Kinetics and Equilibrium Review

The stability of a compound is dependent on the amount of energy absorbed or released during the formation of the compound from its elements. If energy is absorbed – endothermic; the heat term is a reactant; sign is (+) If energy is released – exothermic; the heat term is a product; sign is (-)

Collision theory states that a reaction is most likely to occur if reactant particles collide with the proper energy and orientation. We call these “effective collisions.” More effective collisions means higher reaction rates.

The rate of a chemical reaction is the change in concentration of a reactant or product per unit time. They only test this in a qualitative way; the rate increases or decreases depending on reaction conditions.

The rate of a chemical reaction depends on several factors: temperature, concentration, nature of reactants, surface area, and the presence of a catalyst. There are always 1-2 questions on every exam on these factors: Temperature: increased kinetic energy means more collisions. Concentration: more molecules per unit volume means higher reaction rates. Surface area: smaller particles, more surface area, higher reaction rates Catalysts: lower activation energy, making effective collisions more likely.

Some chemical and physical changes can reach equilibrium. Physical changes: phase changes, solubility, vapor pressure of liquids Chemical changes: most reactions are reversible; we can speak about the forward and reverse directions.

At equilibrium the rate of the forward reaction equals the rate of the reverse reaction while the measurable quantities of reactants and products remain constant. This definition appears somewhere nearly every year.

LeChatelier’s principle can be used to predict the effects of stress (change in pressure, volume, concentration, and temperature) on a system at equilibrium. Increased pressure favors direction with the least number of moles of gas. If equal moles of gas are found on the product and reactant side, there is no effect. Pressure only affects gases; solutions are unaffected.

Le Chatelier’s Principle Concentration Increasing a reactant or product concentration creates conditions that lead to higher concentrations in the opposite direction; add reactant, more products, add product, more reactants. Temperature Heat can be a reactant (endothermic) or a product (exothermic). The endothermic direction is favored when the temperature is increased. For an exothermic reaction, the reverse direction would be favored.

Energy released or absorbed during a chemical reaction is equal to the difference between the potential energy of the products and the potential energy of the reactants. This is ΔH, the heat (enthalpy) of reaction. You need to know how ΔH is represented on an energy diagram. Heat of reaction is tested nearly every year, and many students don’t get it.

A catalyst provides an alternate reaction mechanism, which has lower activation energy than an uncatalyzed reaction. Catalysts have no effect on equilibrium; only the initial rates of reaction are affected. Be able to discern the activation energy on a potential energy diagram. Often they like you to draw a line for a catalyzed reaction. Be sure that your line begins with the potential energy of the reactants and ends with the potential energy of the production

Catalysts and Energy Diagrams

Entropy is a measure of the randomness or disorder of a system. A system with greater disorder has greater entropy. The following processes have increased entropy: Solutes dissolving in solvents Phase changes that occur with increasing temperature (sublimation, melting, vaporization). They often use equations to express this: H 2 O(s)  H 2 O(g) Reactions that produce more moles of gas in the product side

Systems in nature tend to undergo changes in energy and entropy. Reactions that release energy (exothermic) and have increased disorder (positive entropy) are always favored. Reactions that absorb energy (endothermic) and have decreased disorder (negative entropy) are never favored.