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)
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.
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 & iron chloride are produced.
4. What is a
balanced chemical equation? Why should chemical equations be balanced?
Answer: It is an equation which has an equal number of atoms of all the
elements on both sides of the chemical equation. Chemical reaction should be
balanced to follow law of conservation of mass.
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 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.
Answer:
(a) 3H2 (g)
+ N2 (g) → 2NH3 (g)
(b)
2H2S
(g) + 3O2 (g) → 2H2O (l) + 2SO2 (g)
(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
Answer:
(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
Answer:
(a) Ca(OH)2 +
CO2 → CaCO3 + H2O
(b)
Zn + 2AgNO3 → Zn(NO3)2 +
2Ag
(c) Al + 3CuCl2 → 2AlCl3 +
3Cu
(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 bromide(s)
(b) Magnesium(s)
+ Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
(c) Zinc
carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
(d) Hydrogen(g)
+ Chlorine(g) → Hydrogen chloride(g)
Answer:
(a) 2KBr (aq)
+ BaI2 (aq) → 2KI (aq) + BaBr2 (s).
It is Double displacement reaction.
(b) ZnCO3 (s)
→ ZnO (s) +
CO2 (g)
It is Decomposition reaction.
(c) H2 (g)
+ Cl2 (g) → 2HCl (g)
It is Combination reaction.
(d) Mg (s) +
2HCl (aq) → MgCl2 (aq) + H2 (g)
It is Displacement Reaction.
9. What does
one mean by exothermic and endothermic reactions? Give examples.
Answer:
Exothermic reactions: Chemical
reactions that release energy in the form of heat, light, or sound. E.g.
C (g) + O2 (g) → CO2 +
Heat
Endothermic reactions: Reactions
that absorb energy or require energy in order to proceed. E.g.
N2 (g) + O2 (g) + Heat → 2NO
10. Why is
respiration considered an exothermic reaction? Explain.
Answer: 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 these reactions.
Answer:
In decomposition reactions, a compound breaks down to form two or more
substances. These reactions need energy to proceed. In combination reactions,
two or more substances combine to give a new substance with the release of
energy.
Decomposition
Reaction: E.g.
CaCO3 (s) + Heat → CaO (s) + CO2 (g)
Combination
Reaction: E.g.
CaO (s) + H2O (l) → Ca(OH)2 (aq)
12. Write one
equation each for decomposition reactions where energy is supplied in the form
of heat, light or electricity.
Answer:
CaCO3 (s) + Heat → CaO (s) + CO2 (g)
2AgCl (s) + Sunlight → 2AgS (s) + Cl2 (g)
2H2O (l) + Electricity → 2H2 (g) + O2 (g)
13. What is the
difference between displacement and double displacement reactions? Write
equations for these reactions.
Answer: In a
displacement reaction, a more reactive element replaces a less reactive element
from a compound. E.g.
CuSo4 (aq) + Zn (s) → ZnSO4 (aq)
+ Cu (s)
In a
double displacement reaction, two atoms or a group of atoms switch places to
form new compounds. E.g.
Na2SO4 (aq) + BaCl2 (aq)
→ BaSO4(s)
+ 2NaCl (aq)
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:
2AgNO3 (aq)
+ Cu (s) → Cu(NO3)2 (aq)
+ 2Ag (s)
Silver
Nitrate + Copper → Copper Nitrate + Silver
15. What do
you mean by a precipitation reaction? Explain by giving examples.
Answer:
It is a
reaction in which an insoluble solid (precipitate) is formed. E.g.
Na2CO3 (aq) + CaCl2 (aq)
→ CaCO3 (s)
+ 2NaCl (aq)
Here,
calcium carbonate is a precipitate.
16. Explain
the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b)
Reduction
Answer:
(a) Oxidation: It is a
chemical reaction in which gain of oxygen or loss of hydrogen takes place. E.g.
2Mg (s) + O2 (g) + Heat → 2MgO (s)
2Cu (s) + O2 (g) + Heat → 2CuO (s)
(b) Reduction: It is a
chemical reaction in which loss of oxygen or gain of hydrogen takes place. E.g.
CuO + H2 + Heat → Cu + H2O
ZnO + C + Heat → Zn + CO
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: X is
copper and black-coloured compound is copper oxide (CuO).
18. Why do we
apply paint on iron articles?
Answer: Iron
articles are painted to prevent rusting. When painted, the contact of iron
articles from moisture and air is cut off.
19. Oil and
fat containing food items are flushed with nitrogen. Why?
Answer: Nitrogen acts as an antioxidant. It prevents oxidation of food items.
20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Answer:
(a) Corrosion: A process
where materials, usually metals, deteriorate due to a chemical reaction with
air, moisture, chemicals, etc. E.g. Rusting of iron.
4Fe + 3O2 + nH2O → 2Fe2O3.2H2O
(b) Rancidity: A process
of oxidation of fats and oils resulting in the change in taste and smell.
E.g. taste and smell of butter changes when kept for long.