Understanding Reaction Rates and Equilibrium in GCSE Chemistry

Reaction Rates and Factors Affecting Them

The rate of a chemical reaction is a measure of how quickly reactants are converted into products. It can be calculated by monitoring the concentration change of reactants or products over time.

Several factors affect reaction rates, according to the collision theory:

• Temperature: Higher temperatures increase particle kinetic energy and frequency of effective collisions, accelerating the reaction.
• Concentration/Pressure: Higher concentrations or pressures for gases increase the number of particle collisions per unit time, speeding up the reaction.
• Surface area: Increasing the surface area of solid reactants provides more sites for collisions to occur, increasing the reaction rate.
• Catalysts: Catalysts lower the activation energy barrier, allowing more particles to undergo successful collisions and react faster.

Calculating Reaction Rates

Problem: In a reaction, the concentration of a reactant decreases from 1.5 mol/dm³ to 0.8 mol/dm³ in 120 seconds. Calculate the average rate of reaction.

Solution:

1. Concentration change = 1.5 - 0.8 = 0.7 mol/dm³
2. Time taken = 120 seconds
3. Rate = Concentration change / Time taken = 0.7 / 120 = 0.00583 mol/dm³/s

Reversible Reactions and Dynamic Equilibrium

Many reactions are reversible, with products reacting to re-form reactants. At equilibrium, the forward and reverse reaction rates are equal. This dynamic equilibrium can be disturbed by changing conditions like temperature, pressure, or concentrations.

Le Chatelier's Principle states that a system at equilibrium will shift to counteract any applied stress. For example, increasing the pressure on a gaseous equilibrium drives the reaction towards the side with fewer gas molecules.

Predicting Equilibrium Shifts (HT)

For the reversible reaction: N2(g) + 3H2(g) ⇌ 2NH3(g)

If the temperature is increased, the equilibrium will shift to:

1. The endothermic (heat-absorbing) direction, forming more products to absorb the extra heat energy.
2. Therefore, the equilibrium shifts right, producing more ammonia (NH3).

Understanding these core concepts is crucial for success in GCSE Chemistry exams related to rates of reaction and chemical equilibria.