IGCSE Chemistry Paper-1: Specimen Questions with Answers 7 - 8 of 99

Question number: 7

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Question

Three electrolysis cells are set up. Each cell has inert electrodes. The electrolytes are listed below.

  • cell 1 aqueous sodium chloride
  • cell 2 dilute sulfuric acid
  • cell 3 molten lead (II) bromide

    In which of these cells is a gas formed at both electrodes?

Choices

Choice (4)

a.

3 only

b.

1 and 2

c.

1 and 3

d.

2 only

Answer

b.

Explanation

Electrolysis of aqueous sodium chloride yields hydrogen and chlorine, with aqueous sodium hydroxide remaining in solution. ‎‎The reaction at the cathode is:

H2O (l) +2e−→H2 (g) +2OH−

The reaction at the anode is:

Cl−→12Cl2 (g) +1e−

The overall reaction is as follows:

NaCl (aq) +H2O (l) →Na+ (aq) +OH− (aq) +H2 (g) +12Cl2 (g) ‎* Electrolysis of dilute sulphuric acid yields oxygen:

The positive anode electrode reaction:

2H2O (l) – 4e == > 4H+ (aq) + O2 (g) (oxygen gas)

The negative cathode electrode reaction:

2H+ (aq) + 2e == > H2 (g) (hydrogen gas)

Question number: 8

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Question

Which process is not exothermic?

Choices

Choice (4)

a.

obtaining lime from limestone

b.

radioactive decay of 235U

c.

burning a fossil fuel

d.

reacting hydrogen with oxygen

Answer

a.

Explanation

burning a fossil fuel and a radioactive decay are both exothermic. Reaction of hydrogen and oxygen, or otherwise combustion of hydrogen is also exothermic. Lime (Cao) is obtained from limestone (CaCO3) by thermal decomposition of limestone, i. e. we heat (PROVIDE energy) for this decomposition. hence this process is endothermic, not exothermic.

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