Chapter 17 Equilibrium Reversible Reactions

Collision Model Chemical reactions occur because of collisions between reactants. In that collision old bonds are broken and new bonds are formed. Not all collisions are effective in causing a reaction.

Factors That Effect Collisions Collision Frequency Increase in frequency increases the rate of the reaction. Increased pressure or increased temperature. Collision Energy A minimum kinetic energy is required but too much energy is also detrimental. Collision Geometry Molecules must be oriented in the proper geometry for a reaction to occur.

Factors Effecting Reaction Rate Reaction Concentration Increasing concentration increases the chance of collisions and thus speeds up the reaction Reaction Temperature Increasing temperature increases particle kinetic energy which speeds up most reactions Addition of a Catalyst A catalyst lowers activation energy and thus speeds up the reaction.

Changes in Energy Endothermic reactions absorb energy so that the products have more energy than the reactants. Exothermic reactions release energy so that the products have less energy than the reactants.

Role of a Catalyst

Chemical Equilibrium Most chemical reactions do not continue until all of the reactant is used up. At first the forward reaction proceeds quickly. As the amount of product builds, a reverse reaction starts to occur as products collide and form reactants. When the forward and reverse reactions occur at the same rate, equilibrium is established. The concentration of products and reactants remain the same.

Concentration Graphs

Equilibrium Constant There is a mathematical value which indicates when equilibrium is established. The Law of Chemical Equilibrium states that the molar concentrations of the products divided by the molar concentration of the reactants equals a constant value, Kc. If Kc >> 1 the reaction goes to near completion. If Kc << 1 equilibrium is quickly established in the system.

Equilibrium Constant

Equilibrium Constant For the values plugged into the equation for Kc: Gases are entered as their pressure Aqueous solutions are entered as their molar concentrations Liquids and solids are not entered in the equation as they do not affect equilibria in heterogeneous systems.