Equilibrium Expressions

Equilibrium Reviewed Some chemical systems have little tendency to react. Others go to completion. The majority of reactions reach a state of equilibrium with some of the reactants unconsumed. If the reactants are not all consumed, then the amount of products produced is less than the amount predicted b y the balanced chemical According to the equation for the ammonia-producing reaction, 2 mol of ammonia should be produced when 1 mol of nitrogen and 3 mol of hydrogen react. However, because the reaction reaches a state of equilibrium, less than 2 mol of ammonia are obtained.

The Law of Chemical Equilibrium the law of chemical equilibrium states: at a given temperature, a chemical system might reach a state in which a particular ratio of reactant and product concentrations has a constant value.

The Equilibrium Constant Expression The general equation for a reaction at equilibrium is as follows: aA + bB  cC + dD If the law of chemical equilibrium is applied to this reaction, the following ratio is obtained: Keq = [A] and [B] are the concentrations of the reactants and [C] and [D] are the concentrations of the products; a, b, c, and d are the coefficients from the balanced chemical equation.

WARNING: Be Careful! the NUMERATOR = PRODUCTS the DENOMINATOR = REACTANTS

The Equilibrium Constant The equilibrium constant, Keq, is the numerical value of the ratio of product concentrations to reactant concentrations, with each concentration raised to its coefficient in the balanced equation. The value of Keq is constant only at specified temperatures.

Interpreting the size of Keq Recall that a fraction with a numerator greater than its denominator has a value greater than 1. A fraction with a numerator less than its denominator has a value less than 1. For example, compare the ratios 5/1 and 1/5. Five is a larger number than one-fifth. Because the PRODUCT concentrations are in the numerator of the equilibrium expression, a numerically large Keq means that the equilibrium mixture contains more products than reactants. Similarly, a Keq less than 1 means that the equilibrium mixture contains more reactants than products. Keq >1: Products are favored at equilibrium Keq <1: Reactants are favored at equilibrium

Expressions for Homogenous Equilibria How would you write the equilibrium constant expression for this reaction in which hydrogen and iodine react to form hydrogen iodide? H2 (g) + I2 (g)  2 HI (g) This reaction is a homogeneous equilibrium, which means that all the reactants and products are in the same physical state. For gases, instead of molar concentration expressed with [], we use partial pressures of the gases, expressed with (). The expression would become: Keq =

Expressions for Heterogeneous Equilibria When the reactants and products are in more than one physical state, the equilibrium is called heterogeneous equilibrium. For example, when ethanol is placed in a closed flask, a liquid-vapor equilibrium is established: C2H5OH (l)  C2H5OH (g) To write the equilibrium constant expression for this process, you would form a ratio of the product to the reactant.

What is the “concentration” of a pure liquid? Note that the concentration of liquid ethanol is in the denominator. Liquid ethanol is a pure substance, so its concentration is its density expressed in moles per liter. At any given temperature, density is constant. No matter how much or how little C2H5OH is present, its concentration remains constant. Therefore, the “concentration” of the pure liquid is said to be 1, and the term drops out of the equilibrium expression. Keq = = = (PC2H5OH (g))

States of Matter Summary Solids behave the same way liquids do in terms of “concentrations” at equilibrium – they don’t have one! ONLY gases and aqueous solutions are represented by equilibrium expressions. Remember to use partial pressure and () for gases, and molarity and [] for solutions!

Equilibrium Constants For a given reaction at a given temperature, Keq will always be the same regardless of the initial concentrations of reactants and products.

Equilibrium Characteristics All chemical reactions that reach equilibrium have the following three things in common: The reaction must take place in a closed system – no reactant or product can enter or leave the system. The temperature must remain constant. All reactants and products are present, and they are in constant dynamic motion.