Chemistry 9701 · AS & A Level · 4.1

4.1 — practice question

The solubility patterns of the Group 2 sulfates and the Group 2 chromates follow the same general trend.
(a)[4]

Suggest the trend in solubility for the Group 2 chromates down the group. Explain your answer.

(b(i))[1]

Silver(I) chromate, $\mathrm{Ag_2CrO_4}$, has only slight solubility in water. Write an ionic equation that shows the equilibrium between solid $\mathrm{Ag_2CrO_4}$ and its aqueous solution. Include state symbols.

(b(ii))[3]

The solubility product, $K_{sp}$, of $\mathrm{Ag_2CrO_4}$ is $1.12 \times 10^{-12}$ at $298\,\mathrm{K}$. Calculate the equilibrium concentration of $\mathrm{Ag^+}$, in $\mathrm{mol\,dm^{-3}}$, in a saturated solution of $\mathrm{Ag_2CrO_4}$ at $298\,\mathrm{K}$.

(c(i))[2]

The hydrogenchromate ion, $\mathrm{HCrO_4^-}$, is a weak acid. Its $pK_a$ is $6.49$. Calculate the pH of a $0.0250\,\mathrm{mol\,dm^{-3}}$ solution of $\mathrm{HCrO_4^-}$.

(c(ii))[1]

The ion $\mathrm{HCrO_4^-}$ can show amphoteric behaviour. State the formula of the conjugate acid of $\mathrm{HCrO_4^-}$ and the conjugate base of $\mathrm{HCrO_4^-}$.

(d(i))[1]

State the meaning of first electron affinity.

(d(ii))[1]

Explain why the value for the second electron affinity of sulfur is positive.

(d(iii))[1]

Construct an equation to represent the lattice energy of $\mathrm{Ag_2S}$. Include state symbols.

(d(iv))[3]

Calculate the enthalpy change of atomisation, $\Delta H_{at}$, in $\mathrm{kJ\,mol^{-1}}$, for silver using relevant data from Table 2.1. You may find it helpful to draw a labelled Born-Haber cycle. Show your working.

(e)[1]

Suggest how the magnitude of the lattice energy of $\mathrm{Ag_2S(s)}$ compares with that of $\mathrm{Cu_2S(s)}$. Explain your answer.

Worked solution & mark scheme

This 18-mark question has a full step-by-step worked solution and mark scheme. One marking point: (solubility) decreases down the group

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