Chemical energetics (AS): Practice Questions

Which of the following processes is exothermic?

A reaction has a standard enthalpy change \(\Delta H^\ominus = +40\,kJ\,mol^{-1}\) and a standard entropy change \(\Delta S^\ominus = +150\,J\,K^{-1}\,mol^{-1}\).
Determine the sign of the Gibbs free energy change, \(\Delta G^\ominus\), at 298 K and state if the reaction is feasible at this temperature.

The process of boiling water is represented as \(H_2O(l) \rightarrow H_2O(g)\). Given that \(\Delta H^\ominus = +40.7\,kJ\,mol^{-1}\) and \(\Delta S^\ominus = +109\,J\,K^{-1}\,mol^{-1}\).
At what approximate temperature, in Kelvin, is the system in equilibrium (where steam and water coexist at 101 kPa)?

Which of the following statements correctly defines the standard enthalpy change of formation, \(\Delta H_f^\ominus\)?

Which statement correctly compares the enthalpy changes of hydration?

Define the term standard enthalpy change of formation, \(\Delta H_f^\ominus\).

In terms of bond breaking and bond making, explain why a chemical reaction is classified as exothermic.

(a) Define the term standard enthalpy change of combustion, \(\Delta H_c^\ominus\).

(b) The reaction between hydrazine, \(N_2H_4\), and hydrogen peroxide, \(H_2O_2\), is used in rocket propulsion. The equation for the reaction is shown below:
\(N_2H_4(l) + 2H_2O_2(l) \rightarrow N_2(g) + 4H_2O(g)\)

Use the following standard enthalpy changes of formation, \(\Delta H_f^\ominus\), to calculate the standard enthalpy change of this reaction, \(\Delta H_r^\ominus\), in \(kJ \cdot mol^{-1}\).

\(\Delta H_f^\ominus [N_2H_4(l)] = +50.6\,kJ \cdot mol^{-1}\)
\(\Delta H_f^\ominus [H_2O_2(l)] = -187.8\,kJ \cdot mol^{-1}\)
\(\Delta H_f^\ominus [H_2O(g)] = -241.8\,kJ \cdot mol^{-1}\)

A student uses a calorimeter to determine the enthalpy change of solution, \(\Delta H_{sol}^\ominus\), of ammonium nitrate, \(NH_4NO_3\).

(a) When 8.00 g of \(NH_4NO_3\) (\(M_r = 80.0\)) is dissolved in 100.0 \(cm^3\) of water, the temperature decreases from 22.5 \(^\circ C\) to 16.8 \(^\circ C\).
Calculate the heat energy change, \(q\), for this reaction in Joules.
(Assume the density of the solution is 1.00 \(g \cdot cm^{-3}\) and its specific heat capacity is 4.18 \(J \cdot g^{-1} \cdot K^{-1}\))

(b) Calculate the value of \(\Delta H_{sol}^\ominus\) for \(NH_4NO_3\) in \(kJ \cdot mol^{-1}\). Include the correct sign.

(c) Explain why the \(\Delta H_{sol}^\ominus\) of an ionic solid can be either endothermic or exothermic by referring to lattice energy and enthalpy change of hydration.