A-Level Chemistry – Physical: Energetics and Kinetics

In a calorimetry experiment, $0.0500\,\text{mol}$ of a solid reacts completely in $100\,\text{g}$ of aqueous solution. The temperature of the solution rises by $6.8\,^\circ\text{C}$.(a) Calculate the heat released by the reaction.(b) Calculate the molar enthalpy change of the reaction, in $\text{kJ mol}^{-1}$, including its sign.

State the definition of the standard enthalpy change of combustion of a substance.

Given the following enthalpy changes:$\text{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g)$, $\Delta H_1 = -394\,\text{kJ mol}^{-1}$$\text{H}_2(g) + \tfrac{1}{2}\text{O}_2(g) \rightarrow \text{H}_2\text{O}(l)$, $\Delta H_2 = -286\,\text{kJ mol}^{-1}$$\text{C}_2\text{H}_4(g) + 3\text{O}_2(g) \rightarrow 2\text{CO}_2(g) + 2\text{H}_2\text{O}(l)$, $\Delta H_3 = -1411\,\text{kJ mol}^{-1}$Use Hess’s law to calculate the standard enthalpy of formation of ethene, $\text{C}_2\text{H}_4(g)$.

Use the following mean bond enthalpies to calculate the enthalpy change for the complete combustion of methane, $\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g)$:$\text{C-H} = 414$, $\text{O=O} = 498$, $\text{C=O} = 804$, $\text{O-H} = 463\,\text{kJ mol}^{-1}$.

Define lattice enthalpy (of formation) and state whether it is always exothermic or endothermic, giving a reason.

Use the following data to calculate the lattice enthalpy of formation of sodium chloride, $\text{NaCl}(s)$:Enthalpy of formation of $\text{NaCl}(s)$: $-411\,\text{kJ mol}^{-1}$Enthalpy of atomisation (sublimation) of $\text{Na}(s)$: $+107\,\text{kJ mol}^{-1}$First ionisation energy of $\text{Na}(g)$: $+496\,\text{kJ mol}^{-1}$Bond dissociation enthalpy of $\text{Cl}_2(g)$: $+242\,\text{kJ mol}^{-1}$First electron affinity of $\text{Cl}(g)$: $-349\,\text{kJ mol}^{-1}$

For sodium chloride, the lattice enthalpy calculated from the Born-Haber cycle (an experimental value) agrees closely with the theoretical lattice enthalpy calculated from a purely ionic (point-charge) model. Explain what this agreement suggests about the type of bonding in NaCl.

For silver chloride, $\text{AgCl}$, the experimental (Born-Haber) lattice enthalpy is considerably more exothermic than the theoretical lattice enthalpy calculated from a purely ionic model. Explain this difference in terms of ionic polarisation.

For a reaction, $\Delta H = -92.0\,\text{kJ mol}^{-1}$ and $\Delta S = -198\,\text{J K}^{-1}\text{mol}^{-1}$.(a) Calculate $\Delta G$ at $298\,\text{K}$ and state whether the reaction is feasible (spontaneous) at this temperature.(b) Calculate the temperature above which the reaction is no longer feasible.

Explain, in terms of the number and arrangement of particles, why a reaction in which the number of moles of gas increases has a positive entropy change, $\Delta S$.

The Arrhenius equation is $k = Ae^{-E_a/RT}$. For a reaction, $E_a = 75.0\,\text{kJ mol}^{-1}$ and $A = 4.0\times10^{13}\,\text{s}^{-1}$.(a) Calculate the rate constant $k$ at $300\,\text{K}$.(b) Calculate the rate constant $k$ at $310\,\text{K}$.(c) Calculate the ratio $\dfrac{k(310\,\text{K})}{k(300\,\text{K})}$.

Using the Arrhenius equation, explain why a relatively small increase in temperature (e.g. $10\,\text{K}$) can cause a large increase in the rate constant of a reaction.