Chapter 15 Chemical Equilibrium

Overview 15.1 The Concept of Equilibrium When a reaction takes place, both the forward process (the reaction as we have written it) and the reverse reaction occur. Equilibrium is the point at which both processes are occurring at the same rate, with no net change of reactant or product concentration. Equilibrium can be achieved starting with only reactants, only products, or some of each. You will use a simulation in this section to plot concentrations of reactants and products as equilibrium is achieved The Equilibrium Constant There is a constant relationship between equilibrium concentrations of reactants and products at a given temperature. You will learn to write equilibrium expressions for chemical reactions, and a simulation will illustrate the significance of an equilibrium constant's magnitude Heterogeneous Equilibria Some equilibria involve substances all in the same phase. Many others involve substances in different phases. The concentrations of liquids and solids do not appear in equilibrium expressions for a reaction, and you'll see some examples of heterogeneous equilibria and their equilibrium expressions Calculating Equilibrium Constants Often, only one of the species present in an equilibrium mixture can be measured. We can generally use the stoichiometry of the reaction to deduce the concentrations of the other species in the chemical equation. We can also calculate the equilibrium constant for such a reaction Applications of Equilibrium Constants Equilibrium constants can be used to predict the equilibrium concentrations of reactants and products. You will learn how to use K values to calculate the composition of an equilibrium mixture for several different situations Le Châtelier's Principle An important principle throughout chemistry, Le Châtelier's principle is introduced in this section. Two animations illustrate the effects of certain types of stress that can be applied to a system at equilibrium.

15.1 The Concept of Equilibrium The reaction of pure compound A, with initial concentration [A] 0. After a time the concentrations of A and B do not change. The reason is that the rates of the forward reaction (k f [A]) and the reverse reaction (k r [B]) become equal.

15.2 The Equilibrium Constant For the general reaction aA + bB ↔ cC + dD, the concentrations of reactants and products at equilibrium are related by the equilibrium- constant expression or simply the equilibrium expression

15.3 Heterogeneous Equilibria Equilibria that involve more than one phase are called heterogeneous equilibria. When writing the equilibrium expression for a heterogeneous equilibrium, we do not include the concentrations of solids or liquids. Regardless of how much solid or liquid is present, its concentration in terms of moles per liter remains constant. Ex: SnO 2 (s) + 2CO (g) ↔ Sn (s) + 2 CO 2 (g) Kc = [CO 2 (g)] 2 /[CO (g)] 2

15.4 Calculating Equilibrium Constants ICE box!!! Initial(Initial molarity/pressure of reactants and products) Change(Change in molarity/pressure, calculated from the stoichiometry of the reaction) Equilibrium(Molarity/Pressure at Equilibrium = Initial – change)

Example mol of N 2 O 4 gas is placed in a 2.00-L vessel and allowed to equilibrate at 110°C. At equilibrium the concentration of NO 2 is M. Calculate the value of K c for the reaction at this temperature. N 2 O 4 (g)  2 NO 2 (g) Start(0.100 mol/2.00L) = M 0 M (If there is none mentioned in the problem, assume there is none present) Change M Equilibrium0.072 M

Example continued N 2 O 4 (g)  2 NO 2 (g) Change in N 2 O 4 (g) can be calculated from the stoichiometry of the equation Then the rest of the box can be filled in Start(0.100 mol/2.00L) = M 0 M Change(-0.072M/2) = M M Equilibrium0.014 M0.072 M

Example finished Start(0.100 mol/2.00L) = M 0 M Change(-0.072M/2) = M M Equilibrium0.014 M0.072 M

15.6 Le Châtelier's Principle Le Châtelier's Principle. The principle can be stated as follows: When a system at equilibrium is stressed, the equilibrium will shift in such a way as to minimize the effect of the stress. Figure When H 2 is added to an equilibrium mixture of N 2, H 2, and NH 3, a portion of the H 2 reacts with N 2 to form NH 3, thereby establishing a new equilibrium position.

Equilibrium & Temperature The heat given off by an exothermic chemical reaction can be thought of as a product of the reaction. Likewise, the heat consumed by an endothermic process can be thought of as a reactant. The addition of a catalyst changes the rate at which equilibrium is established, but it does not affect the position of the equilibrium.