Alkenes

6 marks · Normal

(a)

A student finds that a colourless, flammable liquid decolourises bromine water and burns with a smoky flame.

Explain what these two observations tell you about the type of hydrocarbon present.

[2 marks]

(b)

The student tests another liquid and observes that it does not decolourise bromine water and burns with a clean flame.

What type of hydrocarbon is this? Explain your reasoning.

[2 marks]

(c)

Write the displayed formula for but-1-ene, the alkene that would show these observations.

[2 marks]

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.

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(a) The liquid is an alkene. Decolourising bromine water means it is unsaturated (has a C=C double bond), and a smoky flame suggests incomplete combustion due to higher carbon content.

(b) The second liquid is an alkane. It does not decolourise bromine water, showing it is saturated (no C=C bond), and a clean flame is typical for alkanes, which burn more completely.

Must show: displayed formula

Show mark scheme and worked solution

Marking points

R1 (a) · 1 mark · Identifies the hydrocarbon as an alkene/unsaturated

E1 (a) · 1 mark · Supports with both observations: decolourises bromine (C=C double bond), smoky flame (higher C by mass, incomplete combustion typical of alkene)

R2 (b) · 1 mark · Identifies the hydrocarbon as an alkane/saturated

E2 (b) · 1 mark · Explains: does not decolourise bromine water (no C=C), burns cleanly (typical for alkanes)

S1 (c) · 2 marks · Complete displayed formula (all atoms and bonds, double bond at correct position)

Worked solution

a) The decolourisation of bromine water shows the compound is unsaturated (an alkene). Burning with a smoky flame is common for alkenes due to higher C:H ratio.

b) No reaction with bromine water means the hydrocarbon is saturated (an alkane). A clean flame on burning supports this.

c) Displayed formula for but-1-ene: