Question

Discuss the factors that affect the velocity (rate) of a chemical reaction.

Hint:

List and explain concentration, temperature, nature of reactants, surface area, catalyst and light as the main rate-controlling factors.

Answer 1

The rate (velocity) of a chemical reaction depends on the following main factors.

Step 1: Concentration of reactants. According to the law of mass action, the rate of a reaction is proportional to the concentration of the reactants. Increasing the concentration increases the number of reactant molecules per unit volume, so more effective collisions take place and the rate increases.
Step 2: Temperature. A rise in temperature increases the kinetic energy of the molecules, so a larger fraction of molecules have energy greater than the activation energy. As a rough rule the rate roughly doubles for every \(10\,^\circ\text{C}\) rise (temperature coefficient).
Step 3: Nature of reactants. Ionic reactions are generally fast, while reactions involving breaking of strong covalent bonds are slower. The physical state (solid, liquid, gas) also matters.
Step 4: Surface area. For heterogeneous reactions, a greater surface area (for example a powdered solid) exposes more particles for collision and speeds up the reaction.
Step 5: Presence of a catalyst. A positive catalyst provides an alternative path with lower activation energy, greatly increasing the rate; a negative catalyst (inhibitor) decreases the rate.
Step 6: Effect of light (for photochemical reactions). Some reactions, such as the combination of \(\text{H}_2\) and \(\text{Cl}_2\), are accelerated by light because light supplies the energy needed to start the reaction.
\[\boxed{\text{Rate depends on concentration, temperature, nature, surface area, catalyst and light.}}\]