two or more simpler substances.

For example:

in a combination reaction, two or more substances simply combine to form a new substance.

For example:

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

Ans:

13. What is the difference between displacement and double displacement reactions? Write relevant equations for the above.

Ans:

In displacement reactions, a more reactive metal displaces a less reactive metal from its solution.

For example,

Fe(s) + CuSO4(aq) → Cu(s) + FeSO4(aq)

In double displacement reactions, two reactants in solution exchange their ions.

For example,

AgNO3(aq) + NaCl (aq) → AgCl(s) + NaNO3 (aq)

14. In the refining of Silver, the recovery of silver from Silver nitrate solution involves displacement reaction by Copper metal. Write down the reaction involved.

Ans:

Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)

15. 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 precipitation reaction.

For Example:

Na2CO3 (aq) + CaCl2 (aq) → CaCO3 (s) + 2NaCl (aq)

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

16. Explain the following in terms of gain of oxygen with two examples each.

(a) Oxidation

(b) Reduction

Ans:

(a) Oxidation : The addition of oxygen to a substance is called oxidation.

Example :

4Na(s) + O2(g) → 2Na2O(s)

H2S + O2 → H2O + SO2

(b) Reduction : The removal of oxygen from a substance is called reduction.

Example :

CuO(s) + H2(g) → Cu(s) + H2O(l)

2HgO → 2Hg + O2

17. 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.

Ans:

Element ‘X’ is copper (Cu).

The black coloured compound is copper oxide (CuO). The reaction involved is

2Cu(s) + O2(g) → 2CuO(s)

18. Why do we apply paint on iron articles?

Ans:

Paint does not allow iron articles to come in contact with air, water and saves iron articles rusting.

19) Oil and Fat containing food items are flushed with Nitrogen. Why?

Ans:

Because nitrogen acts as an antioxidant and it prevents them from oxidation.

20. Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

Ans:

The gradual destruction of pure metals by the action of air, moisture or a chemical (such as an acid) on their surface is called corrosion.

For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.

4Fe + 3O2 + nH2O → 2Fe2O3.nH2O

Rancidity is the process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

For example, the taste and smell of butter changes when kept for long.