Chemistry 9701 · AS & A Level · 3.2

3.2 — practice question

When heated, Group 2 hydrogencarbonate solutions, $\mathrm{M(HCO_3)_2}$, break down to produce the matching metal carbonate, carbon dioxide, and water.
(a(i))[1]

Write the equation for the decomposition of strontium hydrogencarbonate, $\mathrm{Sr(HCO_3)_2}$.

(a(ii))[2]

Thermal stability of Group 2 carbonates rises down the group. Explain this pattern.

(b)[4]

The hydroxides and fluorides of Group 2 elements follow similar solubility patterns. Describe how the solubility of the fluorides of calcium, strontium and barium changes. Explain your answer.

(c(i))[1]

Define enthalpy change of hydration, $\Delta H_{\mathrm{hyd}}$.

(c(ii))[2]

State the main factors that influence the size of the enthalpy change of hydration. Explain your answer.

(d)[2]

Table 1.1 lists several energy changes. Use the data in Table 1.1 to work out the enthalpy change of solution, $\Delta H_{\mathrm{sol}}$, for $\mathrm{MgF_2(s)}$. Drawing a labelled energy cycle may help. Show your working.

(e(i))[2]

Mercury(I) fluoride, $\mathrm{Hg_2F_2}$, has only limited solubility in water. In $\mathrm{Hg_2F_2}$ the cation is the diatomic ion $\mathrm{Hg_2^{2+}}$, which contains a covalent $\mathrm{Hg{-}Hg}$ bond. Write the solubility product expression, $K_{sp}$, for $\mathrm{Hg_2F_2}$. Include the units.

(e(ii))[1]

The solubility of $\mathrm{Hg_2F_2}$ is $9.20 \times 10^{-3}\ \mathrm{mol\ dm^{-3}}$ at $298\ \mathrm{K}$. Calculate the value of $K_{sp}$ for $\mathrm{Hg_2F_2}$ at $298\ \mathrm{K}$.

Worked solution & mark scheme

This 15-mark question has a full step-by-step worked solution and mark scheme. One marking point: Balanced form: $\mathrm{Sr(HCO3)2 \rightarrow SrCO3 + CO2 + H2O}$

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