Energetics: Practice Questions

In terms of energy and bond processes, which statement accurately describes bond-breaking?

In an energy level diagram for an endothermic reaction, how are the relative energy levels of the reactants and products represented?

A student added \(1.30\, g\) of zinc powder (\(A_r = 65\)) to \(50.0\, cm^3\) of \(0.300\, mol/dm^3\) copper(II) sulfate solution. The temperature of the mixture rose by \(8.4\, ^\circ C\).

Calculate the molar enthalpy change (\(\Delta H\)) for the reaction in \(kJ/mol\).
(Assume the density of the solution is \(1.00\, g/cm^3\) and the specific heat capacity is \(4.18\, J/g/^\circ C\). Ignore the mass of the zinc.)

Which of the following statements correctly describes an exothermic reaction?

A student uses a simple calorimeter to heat \(100\,g\) of water. The temperature of the water increases from \(18.5^\circ C\) to \(42.5^\circ C\). Calculate the heat energy change (\(Q\)) in Joules.
(Specific heat capacity of water = \(4.18\,J/g/^\circ C\))

Identify whether a chemical reaction is exothermic or endothermic if the temperature of the reaction mixture increases, and explain your choice.

Using the expression \(Q = mc\Delta T\), calculate the heat energy change in Joules when \(100\,g\) of water is heated by \(15\,^\circ C\). (Specific heat capacity of water, \(c = 4.18\,J/g/^\circ C\)).

A displacement reaction releases \(4.2\,kJ\) of heat energy when \(0.02\,mol\) of a reactant is consumed. Calculate the molar enthalpy change (\(\Delta H\)) in \(kJ/mol\), ensuring the correct sign is included.

A student uses a simple calorimeter to determine the enthalpy change of neutralisation for the reaction between dilute hydrochloric acid (\(HCl\)) and aqueous sodium hydroxide (\(NaOH\)).
The student mixes \(25.0\, cm^3\) of \(1.00\, mol/dm^3\) \(HCl\) with \(25.0\, cm^3\) of \(1.00\, mol/dm^3\) \(NaOH\). The initial temperature of both solutions was \(18.5\, ^\circ C\). The maximum temperature reached after mixing was \(25.0\, ^\circ C\).
(Assume the density of the final solution is \(1.00\, g/cm^3\) and the specific heat capacity is \(4.18\, J/g/^\circ C\)).

a) Calculate the heat energy change (\(Q\)) for this reaction in Joules.
b) Calculate the number of moles of \(HCl\) used in the reaction.
c) Use your answers to parts (a) and (b) to calculate the molar enthalpy change (\(\Delta H\)) for the reaction in \(kJ/mol\). Include a sign in your answer.

The reaction between hydrazine (\(N_2H_4\)) and oxygen is highly exothermic and can be represented by the following equation:
\(N_2H_4(g) + O_2(g) \rightarrow N_2(g) + 2H_2O(g)\)
The enthalpy change (\(\Delta H\)) for this reaction is \(-585\, kJ/mol\).

Use the following average bond energies to calculate the bond energy of the \(N-N\) single bond in hydrazine.
\(N-H\): \(391\, kJ/mol\)
\(O=O\): \(498\, kJ/mol\)
\(N \equiv N\): \(945\, kJ/mol\)
\(O-H\): \(464\, kJ/mol\)

a) Calculate the energy released when the bonds in the products are formed.
b) Determine the energy required to break all the bonds in the reactants, expressing it in terms of the \(N-N\) bond energy (\(x\)).
c) Calculate the value of the \(N-N\) bond energy in \(kJ/mol\).
d) In the space provided, draw an energy level diagram for this reaction. Label the activation energy (\(E_a\)) and the enthalpy change (\(\Delta H\)).