18.0 REACTION RATES AND REVERSIBLE REACTIONS

Why does iron rust slowly, but gasoline burns in an instant? Why do we heat food to cook it faster? Why can't we get 100% yield in some chemical reactions? The answers lie in the study of reaction kinetics (rates) and chemical equilibrium. This chapter explores the factors that control how fast reactions occur, the collision theory that explains them, and the concept of equilibrium in reversible reactions, including Le Chatelier's Principle and its crucial industrial applications.

18.1 FACTORS AFFECTING REACTION RATES

The rate of reaction measures how quickly reactants are converted into products. Several factors can influence this rate.

18.1.1 Concentration (of reactants in solution)

• Effect: Increasing the concentration of reactants increases the reaction rate.
• Explanation (Collision Theory): Higher concentration means more reactant particles in a given volume. This leads to a higher frequency of successful collisions per unit time.
• Example: Magnesium ribbon reacts more vigorously with 2 mol/dm³ HCl than with 0.5 mol/dm³ HCl.

18.1.2 Pressure (for gases)

• Effect: Increasing the pressure of gaseous reactants increases the reaction rate.
• Explanation: Increasing pressure effectively increases the concentration of gas particles (they are forced closer together). This increases the frequency of collisions.
…