Reaction rates and equilibrium Chapter 18

Rate Measure of the speed of any change that occurs within a interval of time. In chemistry, rate is usually expressed as the amount of reactant changing per unit time. Figure 18.3 page 542

Collision theory Atoms, ions, and molecules can react to form products when they collide with one another, provided that the colliding particles have enough kinetic energy. Particles lacking necessary kinetic energy to react bounce apart unchanged when they collide. Two particle collision because probability of 3 particles colliding simultaneously is low. If enough energy is applied to a molecule, the bonds holding molecule together can break apart. Figure 18.4 page 543

Activation energy Minimum amount of energy that colliding particles must have in order to react. Barrier for reactants to cross  products Figure 18.5 page 543

Factors affecting reaction rates Temperature: Concentration: Particle size: Catalysts: Figure 18.8 page 547

Reversible reactions Reaction in which conversion of reactants to products and conversion of products to reactants occurs simultaneously. Forward: Reverse: Overall:

Chemical equilibrium When the rates of the forward and reverse reactions are equal and the reaction has reached a state of balance. At chemical equilibrium, no net change occurs in the actual amounts of the components of the system. Dynamic state: both forward and reverse reactions continue, but because rates are equal  no net change occurs in concentrations Rates are equal, but concentrations of products/reactants are not.

Chemical equilibrium Equilibrium position indicates whether reactants or products are favored in a reverse reaction. In principle, almost all reactions are reversible to some extent, under the right conditions. Figure 18.9 page 549 and figure 18.10 page 550

Le chÂtelier’s Principle If a stress is applied to a system in dynamic equilibrium, the system changes in a way to relieve the stress Types of stress:

Equilibrium constants Ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation. Keq >1 products favored Keq < 1 reactants favored