Rates of Reactions

The rate of reaction refers to how quickly or slowly the reactants are converted into products. This rate can be affected by several factors including the concentration or pressure of the reactants, temperature, surface area, and the presence of catalysts.
Rate equations: The rate equation for a reaction provides the relationship between the rate of the reaction and the concentrations of the reactants. For a reaction aA + bB -> cC, the rate equation is usually given as Rate = k[A]^m[B]^n, where k is the rate constant, [A] and [B] are concentrations of A and B respectively, and m and n are the orders of reaction.
Order of reaction: The order of reaction with respect to a specific reactant (e.g. A) gives the effect of a change in the concentration of A on the rate of the reaction. If the rate is directly proportional to the concentration of A, the order with respect to A is one, termed first order. If the rate is independent of the concentration of A, the order is zero. The sum of the orders with respect to each reactant gives the overall order of the reaction.
The rate constant (k): The rate constant is the proportionality factor in the rate equation. Its value is temperature dependent and is explored in the Arrhenius Equation.
Factors affecting rate of a reaction: A higher concentration or pressure of reactants, increases the rate of reaction. Reactions also occur faster at higher temperatures, due to increased kinetic energy of the molecules, leading to more effective collisions. Increasing the surface area of a solid reactant increases the rate of reaction by providing more area for collisions. Catalysts increase the rate of a reaction by providing an alternative reaction pathway with a lower activation energy.
Collision theory: Reactions only occur when reactants collide with each other with sufficient energy (greater than or equal to the activation energy) and with the correct orientation.
The Maxwell-Boltzmann distribution: An understanding of this curve is important for understanding the effects of temperature on the rate of reaction. This curve represents the distribution of energies of the molecules in a sample at a particular temperature.
Activation energy and catalysts: Activation energy is the minimum energy required for a reaction to occur. Catalysts increase the rate of a reaction by providing an alternative reaction pathway with a lower activation energy.
Experimental determination of reaction rates: Monitoring concentration, mass or volume of reactants/products over time can get the rate of a reaction. Alternatively, initial rates method can be used where the concentrations of reactants are varied one at a time to determine the order of reaction.
The Arrhenius equation — k= Ae^(-Ea/RT) — links the rate constant of a reaction with the temperature, activation energy and a constant (A), known as the Arrhenius constant. It shows that as temperature increases, the rate constant increases, and hence the rate of reaction increases.