Understanding Rate and Extent of Chemical Reactions

Rate of Chemical Reactions

The rate of a chemical reaction is a measure of how quickly reactants are consumed or products are formed over time. It can be calculated as:

Rate = ΔConcentration / ΔTime

Factors Affecting Reaction Rates

According to the collision theory, reaction rates depend on successful collisions between reactant particles with sufficient energy (activation energy) to break bonds. Key factors are:

• Temperature: Higher temperatures increase particle kinetic energy and collision frequency, increasing the reaction rate.
• Concentration or Pressure: Higher concentrations or pressures increase the frequency of collisions between particles, increasing the rate.
• Surface Area: Larger surface areas expose more particles to react, increasing the rate.
• Catalyst: Catalysts provide an alternative pathway with lower activation energy, increasing the rate without being consumed.

Worked Example

Problem: Calculate the rate of reaction from the data: [H₂] = 0.2 mol dm⁻³ at t = 0 s and [H₂] = 0.1 mol dm⁻³ at t = 20 s.

Solution:

• Rate = Δ[H₂] / Δt
• Δ[H₂] = 0.1 - 0.2 = -0.1 mol dm⁻³
• Δt = 20 s
• Rate = -0.1 / 20 = -0.005 mol dm⁻³ s⁻¹

Reversible Reactions and Equilibrium

Some reactions are reversible, meaning the products can re-form reactants. At equilibrium, the forward and reverse reaction rates are equal.

Le Chatelier's Principle states that if a stress is applied to a system at equilibrium, the system will respond to minimize that stress. Changing conditions like temperature, pressure, and concentrations can shift the equilibrium position.

• Increasing temperature favors the endothermic direction.
• Increasing pressure favors the side with fewer gas molecules.
• Increasing concentration favors the reverse reaction.

HIGHER TIER: Understanding reaction rates, equilibrium principles, and their applications is essential for success in GCSE Chemistry assessments.