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

Atoms, molecules and stoichiometry — practice question

A quantity of $0.025\,\text{mol}$ of $\text{HI(g)}$ is placed in a sealed vessel and allowed to settle at dynamic equilibrium. The vessel has a total pressure of $100\,\text{kPa}$. Equation 1: $\text{2HI(g)} \rightleftharpoons \text{H}_2\text{(g)} + \text{I}_2\text{(g)}$
(a(i))[2]

Explain what is understood by dynamic equilibrium.

(a(ii))[1]

Describe one difference between the reaction mixture at the start and the mixture once equilibrium has been reached.

(a(iii))[1]

Write an expression for $K_p$ for the reaction in equation 1.

(a(iv))[2]

At equilibrium, the partial pressure of $\text{HI(g)}$ equals $86.4\,\text{kPa}$. Calculate how many moles of $\text{HI(g)}$ are present in the equilibrium mixture. Show your working.

(b)[2]

Use equation 1 and the bond energy values in Table 3.1 to calculate the enthalpy change, $\Delta H$, for the thermal decomposition of $1$ mole of $\text{HI(g)}$. Show your working.

(c)[1]

Describe how increasing pressure affects the value of $K_p$ for the decomposition of $\text{HI(g)}$.

(d(i))[1]

Identify the kind of reaction that takes place when $\text{NaI(s)}$ reacts with concentrated $\text{H}_2\text{SO}_4$ to form $\text{HI(g)}$.

(d(ii))[1]

Write an equation for how $\text{HI(g)}$ reacts with concentrated $\text{H}_2\text{SO}_4$.

(d(iii))[1]

Explain why $\text{HI(g)}$ reacts with concentrated $\text{H}_2\text{SO}_4$ but $\text{HCl}$ does not.

Worked solution & mark scheme

This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: At equilibrium, the forward and reverse reactions proceed at the same rate

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