Chemistry 9701 · AS & A Level · Chemical energetics

Chemical energetics — practice question

Nitrogen monoxide, NO, is produced in a reversible reaction when air is heated to the temperature reached inside a car engine.
(a(i))

Suggest a ‘dot-and-cross’ electronic structure for nitrogen monoxide.

(a(ii))

The enthalpy change of formation of nitrogen monoxide is $+90\ \text{kJ mol}^{-1}$. What is the enthalpy change for the reaction below? $2\text{NO(g)} \rightarrow \text{N}_2\text{(g)} + \text{O}_2\text{(g)}$

(a(iii))

Explain why nitrogen monoxide is formed in the car engine.

(a(iv))[5]

Using the bond enthalpy data in the Data Booklet and your answer to (ii), calculate a value for the bond energy of nitrogen monoxide.

(b(i))

At $800\ \text{K}$, nitrogen monoxide reacts with hydrogen according to the equation below: $2\text{H}_2\text{(g)} + 2\text{NO(g)} \rightarrow 2\text{H}_2\text{O(g)} + \text{N}_2\text{(g)}$ The table below shows how the initial rate of this reaction varies with the partial pressures of the reactants. Determine the order of the reaction with respect to each reactant, explaining your reasoning.

(b(ii))[1]

Write down the rate equation and the units of the rate constant.

(b(iii))[2]

This mechanism has been suggested for this reaction: step 1: $\text{NO} + \text{NO} \rightarrow \text{N}_2\text{O} + \text{O}$ step 2: $\text{H}_2 + \text{O} \rightarrow \text{H}_2\text{O}$ step 3: $\text{H}_2 + \text{N}_2\text{O} \rightarrow \text{N}_2 + \text{H}_2\text{O}$ Show how the overall stoichiometric equation is obtained from the three equations for the separate steps listed above.

(b(iv))[2]

Suggest which one of the three reactions in the mechanism is the rate-determining step. Explain your answer.

(c(i))[1]

The half-reaction data below relate to the reaction between $\text{HNO}_3$ and an excess of $\text{FeSO}_4$. $\text{Fe}^{3+} + e^- \rightarrow \text{Fe}^{2+} \quad E^\circ = +0.77\ \text{V}$ $3\text{H}^+ + \text{NO}_3^- + 2e^- \rightarrow \text{HNO}_2 + \text{H}_2\text{O} \quad E^\circ = +0.94\ \text{V}$ $\text{HNO}_2 + \text{H}^+ + e^- \rightarrow \text{NO} + \text{H}_2\text{O} \quad E^\circ = +0.99\ \text{V}$ Suggest the formula of the final nitrogen-containing product of this reaction.

(c(ii))[1]

Write an equation for the production of this nitrogen-containing product.

(c(iii))[1]

Nitrogen monoxide forms a dark brown complex with an excess of $\text{FeSO}_4\text{(aq)}$. What type of bonding is used in forming the complex?

(c(iv))[1]

Suggest a formula for this complex.

Worked solution & mark scheme

This 14-mark question has a full step-by-step worked solution and mark scheme. One marking point: Accurate overlapping orbital diagram with the shared electrons between N and O

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