Chemical Equilibria: General Concepts

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AP* Chemistry CHEMICAL EQUILIBRIA: GENERAL CONCEPTS
THE NATURE OF THE EQUILIBRIUM STATE: Equilibrium is the state where the rate of the forward reaction is equal to the rate of the reverse reaction. At these conditions, concentrations of all reactants and products remain constant with time once equilibrium has been established at constant temperature. (In stoichiometry, we dealt with equations that went to completion; often equilibrium equations are going to fall short of this goal.) Reactions are reversible. This is indicated by double arrows. • dynamic-- indicates that the reaction is proceeding in the forward and in the reverse direction and once equilibrium is established, the rate of each direction is equal. This keeps the concentration of reactants and products equal. the nature and properties of the equilibrium state are the same, no matter what the direction of approach. Examples: Look at the following plot of the reaction between steam and carbon monoxide in a closed vessel at a high temperature where the reaction takes place rapidly.

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H2O(g) + CO(g)

H2(g) + CO2(g)

THE EQUILIBRIUM POSITION: Whether the reaction lies far to the right or to the left depends on three main factors. • Initial concentrations (more collisions--faster reaction) • Relative energies of reactants and products (nature goes to minimum energy) • Degree of organization of reactants and products (nature goes to maximum disorder) • The significance of K: K > 1 means that the reaction favors the products at equilibrium K < 1 means that the reaction favors the reactants at equilibrium THE EQUILIBRIUM EXPRESSION: A general description of the equilibrium condition proposed by Gudberg and Waage in 1864 is known as the Law of Mass Action. Equilibrium is temperature dependent, however, it does not change with concentration or pressure. • equilibrium constant expression--for the general reaction aA + bB cC + dD Equilibrium constant: K = [C]c[D]d [A]a[B]b * Note* K, Kc, Keq may all be used here!

*AP is a registered trademark of the College Board, which was not involved in the production of, and does not endorse, this product.© 2008 by René McCormick. All rights reserved.

The product concentrations appear in the numerator and the reactant concentrations in the denominator. Each concentration is raised to the power of its stoichiometric coefficient in the balanced equation. [ ] indicates concentration in Molarity (mol/L) Kc--is for concentration (aqueous) Kp--is for partial pressure (gases) “K” values are often written without units • • • • USING EQUILIBRIUM CONSTANT EXPRESSIONS Pure solids--do not appear in expression—you’ll see this in Ksp problems soon! Pure liquids--do not appear in expression—H2O(l) is pure, so leave it out of the calculation Water--as a pure liquid or reactant, does not appear in the expression. (55.5 M will not change significantly) o Weak acid and weak base equations are heterogeneous [multi-states of matter; pure liquid and aqueous components] equilibria. o Solubility of salts also fits into this category. The initial solid component has a constant concentration and is therefore left out of the equilibrium expression.

Exercise 1

Writing Equilibrium Expressions

Write the equilibrium expression for the following reaction: 4 NH3(g) + 7 O2(g) 4 NO2(g) + 6 H2O(g)

K = [NO2]4[H2O]6 [NH3]4[O2]7 Exercise 2 Equilibrium Expressions for Heterogeneous Equilibria

Write the expressions for K and Kp for the following processes: a. The decomposition of solid phosphorus pentachloride to liquid phosphorus trichloride and chlorine gas. b. Deep blue solid copper(II) sulfate pentahydrate is heated to drive off water vapor to form white solid copper(II) sulfate. A: K = [Cl2] Kp = PCl2 B: K = [H2O]5 Kp = PH2O5

. Chemical Equilibria: General Concepts

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CHANGING STOICHIOMETRIC COEFFICIENTS when the stoichiometric coefficients of a balanced equation are multiplied by some factor, the K is raised to...
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