International A LevelChemistryEdexcelTopic Questions1. Structure, Bonding & Introduction to Organic Chemistry1.9 Alkanes

Alkanes (Edexcel International A Level Chemistry)

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1a
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Using excess oxygen, 25 cm3 of a gaseous hydrocarbon CxHy was burned completely.

After cooling to room temperature the total gas volume was measured and found to be 75 cm3 less than the total gas volume before the mixture was ignited.

When the product gases were shaken with potassium hydroxide solution, the total gas volume decreased by a further 100 cm3.

All gas volumes were measured at room temperature and pressure.

A general equation for the combustion of a hydrocarbon is

straight C subscript straight x straight H subscript straight y space plus space open parentheses straight x plus y over 4 close parentheses space straight O subscript 2 space rightwards arrow space xCO subscript 2 plus space open parentheses y over 2 close parentheses space straight H subscript 2 straight O

Determine the molecular formula of CxHy. You must show your working.

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1b2 marks

When CxHy was added to a little bromine water and the mixture shaken, the bromine water remained yellow.

Suggest two possible structures for CxHy.

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1a2 marks

Heptane, C7H16 , is an alkane found in crude oil.

Heptane can undergo incomplete combustion.

i)

Give a reason why incomplete combustion sometimes occurs.

(1)

i)
Write the equation for the incomplete combustion of heptane, forming carbon monoxide and water as the only products.
State symbols are not required.

(1)

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1b4 marks

Heptane is reformed into branched-chain and cyclic hydrocarbons that are used in petrol.

i)

Draw the skeletal formulae of a branched-chain alkane and a cycloalkane, each containing seven carbon atoms.

(2)

Branched-chain alkane
Cycloalkane

ii)

Write the equation for the reforming of heptane into a cycloalkane.

Use molecular formulae.

(1)

iii)
Give a reason for adding cycloalkanes to petrol.
(1)

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1c9 marks

Heptane, C7H16 , reacts with chlorine in the presence of ultraviolet radiation.

i)

State the type and mechanism of this reaction.

(2)

ii)

Give the mechanism for the reaction to produce C7H15Cl, C14H30 and HCl as the only products.

Include the name of each of the steps in your mechanism.
Curly half-arrows are not required.

(7)

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