Chemistry 9701 · AS & A Level · Atoms, molecules and stoichiometry

Atoms, molecules and stoichiometry — practice question

Trihalomethanes are organic compounds in which three hydrogen atoms in methane are substituted by halogen atoms, for example $\text{CHF}_3$.
(a)[3]

The equation shows a reaction to make $\text{CHF}_3$. $\text{CHI}_3(s) + 3\text{AgF}(s) \rightarrow \text{CHF}_3(g) + 3\text{AgI}(s)$ Use the data to calculate the enthalpy change of reaction, $\Delta H_r$, for this production of $\text{CHF}_3$.

(b(i))[3]

CHF$_3$ is not an ideal gas. State three basic assumptions scientists use for ideal-gas behaviour.

(b(ii))[2]

Explain why CHF$_3$ does not behave as an ideal gas at pressures above $300\,\text{atm}$.

(c(i))[1]

Another trihalomethane, CHCl$_3$, reacts with O$_2$ to form carbonyl dichloride. HCl$(g)$ is also given off as a product in this reaction. Write an equation for this reaction of CHCl$_3$ with O$_2$.

(c(ii))[2]

The change from CHCl$_3$ to carbonyl dichloride can be followed using infra-red spectroscopy. The infra-red spectrum of carbonyl dichloride is shown. On the infra-red spectrum of carbonyl dichloride identify with an X the absorption that would not be present in an infra-red spectrum of CHCl$_3$. Explain your answer.

(c(iii))[1]

Suggest another difference between the infra-red spectra of CHCl$_3$ and carbonyl dichloride.

Worked solution & mark scheme

This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: Correct Hess cycle equation for $\Delta H_r$

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