Chemical Equilibrium What does equilibrium mean Equilibrium Dynamic

Chemical Equilibrium What does equilibrium mean? .

Equilibrium § § Dynamic state of equilibrium. State of balance between 2 opposing reactions occurring at the same time and same rate. In a closed system as products are produced they will react in the reverse reaction until the rates of the forward and reverse reactions are equal. (Rate forward = Rate reverse) Concentration of reactants and products remain constant.

Types of Equilibrium Phase Equilibrium � Rate of one phase change is equal to the rate of the opposing phase change, � Occurs when two phases exist at the same temperature. � Examples:

Heating and Cooling Curves

Solution Equilibrium � Rate of dissolving = rate of crystallization � Saturated Solution

� Chemical § § Equilibrium Occurs when two or more substances are brought together to yield a chemical reaction and the Rate of forward reaction is equal to the rate of the reverse reaction. Concentration of reactants and products are constant.

Le Chatelier’s Principle § § § If a stress is applied to a system at equilibrium the equilibrium will shift in the direction that minimizes the stress. A new point of equilibrium is established with new concentrations of reactants and products. Stresses include: change in concentration of reactants or products, temperature and pressure.

Haber Reaction � Equation � N 2 for the formation of ammonia + 3 H 2 ⇄ 2 NH 3 + heat

Change in concentration � If the concentration of a reactant or product increases you should shift away from the increase. � If the concentration of a reactant or product decreases shift toward the decrease.

Practice: N 2 Stress Add N 2 Add H 2 Add NH 3 Remove N 2 Remove H 2 Remove. NH 3 + 3 H 2 ⇄ 2 NH 3 + heat Shift [N 2] [H 2] [NH 3] Keq

Change in Pressure § § Affects gases only. An increase in pressure shifts in the direction of the smaller volume of gases diminishing the stress. A decrease in pressure shifts in the direction of the larger volume of gases diminishing the stress. If there is no change in the volume of gases involved in the chemical reaction a change in pressure has little affect on the equilibrium.

Practice N 2(g) Stress Increase pressure Decrease pressure Shift + 3 H 2(g) ⇄ 2 NH 3(g) + heat [N 2] [H 2] [NH 3] keq

Change in Temperature � An increase in temperature speeds up the rate of both an endothermic and exothermic reaction but unequally. � However an increase in temperature favors the endothermic reaction. (shift away from the heat) � A decrease in temperature favors the exothermic reaction. (shift towards the heat) � Temperature is the only thing that effects the keq. The keq changes in the same direction as the products of the forward reaction.

Practice N 2(g) Stress Increase temperature Decrease temperature + 3 H 2(g) ⇄ 2 NH 3(g) + heat Shift [N 2] [H 2] [NH 3] keq

Effect of a Catalyst � An addition of a catalyst speeds up a reaction by lowering the activation energy and providing an alternate pathway for the reaction. � A catalyst will increase the rate of the forward and reverse reaction equally. � There is no net change in the equilibrium concentrations. � Equilibrium is reached quicker.

Equilibrium Constant (Mass action expression) � a. A + b. B ⇌ c. C + d. D � Keq = [C]C [D]d � � � /[A] a [B]b [ ] are the concentrations are equilibrium. Expressed in molarity. (moles/liter) Only include gases and aqueous ions. Do not include pure solids and liquids they do not affect the keq. The only stress that changes the keq is a change in temperature.

Meaning of keq � Keq has no units � Keq > 1 favors the formation of products (forward reaction; ∆G = negative value— spontaneous) � Keq < 1 favors the formation of reactants (reverse reaction; ∆G = positive value—not spontaneous)

Types of keq � Ksp: solubility product constant (the larger the ksp the more soluble the salt) � Ka: ionizaion constant for an acid(the larger the ka he stronger the acid) � Kb: ionization constant for a base (the larger the kb the stronger the base) � Kw: ionization constant for water (used in ph problems—next unit)