Reaction rates & equilibria A survey

Collision Theory Products form when reactants collide with each other, providing they have enough energy to overcome any activation requirements (energy of activation) This may be expressed as amount of reactants changing per unit time Activated complex: an unstable arrangement of atoms that forms momentarily at the peak of the activation energy barrier; may also be called the transition state

Factors that affect reaction rate Temperature: usually raising temperature increases reaction rates Concentration: in a fixed volume, increased concentration usually increases reaction rates Particle size: an increase in surface area increase reaction rate Catalysts: create a lower energy pathway Inhibitor: a substance that interferes with a catalyst

Equilibria Most reactions are reversible, but degree is variable Chemical equilibrium: when the forward and backward reaction rates are equal, so that no further change is occuring Equilibrium position: this indicates whether the reaction tends to go forward or backward…shown by the lengths of the direction arrows between reactants and products

Factors affecting equilibrium LeChatelier’s Principle If a stress is applied to a system, the system will change in some way to relieve the stress Concentration: adding or removing either a product or reactant Temperature: depends upon whether heat is a product or reactant Pressure: only affects gaseous equilibria; depends upon volumes on each side of equation

Equilibrium Constants aA + bB  cC + dD Equilibrium constant is the ration of product concentrations to reactant conc. Keq = [C]c · [D]d/ [A]a · [B]b If Keq >1, products favored at equilibrium If Keq <1, reactants favored at equilibrium

Solubility Equilibria Solubility Product Constant: Ksp Salt: AB(s)  A+ (aq) + B- (aq) Keq = [A][B]/[AB], but since AB is a solid, we consider it to be a constant, so: Ksp = [A][B] The smaller the numerical value of Ksp, the lower the solubility of the compound.

Common Ion Effect The addition of a common ion, found in both salts in a solution, will cause the solubility of one of the salts to decrease