Question 103: [Organic > Hydrocarbons]

The complete combustion of alkanes to produce carbon dioxide and water is an important exothermic reaction.

Which line on the graph shows the relationship between the number of carbon atoms in the alkane and the number of moles of oxygen gas needed for complete combustion of the alkane?

Reference: Past Exam Paper –June 2003 Paper 1 Q23

Solution 63:

Answer: B.

1 mole of oxygen gas has 2 oxygen atoms. In the combustion of 1 carbon atom, 1 mole of CO₂ is formed. This requires 1 carbon atom and 2 oxygen atoms. Thus 2 carbon atoms requires 2 moles of oxygen gas (which actually has 4 atoms of oxygen), to form 2 moles of CO₂. Hence the number of moles of oxygen gas and number of carbon atoms are directly proportional, in a 1:1 ratio. This is only depicted by the line in option B. Option A and C are curved thus do not show a constant proportionality, thus are eliminated. Option D suggests the amount of oxygen gas required for the combustion of any number of carbon atoms is the same, which is illogical, thus is eliminated.

Question 38: [Organic > Hydrocarbons]:

When octane is subjected to catalytic cracking, which compounds can be obtained?

1 CH₂=CH₂

2 CH₃CH₂CH=CH₂

3 CH₃(CH₂)₄CH₃

Reference: Past Exam Paper – November 2002 Paper 1 Q38

Solution 38:

Answer: A.

All these products are hydrocarbons with fewer carbon atoms than octane (8) thus all can be produced, hence correct option is A.

Question 26: [Organic > Hydrocarbons]:

Oxidation of an alkene Y gives a diol; further oxidation gives a diketone.

What could be Y?

A CH₃CH=C(CH₃)₂

B (CH₃)₂CHCH=CH₂

C C₆H₅CH=CHC₆H₅

D (C₆H₅)₂C=CHCH₃

Reference: Past Exam Paper – November 2002 Paper 1 Q26

Solution 26:

Answer: C.

To form a diol the alkene would require that the unsaturated carbons each have at least 1 hydrogen atom bonded to it. 

Option A and D do not provide this hence are eliminated. 

In option B the compound has 2 hydrogen atoms bonded to 1 of the unsaturated carbons this will cause it to become partially a primary alcohol and further oxidation will lead to fusion of a ketone from one side and a carboxylic acid on the other not a diketone. Hence option B is also incorrect. 

Option C provides a secondary alcohol on both sides hence will produce a diketone, making it the correct option.

Question 22: [Organic > Hydrocarbons]:

Chloroethane is used as a starting material for the production of ‘time-release capsules’ in pharmaceutical products. One way of preparing chloroethane is to react chlorine and ethane in the presence of ultraviolet light.

Which statement is correct about the first stage of the mechanism of this reaction?

A The Cl – Cl bond is split homolytically.

B The Cl – Cl bond is split heterolytically.

C The C – H bond is split homolytically.

D The C – H bond is split heterolytically.

Reference: Past Exam Paper – November 2002 Paper 1 Q22

Solution 22:

Answer: A.

This reaction is a free radical substitution, where an alkane reacts with a halogen to form a halogenoalkane. The first step is known as chain initiation in which the halogen’s diatomic molecule splits homolytically to form 2 halogen free radicals. In this case the Cl₂ molecule splits.