(a)(i) What is meant by the rate of a chemical reaction? (ii) Explain in terms of the vision theory, the effect of temperature increase on reaction rate. (b...

(a)(i) The rate of a chemical reaction is the change in concentration (or amount) of a reactant used up, or product formed, per unit time.

(a)(ii) Interpreting “vision theory” as collision theory: particles must collide for a reaction to occur. Increasing temperature increases the particles’ average kinetic energy, so they move faster and collide more frequently. It also means that a greater proportion of collisions have energy at least equal to the activation energy. Therefore, there are more successful collisions each second and the reaction rate increases.

(b)(i)

\[2\text{H}_2\text{O}_{2(aq)} \rightarrow 2\text{H}_2\text{O}_{(l)}+\text{O}_{2(g)}\]

(b)(ii) Use manganese(IV) oxide, \(\text{MnO}_2\), as the catalyst.

1. Place equal volumes and concentrations of hydrogen peroxide solution into two identical conical flasks at the same temperature.
2. Fit each flask with a bung and delivery tube connected to a gas syringe.
3. Add a measured mass of \(\text{MnO}_2\) to one flask, immediately fit the bung, and start the timer. Leave the other flask without catalyst as a control.
4. Record the volume of oxygen collected in each gas syringe at regular time intervals.

The flask containing \(\text{MnO}_2\) produces oxygen more rapidly: it will give a steeper volume-of-oxygen-against-time graph and reach a given oxygen volume in a shorter time. This shows that manganese(IV) oxide increases the rate of decomposition. Keeping the volume and concentration of hydrogen peroxide, temperature, and apparatus the same makes this a fair comparison.

(b)(iii) The reaction is exothermic, so the products have lower energy than the reactants. A catalyst provides an alternative pathway with a lower activation energy, but it does not change the enthalpy change, \(\Delta H\).

(c)(i) Enthalpy change alone cannot predict spontaneity because spontaneity depends on both enthalpy change and entropy change, as well as temperature:

\[\Delta G=\Delta H-T\Delta S\]

A process is thermodynamically spontaneous when \(\Delta G<0\). An endothermic reaction, with positive \(\Delta H\), can still be spontaneous if the entropy increase is sufficiently large at a suitable temperature. Conversely, a favourable enthalpy change alone does not guarantee spontaneity under all conditions. Also, spontaneity does not mean that a reaction is fast: hydrogen peroxide decomposition may be thermodynamically feasible but slow without a catalyst because of its activation energy.

Examination reminder: Use \(\Delta H\) to describe heat energy change, but use \(\Delta G\), including \(\Delta S\) and \(T\), to decide thermodynamic spontaneity.