Alkenes

5 marks · Easy

The table below shows two hydrocarbon molecules.

Molecule	Structural formula
A	CH_3CH=CH_2
B	CH_3CH_2CH_3

(a)

Which molecule is unsaturated? Explain your answer.

[2 marks]

(b)

State how the presence of a C=C bond affects the chemical properties of alkenes.

[1 mark]

(c)

Describe the physical state of molecule A at room temperature.

[1 mark]

(d)

Compare the boiling points of molecule A and molecule B.

[1 mark]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

(a) Molecule A; it contains a C=C double bond · must show Identification of molecule A, Explanation: presence of C=C double bond means unsaturaton

(b) It makes alkenes more reactive; allows addition reactions. · must show Alkenes are more reactive than alkanes, C=C bond allows addition reactions

(d) They have similar boiling points. · must show similar boiling points

Show mark scheme and worked solution

Marking points

A1 (a) · 1 mark · Correctly identifies molecule A as unsaturated.

A2 (a) · 1 mark · Correctly explains that molecule A contains a C=C bond making it unsaturated.

B1 (b) · 1 mark · States that the C=C bond makes alkenes more reactive / allows addition reactions.

C1 (c) · 1 mark · Correctly states molecule A is a gas at room temperature.

D1 (d) · 1 mark · States that boiling points of A and B are similar.

Worked solution

a) Molecule A is unsaturated because it has a C=C double bond; molecule B only has single C–C bonds and is saturated.

b) The C=C bond makes alkenes more reactive than alkanes and allows them to take part in addition reactions.

c) Molecule A (propene) is a gas at room temperature.

d) The boiling points of molecule A and B are very similar because both are small molecules with similar molecular masses.