Reaction Rates and Factors Affecting Chemical Change

Reaction Rates

The rate of a chemical reaction is the speed at which reactants are consumed or products are formed over time. It can be calculated using the equation:

Rate = ΔConcentration / ΔTime

Factors Affecting Reaction Rates

Several factors influence the rate of a chemical reaction:

1. Temperature: Increasing temperature increases the kinetic energy and frequency of collisions between reactant particles, speeding up the reaction.
2. Concentration/Pressure: Higher concentrations or pressures increase the number of reactant particles per unit volume, leading to more frequent collisions and faster reactions.
3. Surface Area: Larger surface areas expose more reactant particles, increasing the chances of successful collisions.
4. Catalysts: Catalysts provide an alternative pathway with lower activation energy, allowing reactions to proceed faster at the same temperature.

Collision Theory and Activation Energy

The collision theory explains how reactions occur through successful collisions between reactant particles with sufficient kinetic energy to overcome the activation energy barrier. Catalysts work by lowering this activation energy, allowing more reactant particles to undergo successful collisions.

Worked Example

Problem: A reaction between gases A and B was conducted at two different temperatures, and the rates were measured. At 25°C, the rate was 0.02 mol L⁻¹ s⁻¹, while at 50°C, the rate was 0.08 mol L⁻¹ s⁻¹. Explain the difference in rates using collision theory.

Solution:

• At higher temperatures, reactant particles have greater kinetic energy.
• This leads to more frequent collisions between A and B particles with sufficient energy to overcome the activation energy barrier.
• Therefore, more successful collisions occur per unit time, resulting in a faster reaction rate at 50°C compared to 25°C.

Reversible Reactions and Dynamic Equilibrium

Many reactions are reversible, with products being able to reform reactants. At dynamic equilibrium, the forward and reverse reaction rates are equal, resulting in constant concentrations of reactants and products.

Le Chatelier's Principle (HT Only)

Le Chatelier's Principle states that when a system at equilibrium is disturbed by a change in temperature, pressure, or concentration, the equilibrium will shift to counteract the change and re-establish equilibrium. This principle can predict the effects of such changes on the position of equilibrium.