(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 of a reactant or product per unit time.

(a)(ii) According to collision theory, reactant particles must collide effectively before products can form. On increasing the temperature, particles acquire greater kinetic energy. They move faster, collide more frequently, and a greater proportion of the collisions have energy equal to or greater than the activation energy. Hence, the number of effective collisions per second increases and the reaction rate increases.

(b)(i)

(b)(ii) Place equal volumes and concentrations of hydrogen peroxide solution in two identical conical flasks fitted with bungs and delivery tubes leading to gas syringes. Add the same measured mass of manganese(IV) oxide, \(\mathrm{MnO_2}\), to one flask and immediately replace the bung. Do not add \(\mathrm{MnO_2}\) to the other flask, which serves as the control. Record the volume of oxygen collected in each gas syringe at equal time intervals.

The flask containing \(\mathrm{MnO_2}\) produces oxygen much more rapidly, shown by a steeper volume of oxygen against time curve and by vigorous effervescence. The gas relights a glowing splint, confirming that it is oxygen. \(\mathrm{MnO_2}\) is a catalyst because it increases the rate without being used up chemically.

(b)(iii) The catalysed pathway has a lower activation energy, while the enthalpy change, \(\Delta H\), remains negative because the products are at a lower energy than the reactants.

(c)(i) Enthalpy change alone cannot predict spontaneity because spontaneity also depends on the entropy change and on temperature. At constant temperature and pressure, a reaction is spontaneous only when:

Thus, an endothermic reaction may be spontaneous if the increase in entropy is sufficiently large at a suitable temperature, whereas an exothermic reaction may be non-spontaneous if the entropy change is sufficiently unfavourable.