Chemical Equilibrium Lesson 2 Equilibrium Law Constants Developing

Chemical Equilibrium Lesson # 2 Equilibrium Law & Constants

Developing the Equilibrium Constant • In equilibrium, we know that the rate of forward reaction is equal to the rate of the reverse reaction. • The equilibrium law is the mathematical description of a chemical system at equilibrium. • Consider reaction A + B = 2 C: *Note that we can only convert molar coefficients to powers if the reaction is a simple one-step reaction (elementary process). *

Dynamic Equilibrium • In general for: a. A + b. B = c. C + d. D. K = [C]c [D]d [A]a [B]b • K is unitless (unlike rate values of k), but varies with temperature.

Example 1 • Write the equilibrium equation for the reaction described by the following balanced equation: • 4 NH 3 (g) + 7 O 2 (g) = 4 NO 2 (g) + 6 H 2 O (g) Assume this is a one-step reaction.

Example 2 In a closed vessel at 500°C, nitrogen and hydrogen gases combine in an equilibrium reaction to form ammonia gas. N 2 (g) + 3 H 2 (g) = 2 NH 3 (g). The equilibrium concentration of nitrogen is 1. 50 x 10 -5 mol/L, hydrogen is 3. 45 x 10 -1 mol/L and ammonia is 2. 00 x 10 -4 mol/L. Calculate the equilibrium constant, K, for this chemical reaction under these conditions. Assume this is a one-step reaction.

Heterogeneous Equilibria • So far all examples discussed have been ones of homogeneous equilibrium, in which all reactants and products in the system were in the same state of matter. • When the reactants and products exist in more than one state, the system is a heterogeneous equilibrium. • It is important to note that equilibrium position of a heterogeneous equilibrium does not depend on the quantities of pure solids or liquids – only aqueous or gaseous entities. • This is because the concentration of pure solids and liquids cannot change.

Example 3 a) Consider the reaction: Ca. CO 3 (s) = Ca. O (s) + CO 2 (g). Determine the equilibrium law (equation) for the reaction. b) Solid phosphorus pentachloride can undergo a reversible reaction to form liquid phosphorus trichloride and chlorine gas. Write the equilibrium law for this reaction:

The Magnitude of K • The value of K (its magnitude) can help scientists predict the equilibrium position between reactants and products. • When K >>> 1, the equilibrium favours the products – the equilibrium lies to the right • When K = 1, the equilibrium concentration of the products is similar to that of the reactants – the equilibrium lies in the middle • When K <<< 1, the equilibrium favours the reactants – the equilibrium lies to the left.