National 5 Chemistry SQA

Rates of Reaction

Rates of Reaction

Definition and Importance

• The rate of reaction is a measure of how fast a chemical reaction occurs. It is typically expressed in terms of change in concentration per unit time.
• Understanding how reaction rates change under different conditions helps scientists and engineers optimise processes in industry, biology, and environmental science.

Factors Affecting Reaction Rates

• Concentration of Reactants: As concentration increases, the reaction rate increases. More particles are available to react, leading to a higher chance of collisions and reaction.
• Temperature: As temperature increases, the reaction rate increases. Particles have more energy and collide more frequently, meaning there's a greater chance of successful reactions.
• Pressure (for gases): As pressure increases, the reaction rate increases. Particles are closer together, leading to more collisions and reaction per unit time.
• Catalysts: A catalyst speeds up a reaction by providing an alternate reaction pathway with a lower activation energy.

Calculating Rates of Reaction

• A simple way to measure the rate of reaction is to measure the change in concentration of a reactant or a product per unit time. This is done by taking the gradient of the concentration-time graph.
• For reactions where gases are produced, a common method involves measuring the volume of gas produced over time.

Graphing Rates of Reaction

• Concentration-time graph: Over time, the concentration of reactants decreases, while the concentration of products increases. The gradient of the graph gives the rate of reaction.
• Rate-time graph: This graph reveals how the rate of reaction changes over time. For many reactions, the rate is highest at the start when the concentration of reactants is highest.

Kinetic Particle Theory and Reaction Rates

• An increase in temperature, concentration, or pressure increases the number of successful collisions between reactant particles, which increases reaction rate.
• A successful collision is one where particles collide with sufficient energy (the activation energy) and an orientation that allows a reaction to occur.

Understanding Catalysts

• A catalyst reduces the activation energy required for a reaction by providing an alternate reaction pathway.
• Catalysts remain chemically unchanged at the end of the reaction, meaning they can be reused.
• In biological systems, enzymes serve as catalysts.