SECTION A: PHYSICAL AND GENERAL CHEMISTRY

Question 1

(a) Define the following:

• (i) A standard solution.

• (ii) A reduction-oxidation (redox) reaction.

(b) Structured Calculation

A $1.98\text{ g}$ sample of hydrated oxalic acid ($\text{H}_2\text{C}_2\text{O}_4 \cdot 2\text{H}_2\text{O}$, $\text{RMM} = 126$) was dissolved in distilled water and made up to $250\text{ cm}^3$ in a volumetric flask. $25.0\text{ cm}^3$ of this solution was acidified and found to react with $30.5\text{ cm}^3$ of $0.021\text{ M}$ $\text{MnO}_4^{-}(\text{aq})$.

Calculate the following:

• (i) Number of moles of $\text{MnO}_4^{-}$ ions that reacted with the $\text{C}_2\text{O}_4^{2-}$ ions.

• (ii) Number of moles of $\text{C}_2\text{O}_4^{2-}$ ions in $250\text{ cm}^3$ of the solution.

(2 marks)

(c) Energetics & Cycles

• (i) Draw the Born-Haber cycle for the formation of one mole of magnesium oxide.

  (4 marks)

• (ii) Given the following bond energies in $\text{kJ}\,\text{mol}^{-1}$:

  • $E(\text{C}-\text{H}) = +413$

  • $E(\text{Cl}-\text{Cl}) = +242$

  • $E(\text{C}-\text{Cl}) = +326$

  • $E(\text{H}-\text{Cl}) = +431$

  Calculate the enthalpy change for the reaction:

  $$\text{CH}_4 + \text{Cl}_2 \longrightarrow \text{CH}_3\text{Cl} + \text{HCl}$$

  (5 marks)

(d) Solid State Chemistry

What is the difference between a crystal structure and a unit cell?

(2 marks)

(e) Chemical Equilibria

The reaction:

is in equilibrium at $600^\circ\text{C}$. If the partial pressures of $\text{N}_2$, $\text{H}_2$, and $\text{NH}_3$ are $1.27$, $0.39$, and $0.18\text{ atm}$ respectively:

• (i) Write an expression for the equilibrium constant, $K_p$, for the reaction.

• (ii) Calculate the equilibrium constant, $K_p$, stating its units.

(3 marks)

(f) Acid-Base Equilibria

Identify the conjugate acid / base pairs in the reaction below:

(2 marks)

(g) Electrochemistry

Given the following half-cell reactions and their electrode potentials:

Write the cell diagram and calculate the emf of the cell.

(2 marks) (Total: 20 marks)