Alkenes

5 marks · Easy

Answer the questions about alkenes below.

(a)

A straight-chain alkene has the formula C4H8. Draw the displayed formula of one isomer of C4H8 and name it.

(i)

Draw the displayed formula.

[1 mark]

(ii)

Name this isomer.

[1 mark]

(b)

The general formula for alkenes is CnH2n. Calculate the molecular formula of the alkene with 7 carbon atoms.

[2 marks]

(c)

Explain how the physical state of the C7 alkene compares to the C4 alkene at room temperature.

[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.i) — · must show Structure for but-1-ene or but-2-ene with correct C=C placement and correct H atoms.

(a.ii) but-1-ene · must show Correct name (but-1-ene or but-2-ene)

(b) C7H14 · must show C7H14

Also accept: Allow but-1-ene or but-2-ene as valid isomers of C4H8.

Common mistakes: Mixing up alkane and alkene formulae.; Confusing isomer with structural formula.

Show mark scheme and worked solution

Marking points

A1 (a(i)) · 1 mark · Correct displayed formula of a straight-chain isomer of C4H8 (but-1-ene or but-2-ene).

A2 (a(ii)) · 1 mark · Correct full name (but-1-ene or but-2-ene) matching the structure drawn.

B1 (b) · 1 mark · Correctly substitutes n = 7 into the general formula.

B2 (b) · 1 mark · Gives correct molecular formula as C7H14. (follow-through)

C1 (c) · 1 mark · Correct explanation: C7 is a liquid, C4 is a gas at room temperature.

Worked solution

a.i) Drawn displayed formula (shown below for but-1-ene):

a.ii) Name: but-1-ene

b) Using CnH2n with n = 7: C7H2x7 = C7H14

c) The C7 alkene (heptene) is a liquid at room temperature, while C4 alkene (butene) is a gas at room temperature.