C.P. Chemistry: Reaction Rates and Equilibrium

Keq

Keq= products over reactants. Brackets= concentration (molarity or M). Keq is dependent on temperature. If temperature changes, Keq will change. Keq indicates which part of the reaction is favored. If Keq is greater than 1, the products are favored at equilibrium (product concentration higher than reactant). If Keq is less than 1, the reactants are favored at equilibrium. If Keq equals 1 the reactants= the products at equilibrium. Solids and liquids are not included in the Keq expressions, since they are pure and they don't change the concentration.

Rate

Measures changes that occur within intervals of time (speed of the chemical reaction). Examples: mi/hr, m/s, mol/s, M/s or M*s-1.

Agitation

Stirring.

Inhibitors

Substances that interfere with the action of a catalyst (slows down a reaction).

Factors Affecting Reaction Rates

1.) Temperature. 2.) Concentration of Reactants. 3.) Particle Size. 4.) Agitation. 5.) Catalysts. 6.) Nature of the Reactants.

Entropy

A measure of the disorder of a system (S). The dissolving of a gas in a solvent always results in a decrease of entropy. Forming a solution always increases entropy. Assuming no charge in a state of matter, the entropy of a system usually increases when the # of gaseous particles is greater than the # of reactant particles.

Non-spontaneous Reaction

A reaction that does not favor the formation of products at specific conditions.

Spontaneous Reaction

A reaction that occurs naturally and favors the formation of products at specific conditions.

Catalysts

A substance that increases the rate of reaction without being used up itself in the reaction. The catalyst lowers the activation energy needed by the reactants for an effective collision. More of the collisions taking place between reactant particles will be effective. Therefore, the addition of a catalyst will increase the rate of a chemical reaction.

Activated Complex

A temporary, intermediate substance. The arrangement of atoms that may form products if the collision has sufficient energy and proper orientation. Sometimes called the transition state.

Concentration

Add reactant= reactants go down, products go up. Add product= r goes up, p goes down. Remove reactant= r goes up, p goes down. Remove product= r goes down, p goes up.

Temperature

As temperature increases, average kinetic energy of the molecules increases (particles have increased motion). More motion results in more collisions between particles. Collisions are also more energetic. More collisions will be effective. Therefore, increasing temperature will increase the rate of a chemical reaction. Example- Glow Sticks.

Collision Theory

Atoms, ions, and molecules must collide in order to react. Reacting substances must collide with the correct orientation. Reacting substances must collide with sufficient energy to form the activated complex.

Delta H

Change of Enthalpy.

Particle Size

If reactant crystals are broken up, the surface area increases. Smaller particles= larger surface area. Since collisions happen on a particles surface, increasing surface area will increase the frequency of collisions between particles. Therefore, as particle size of reactants decrease, reaction rate increases.

Temperature (in equilibrium expressions)

If the temperature increases, equilibrium shifts in favor of endothermic reaction (away from energy). If decreases, the eq. shifts in favor of exothermic reaction (towards energy). With a left shift, the Keq gets smaller. With a right shift, the Keq gets bigger.

Nature of the Reactants

In order for a collision to be effective, activation energy is needed to break the bonds of the reactants. Some bonds are easier to break (lower bond energy), some bonds are more difficult. Compounds with weaker bonds will very often faster than reactants with stronger bonds (more effective collisions). Therefore, reaction rates may differ due to the chemical nature of the reactants.

Pressure

Increase= eq. shifts to the side with less molecules. Decrease= eq. shifts to the side with more molecules. *If same # of molecules on both sides= no effect.

Volume

Increase= eq. shifts to the side with more molecules. Decrease= eq. shifts to the side with less molecules.

Activation Energy (Ea)

The minimum amount of energy that reacting particles must have to form the activated complex (energy needed to break bonds and result in the formation of the product). High Ea= low reaction rate (less effective collisions). Low Ea= high reaction rate (more effective collisions).

Equilibrium Constant

The ratio of product concentrations to reactant concentrations. Each concentration is raised to the power given by the number of moles (coefficient) of the substance in a balanced chemical equation.

Concentration of Reactants

When the concentration (molarity) of reactant particles is increased there will be more particles in a fixed volume (given space). Frequency of collisions between particles will increase. Therefore, as concentration of reactants increases, reaction rate increases.

Equilibrium

When the system is in equilibrium, both reactants and products are present in the container at the same time. There is no net change in amounts of reactants or products at any given point at equilibrium.

Effective Collision

Will result in the breaking of bonds in the reactants and the forming of bonds in the product (a chemical reaction).