Answer: (a) The change in concentration of a reactant or product per unit time; (b) e.g. temperature and size of particles; (c) \(\mathrm{3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2}\), iron is oxidised.
(a) The rate of a chemical reaction is the change in the concentration of a reactant or a product in unit time, i.e. how fast the reactants are used up or the products are formed.
(b) Any two of the following:
- Temperature: the rate increases with rise in temperature. Example: food cooks faster in a pressure cooker, and milk turns sour faster in summer than in winter.
- Size of the particles: smaller particles have more surface area, so they react faster. Example: powdered calcium carbonate (Shahabad tile powder) reacts with dilute HCl faster than pieces of tile.
- Concentration of reactants: a more concentrated reactant reacts faster. Example: CaCO3 reacts faster with concentrated HCl than with dilute HCl.
- Catalyst: a catalyst changes the rate without itself being used up. Example: MnO2 speeds up the decomposition of potassium chlorate.
(c) Fe3O4 has 3 Fe and 4 O, so take 3Fe and 4H2O; then 8 H atoms on the left give 4H2 on the right:
\[\mathrm{3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2}\]
Check: Fe 3 = 3, H 8 = 8, O 4 = 4. Iron is oxidised because it gains oxygen and forms Fe3O4; water is reduced to hydrogen.
