Chemical energetics (AS): Practice Questions

When \(0.10\text{ mol}\) of anhydrous copper(II) sulfate is dissolved in a large excess of water, the temperature increases and the enthalpy change is \(-66.0\text{ kJ mol}^{-1}\). When \(0.10\text{ mol}\) of hydrated copper(II) sulfate, \(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\), is dissolved in the same excess of water, the temperature decreases and the enthalpy change is \(+11.0\text{ kJ mol}^{-1}\).

What is the enthalpy change for the following reaction?
\(\text{CuSO}_4(\text{s}) + 5\text{H}_2\text{O}(\text{l}) \rightarrow \text{CuSO}_4 \cdot 5\text{H}_2\text{O}(\text{s})\)

A reaction pathway diagram shows an initial energy level of reactants at \(+50\text{ kJ mol}^{-1}\), a peak at \(+180\text{ kJ mol}^{-1}\), and the energy level of products at \(-20\text{ kJ mol}^{-1}\).

Which statement about this reaction is correct?

In an experiment to determine the enthalpy change of combustion of methanol, \(\text{CH}_3\text{OH}\), \(0.80\text{ g}\) of methanol was burned to heat \(100\text{ cm}^3\) of water. The temperature of the water rose from \(20.0^{\circ}\text{C}\) to \(45.0^{\circ}\text{C}\).

Specific heat capacity of water = \(4.18\text{ J g}^{-1}\text{ K}^{-1}\).
Density of water = \(1.00\text{ g cm}^{-3}\).
Relative molecular mass of methanol = \(32.0\).

What is the calculated enthalpy change of combustion of methanol from this experiment?

The reaction \(N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)\) has \(\Delta H_r = -92\) kJ mol\(^{-1}\). If the bond energies for \(H–H\) and \(N–H\) are \(436\) kJ mol\(^{-1}\) and \(391\) kJ mol\(^{-1}\) respectively, calculate the bond energy of the \(N\equiv N\) triple bond.

The bond energies for some covalent bonds are given below:
\(\text{C-H}: 410\text{ kJ mol}^{-1}\)
\(\text{Cl-Cl}: 242\text{ kJ mol}^{-1}\)
\(\text{C-Cl}: 338\text{ kJ mol}^{-1}\)
\(\text{H-Cl}: 431\text{ kJ mol}^{-1}\)

What is the enthalpy change, \(\Delta H_r\), for the following reaction?
\(\text{CH}_4(\text{g}) + \text{Cl}_2(\text{g}) \rightarrow \text{CH}_3\text{Cl}(\text{g}) + \text{HCl}(\text{g})\)

Calculate the standard enthalpy change of formation for CO(g) using the following standard enthalpy changes of combustion:
\(\Delta H_c^{\ominus}[\text{C(graphite)}] = -394 \text{ kJ mol}^{-1}\)
\(\Delta H_c^{\ominus}[\text{CO(g)}] = -283 \text{ kJ mol}^{-1}\)

Define standard enthalpy change of combustion and explain why the experimental value for the combustion of ethanol obtained using a simple spirit burner is usually less exothermic than the data book value.

A student uses the relationship \(q = mc\Delta T\) to determine the enthalpy of neutralisation. In an experiment, \(50.0\text{ cm}^3\) of \(1.0\text{ mol dm}^{-3}\text{ HCl}\) is mixed with \(50.0\text{ cm}^3\) of \(1.0\text{ mol dm}^{-3}\text{ NaOH}\). If the temperature rises by \(6.8^{\circ}\text{C}\), calculate the energy released in Joules (assume the density of the solution is \(1.0\text{ g cm}^{-3}\) and specific heat capacity is \(4.18\text{ J g}^{-1}\text{ K}^{-1}\)).

A student performs a calorimetry experiment to determine the enthalpy change of neutralisation, \(\Delta H_{\text{neut}}\), for the reaction between aqueous sodium hydroxide and hydrochloric acid.
(a) Define standard enthalpy change of neutralisation.
(b) In the experiment, \(50.0\text{ cm}^3\) of \(1.00\text{ mol dm}^{-3}\) \(\text{HCl}\) is mixed with \(50.0\text{ cm}^3\) of \(1.10\text{ mol dm}^{-3}\) \(\text{NaOH}\). Both solutions were initially at \(21.5\,^{\circ}\text{C}\). The maximum temperature reached was \(28.2\,^{\circ}\text{C}\). Calculate the heat energy released in Joules, assuming the density of the mixture is \(1.00\text{ g cm}^{-3}\) and its specific heat capacity is \(4.18\text{ J g}^{-1} \text{K}^{-1}\).
(c) Calculate the enthalpy change of neutralisation in \(\text{kJ mol}^{-1}\) for this reaction.

A student is asked to determine the enthalpy change of formation of ethane, \(\text{C}_2\text{H}_6\text{(g)}\), using Hess's Law.
(a) Define standard enthalpy change of formation.
(b) Use the following standard enthalpy changes of combustion to calculate \(\Delta H_f^{\ominus}\) of \(\text{C}_2\text{H}_6\text{(g)}\):
\(\Delta H_c^{\ominus}[\text{C(graphite)}] = -394\text{ kJ mol}^{-1}\)
\(\Delta H_c^{\ominus}[\text{H}_2\text{(g)}] = -286\text{ kJ mol}^{-1}\)
\(\Delta H_c^{\ominus}[\text{C}_2\text{H}_6\text{(g)}] = -1560\text{ kJ mol}^{-1}\)
(c) Construct a labeled Hess's Law cycle for this calculation.