Exploring Rates and Extents of Chemical Reactions

Rates of Chemical Reactions

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

Rate = Δ[Concentration] / Δt

Where Δ[Concentration] is the change in concentration of a reactant or product, and Δt is the change in time.

Factors Affecting Reaction Rates

Several factors influence the rate at which a chemical reaction proceeds:

1. Temperature: Increasing the temperature raises the kinetic energy of particles, leading to more frequent, energetic collisions and a faster reaction rate.
2. Concentration/Pressure: Higher concentrations of reactants result in more frequent collisions per unit time, speeding up the reaction. Similarly, increasing pressure on gaseous reactants boosts their concentration.
3. Surface Area: A larger surface area of solid reactants exposes more particles to collisions, increasing the reaction rate.
4. Catalysts: Catalysts lower the activation energy required for the reaction, allowing more particles to undergo successful collisions and react faster.

Collision Theory and Activation Energy

The collision theory explains reaction rates by considering the energy and orientation of particle collisions. For a reaction to occur, colliding particles must have sufficient kinetic energy (the activation energy, Ea) and the correct orientation.

Worked Example: Effect of Temperature on Reaction Rate

Problem: A reaction has an activation energy of 50 kJ mol⁻¹. Compare the fraction of particles with enough energy to react at 25°C and 50°C.

Solution:

1. At 25°C (298 K), the fraction of particles with energy ≥ 50 kJ mol⁻¹ is very small.
2. At 50°C (323 K), the higher temperature means more particles have energy ≥ 50 kJ mol⁻¹, leading to a higher reaction rate.

Reversible Reactions and Chemical Equilibrium

Many reactions are reversible, with products reforming reactants. At equilibrium, the forward and reverse reaction rates are equal, and the concentrations remain constant.

Le Chatelier's Principle states that if a system at equilibrium is disturbed, the equilibrium shifts to counteract the change. Changes in temperature, pressure, or concentration can disturb the equilibrium.

Worked Example: Applying Le Chatelier's Principle

Problem: For the reaction: N2O4(g) ⇌ 2NO2(g) + heat, predict how adding NO2 affects the equilibrium.

Solution:

1. Adding NO2 increases its concentration, disturbing the equilibrium.
2. To counteract this, the equilibrium shifts left, consuming NO2 to form more N2O4.

Understanding reaction rates and equilibria is crucial in optimizing chemical processes and predicting their behavior under different conditions.