Chapter 7: Reaction Rates and Chemical Equilibrium

Chemical Reactions Requirements

• Molecules must collide with correct orientation.

• Collisions need enough kinetic energy to break bonds.

Factors Affecting Reaction Rate

• Concentration: Higher concentration increases collision rate and reaction speed.

• Temperature: Increasing temperature boosts kinetic energy and collision effectiveness. A 10˚C increase can double reaction rate.

Reaction Rate Definitions

• Rate defined as change in concentration of reactants/products over time: $\text{Rate} = \frac{\Delta [A]}{\Delta \text{time}}$

• Rate decreases over time as reactants are consumed.

Order of Reaction

• Order determined by experiments, typically ranging from 0 to 1 for unimolecular reactions.

• For reactions: $\text{Rate} = k[\text{Reactants}]^n$ where k is rate constant and n is order.

• Units of k differ by reaction order:

  • Zero-order: $M/s$

  • First-order: $1/s$

  • Second-order: $M^{-1}s^{-1}$

Energy Changes in Reactions

• Bond Breaking: Endothermic (energy absorbed, \Delta H > 0).

• Bond Formation: Exothermic (energy released, \Delta H < 0).

• Reaction Coordinate Diagrams depict energy changes involved.

Catalysts

• Catalysts lower activation energy (Ea) of a reaction, increasing reaction rate without changing $\Delta H$.

Chemical Equilibrium

• Equilibrium reached when forward and reverse reaction rates are equal.

• Dynamic state where concentrations of reactants/products remain constant.

• Equilibrium constant $K = \frac{[C]^c [D]^d}{[A]^a [B]^b}$; dimensionless.

Le Châtelier’s Principle

• Disturbances cause system to shift equilibrium to counteract the change:

  • Concentration Change: Adding reactants/products affects the opposite.

  • Temperature Change: Endothermic reactions favor forward shift with heat addition; exothermic reactions favor reverse.

  • Pressure Change: Increasing pressure favors side with fewer moles; decreasing favors side with more moles.