Chemical Kinetics - Factors affecting rate of reaction

The rate of a chemical reaction is influenced by several factors including the concentration of reactants, temperature, surface area, and the presence of a catalyst.

Nature of Reactants: Ionic reactions are generally faster than molecular reactions because they involve simple ion combinations rather than the breaking and forming of covalent bonds. For example, the precipitation of $AgCl$ from $AgNO_3$ and $NaCl$ is almost instantaneous.

Concentration of Reactants: According to the Collision Theory, an increase in concentration leads to an increase in the frequency of effective collisions per unit volume, thereby increasing the rate. This is expressed by the Rate Law: $Rate = k[A]^x[B]^y$.

Surface Area: In heterogeneous reactions involving solids, the rate increases with an increase in surface area. Powdered reactants react faster than large chunks (e.g., powdered $CaCO_3$ reacts faster with $HCl$ than marble chips).

Temperature: Generally, the rate of reaction increases with temperature. For most reactions, the rate constant doubles or triples for every $10^{\circ}C$ rise in temperature. This is quantified by the Temperature Coefficient: $\eta = \frac{k_{T+10}}{k_T}$.

Activation Energy ($E_a$): It is the minimum extra energy required by reactant molecules to undergo a successful collision. A catalyst increases the reaction rate by providing an alternative pathway with a lower $E_a$.

Effect of Radiation: Some reactions, known as photochemical reactions, occur only in the presence of light (photons). For example, the reaction between $H_2$ and $Cl_2$ to form $HCl$ is significantly accelerated by $h\nu$ (ultraviolet light).