Chemistry 9701 · AS & A Level · Nitrogen and sulfur

Nitrogen and sulfur — practice question

Dinitrogen tetraoxide, $\text{N}_2\text{O}_4$, and nitrogen dioxide, $\text{NO}_2$, are in dynamic equilibrium with one another. $\text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g) \qquad \Delta H = +54\,\text{kJ mol}^{-1}$ An energy profile for this reaction is shown.
(a)[2]

Add labelled arrows to the energy profile to show the reaction enthalpy, $\Delta H$, and the activation energy for the forward reaction, $E_a$.

(b(i))[2]

$0.0500\,\text{mol}$ of $\text{N}_2\text{O}_4$ was put into a sealed vessel with volume $1.00\,\text{dm}^3$, at $50^{\circ}\text{C}$ and pressure $1.68 \times 10^5\,\text{Pa}$. The mass of the equilibrium mixture produced was $4.606\,\text{g}$. Calculate the average molecular mass, $M_r$, of the equilibrium mixture produced. Give your answer to three significant figures.

(b(ii))[1]

Let the number of moles of $\text{N}_2\text{O}_4$ that dissociated be $n$. State, in terms of $n$, the amount, in moles, of $\text{NO}_2$ present in the equilibrium mixture.

(b(iii))[1]

The amount of $\text{N}_2\text{O}_4$ left at equilibrium is $(0.05 - n)$. State, in terms of $n$, the total amount, in moles, of gas present in the equilibrium mixture.

(b(iv))[1]

State, in terms of $n$, the mole fraction for $\text{NO}_2$ in the equilibrium mixture.

(b(v))[2]

For this equilibrium mixture, the mole fraction of $\text{NO}_2$ is $0.400$. Use your answers to (ii) and (iv) to find the amount in moles of each gas in the equilibrium mixture. Give your answers to three significant figures.

(b(vi))[1]

Write the expression for the equilibrium constant, $K_p$, for this system.

(b(vii))[3]

Using the total pressure of the mixture, $1.68 \times 10^5\,\text{Pa}$, calculate the value of the equilibrium constant, $K_p$, and include its units.

Worked solution & mark scheme

This 13-mark question has a full step-by-step worked solution and mark scheme. One marking point: Vertical arrow drawn from the $N_2O_4$ level up to the $2NO_2$ level, labelled enthalpy change $\Delta H$

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