1. Chemical Equilibrium

2. Lesson Objectives Describe the nature of a reversible reaction. Define chemical equilibrium. Write chemical equilibrium expressions from balanced chemical equations.

3. Lesson Vocabulary irreversible and reversible reaction  reactants and products  forward and reverse reaction  rate of reaction chemical equilibrium  molar concentration (molarity) equilibrium constant  coefficient

4. Irreversible and reversible reactions CH 4 + 2O 2 → CO 2 + 2 H 2 O reactants products The combustion of methane is an irreversible reaction

6. In a reversible reaction the conversion of reactants to products and the conversion of products to reactants occur simultaneously. H 2 + I 2 ↔ HI The formation of hydrogen iodide is a reversible reaction Forward reaction: H 2 + I 2 → HI Reverse reaction: HI → H 2 + I 2 reversible reaction animation

7. When the rates of the forward and reverse reactions have become equal to one another, the reaction has achieved a state of balance, called Chemical equilibrium. Chemical equilibrium

8. The conditions and properties of a system at equilibrium are summarized below: 1. The system must be closed, meaning no substances can enter or leave the system. 2. Equilibrium is a dynamic process. Even though we don’t observe any changes, both the forward and reverse reactions are still taking place. 3. The rates of the forward and reverse reactions must be equal. 4. The amounts of reactants and products do not have to be equal. However, after equilibrium is attained, the amounts of reactants and products (their concentration) will remain constant.

9. The Equilibrium Constant (K eq ) The rates of the forward and reverse reactions are the same at equilibrium, and so the concentrations of all of the substances are constant. The equilibrium constant (Keq) is the ratio of the concentrations of the products to the concentrations of the reactants, at equilibrium. Each concentration is raised to the power of its coefficient. aA + bB  cC + dD Consider the hypothetical reversible reaction in which reactants A and B react to form products C and D: (lower case letters, a,b,c,d represent the coefficients of each substance). K eq = [C] c [D] d _______ [A] a [B] b The concentrations of each substance, indicated by the square brackets around the formula, are measured in molarity units (mol/L).

10. The value of the equilibrium constant for any reaction does not depend on the starting concentrations, so the equilibrium constant has the same value regardless of the initial amounts of each reaction component. It does, however, depend on the temperature of the reaction.

11. The equilibrium expression only shows those substances whose concentrations are variable during the reaction. Therefore, an equilibrium expression omits pure solids (s) and liquids (l) and only shows the concentrations of gases (g) and aqueous solutions (aq). Determining the Equilibrium Constant 1) Write the equilibrium expressions for the following reactions: H 2 (g) + I 2 (g)  2HI(g) 2NH 3 (g) + 3 CuO(s)  3H 2 O(g) + N 2 (g) + 3Cu(s) HCN(s) + H 2 O(l)  H + (aq) + CN - (aq)

12. 2NO(g) + O 2 (g)  2NO 2 2) Calculate the equilibrium expressions for the following reaction: knowing that at 230°C, the equilibrium concentrations are measured to be [NO] = 0.0542 M, [O2] = 0.127 M, and [NO2] = 15.5 M.

13. Interpretation of the Equilibrium Constant For K>1, we can state that the amount of product is greater than the amount of reactant. The equilibrium is favored to the right. Likewise, for K<1, the amount of reactants is much greater than the products. The equilibrium favored to the left.

14. Lesson Summary A reversible reaction is one in which products are converted to reactants as well as reactants being converted to products. Equilibrium is achieved when the rate of the forward reaction is equal to the rate of the reverse reaction. Once at equilibrium, the concentrations of all substances remain constant, and no net change occurs in the system. An equilibrium constant can be calculated for any reaction. Once a reaction reaches equilibrium, the ratio of the mathematical product of all product concentrations to the mathematical product of all reactant concentrations, each raised to the power of its coefficient, will always be equal to its equilibrium constant at that temperature. Note that solid and liquid reaction components are not included in the calculation of the equilibrium constant, since their concentrations are not dependent on the amount present and thus do not change over the course of the reaction. An equilibrium constant greater than 1 indicates that the products of the reaction as written are favored. An equilibrium constant less than 1 indicates that the reactants are favored.