May/June 2026 Paper 42

This question is about compounds of nitrogen. Ammonia is manufactured by reacting hydrogen with nitrogen in the Haber process. State one source of hydrogen and one source of nitrogen.

Write the symbol equation for the reaction in the Haber process.

State the typical pressure, in kPa, used in the Haber process.

Aqueous ammonia, NH${}_3$(aq), is used to detect aqueous metal ions. The colour of the precipitate formed when NH${}_3$(aq) is added to an aqueous solution indicates the metal ion present. State the colour and give the name of the precipitate formed when NH${}_3$(aq) is added to Cr${}^{3+}$(aq) ions.

When a few drops of NH${}_3$(aq) are added to Zn${}^{2+}$(aq) a white precipitate forms. When a few drops of Zn${}^{2+}$(aq) are added to NH${}_3$(aq) no precipitate forms. Explain these observations.

The reaction between ammonia and fluorine forms nitrogen trifluoride, NF${}_3$(g), and hydrogen fluoride, HF(g).

State how the equation shows this reaction is:

Figure 5.1 shows an incomplete reaction profile diagram for the forward reaction. On Figure 5.1: • write the formulas of the reactants and the products • draw an arrow, labelled $E_a$, to show the activation energy • draw an arrow, labelled $\Delta H$, to show the enthalpy change.

The equation for the forward reaction can be represented as shown.

Use the bond energies in Table 5.1 and the $\Delta H$ value of $-870$ kJ / mol for the forward reaction to calculate the H–F bond energy. Use the following steps. • Calculate the energy needed to break the bonds in the reactants. • Calculate the total energy released when the bonds in the products form. • Calculate the bond energy of the H–F bond.

On Figure 5.2, complete the dot‑and‑cross diagram of a molecule of nitrogen trifluoride.