Chemistry 9701 · AS & A Level · Halogen compounds

Halogen compounds — practice question

Halogenoalkanes have long been used for many years as aerosol propellants, refrigerants and solvents. Fluoroethane, $\text{CH}_3\text{CH}_2\text{F}$, has been used as a refrigerant and can be prepared by reacting ethene with hydrogen fluoride. Your task is to calculate the C-F bond energy in fluoroethane.
(a)[4]

Calculate a value for the bond energy of the C-F bond, using suitable bond energies from the Data Booklet and the equation below. $\text{CH}_2=\text{CH}_2\text{(g)} + \text{HF(g)} \rightarrow \text{CH}_3\text{CH}_2\text{F(g)} \qquad \Delta H^{\circ} = -73\,\text{kJ mol}^{-1}$

(b)[2]

Another halogenoalkane used both as a refrigerant and as an aerosol propellant is dichlorodifluoromethane, $\text{CCl}_2\text{F}_2$. State two reasons why compounds such as $\text{CH}_3\text{CH}_2\text{F}$ and $\text{CCl}_2\text{F}_2$ have been chosen for use as aerosol propellants and refrigerants.

(c)[2]

Using relevant Data Booklet information and your response to (a), suggest why $\text{CCl}_2\text{F}_2$ is responsible for damage to the ozone layer in the stratosphere, whereas $\text{CH}_3\text{CH}_2\text{F}$ is not.

(d(i))[3]

What does the term enhanced greenhouse effect refer to?

(d(ii))[3]

Water vapour is the most abundant greenhouse gas. Which greenhouse gas is second in abundance?

(e)[1]

A greenhouse gas found in very small quantities in the atmosphere is sulfur hexafluoride, $\text{SF}_6$, which is used in high voltage electrical switchgear. What is the shape of the $\text{SF}_6$ molecule?

Worked solution & mark scheme

This 15-mark question has a full step-by-step worked solution and mark scheme. One marking point: The energy needed to break the reactant bonds is $2812\ \mathrm{kJ\ mol^{-1}}$

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