Exploring Reaction Rates and Equilibrium in GCSE Chemistry

Reaction Rates

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

Rate = Δ[concentration] / Δt

Factors Affecting Reaction Rates

Several factors influence the rate of a chemical reaction:

• Temperature: Increasing temperature raises the average kinetic energy, leading to more frequent and energetic collisions between reactant particles. This increases the reaction rate.
• Concentration/Pressure: Higher concentrations of reactants increase the frequency of collisions, speeding up the reaction. For gaseous reactions, higher pressures also increase collision rates.
• Surface Area: Increasing the surface area of solid reactants exposes more particles for collisions, accelerating the reaction.
• Catalysts: Catalysts provide an alternate reaction pathway with lower activation energy, increasing the fraction of successful collisions and boosting the reaction rate.

Collision Theory and Activation Energy

The collision theory explains how reactions occur through collisions between reactant particles with sufficient kinetic energy (activation energy) to initiate bond breaking/forming. Increasing factors that promote more energetic collisions raises the reaction rate.

Worked Example

Problem: For the reaction 2NO(g) + O2(g) → 2NO2(g), the rate was measured at different temperatures. Calculate the rate at 35°C given the data:

Temperature (°C) | Rate (mol L-1 s-1)

• 25 °C | 1.2 x 10-3
• 35 °C | ?
• 45 °C | 5.4 x 10-3

Solution:

• The rate increases as temperature increases, following the Arrhenius equation.
• Using the given rates at 25°C and 45°C, we can estimate the rate at 35°C by interpolation.
• Rate at 35°C ≈ (5.4 - 1.2) x (35 - 25)/(45 - 25) + 1.2 = 2.7 x 10-3 mol L-1 s-1

Reversible Reactions and Equilibrium

Some reactions are reversible, with products re-forming reactants. At equilibrium, forward and reverse rates are equal, and concentrations remain constant. Le Chatelier's Principle states that if conditions change, the system shifts to counteract the change. Changes in temperature, pressure, or concentrations can disturb the equilibrium position.