Chemistry 9701 · AS & A Level · Chemical energetics

Chemical energetics — practice question

When calcium carbonate is heated, it breaks down as follows: $\text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g)$. Table 4.1 gives the values of the Gibbs free energy change, $\Delta G^{\ominus}$, for this reaction at different temperatures. Assume that the standard enthalpy change, $\Delta H^{\ominus}$, and the standard entropy change, $\Delta S^{\ominus}$, for this reaction do not vary over this temperature range.
(a(i))[2]

Use the information in Table 4.1 to draw a graph of $\Delta G^{\ominus}$ against $T$ on the grid.

(a(ii))[2]

Calculate the gradient of your graph. Determine $\Delta S^{\ominus}$ in $\text{J K}^{-1} \text{mol}^{-1}$ for this reaction. Show all working.

(b(i))[1]

Write an ionic equation for the breakdown of the hydrogencarbonate ion.

(b(ii))[2]

Thermal stability of Group 1 hydrogencarbonates increases down the group. Suggest an explanation for the trend in thermal stability of the Group 1 hydrogencarbonates.

(c(i))[2]

Give the definition of a buffer solution.

(c(ii))[2]

Write two equations to show the way equilibrium 5 acts as a buffer solution. Equilibrium 5: $\text{H}_2\text{CO}_3 + \text{H}_2\text{O} \rightleftharpoons \text{HCO}_3^- + \text{H}_3\text{O}^+$

(c(iii))[3]

In a seawater sample, the $[\text{HCO}_3^-]/[\text{H}_2\text{CO}_3]$ ratio is 14.1. Calculate the pH of this sample. $[pK_a: \text{H}_2\text{CO}_3,\ 6.35]$

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