Equilibrium Introduction From kinetics, we know that reactants sometimes collide to give products. But why can t products collide to go back to reactants? Theoretically, all chemical reactions are reversible. In practice though, many are not. If chemical reactions are reversible, it is possible for them to establish an equilibrium. Equilibrium is attained when the rate of the forward reaction equals the rate of the reverse reaction. The reactions must take place in a closed system. Forward Reaction H 2 (g) + I 2 (g) æ Æ 2HI(g) Reverse Reaction H 2 (g) + I 2 (g) æ æ 2HI(g) Equilibrium H 2 (g) + I 2 (g) 2HI(g) Equilibrium reactions are incomplete in that a measurable amount of reactants and products are always present. A complete reaction represents one in which all of the reactant is used up and the reaction stops.
What can we say about the concentrations of products and reactants if the rates of both the forward and reverse reactions are constant? During chemical equilibrium, the concentrations of all reactants and products remain constant with time.
The Equilibrium Constant - Kc or Keq At equilibrium, there is a constant ratio between the concentrations of the products and reactants in any change. Thus for any general equilibrium reaction: ap + bq cr +ds the value for the equilibrium constant is: K c = [R]c [S] d [P] a [Q] b Note: [ ] designates concentration (mol/l) only substances that are gases or aqueous are included in the equilibrium expression Kc varies with temp. So an equilibrium constant is only valid for constant temp. Large K means lots more products, while small K means lots more reactants.
Calculating an Equilibrium Constant If you are given the concentrations of compounds at equilibrium. These values can be substituted into the equilibrium expression to find K A mixture of nitrogen and chlorine gases was kept at a certain temperature in a 5.0 L reaction flask to yield nitrogen trichloride. When the equilibrium mixture was analyzed, it was found to contain 0.0070 mol of N2(g), 0.0022 mol of Cl2(g) and 0.95 mol of NCl3 (g). Calculate the equilibrium constant for this reaction.
Given Concentrations at Equilibrium Calculate Kc if 1.00 mol of CO and 3.00 mol of H2 are placed in a 7.00 L flask, reacted and allowed to come to equilibrium. 0.387 moles of gaseous H2O is found at equilibrium. The other product that is formed in methane.
Calculating Concentrations at Equilibrium Given Kc and initial concentrations, you can calculate the concentration of all substances after equilibrium has been established. Need to make an ICE table (Initial/Change/Equilibrium) The following reaction increases the proportion of hydrogen gas for use as a fuel. CO(g) + H2O(g) H2(g) + CO2(g) This reaction has been studied at different temperatures to find the optimum conditions. At 700 K (kelvin), the equilibrium constant is 8.3. Suppose that you start with 1.0 mol of CO(g) and 1.0 mol of H2O(g) in a 5.0 L container. What amount of each substance will be present in the container when the gases are at equilibrium, at 700 K? The following reaction has an equilibrium constant of 25.0 at 1100 K. H2(g) + I2(g) 2HI(g) 2.00 mol of H2(g) and 3.00 mol of I2(g) are placed in a 1.00 L reaction vessel at 1100 K. What is the equilibrium concentration of each gas? (Hint: Need to use the quadratic formula) At 500 K, 1.00 mol of ONCl(g) is introduced into a 1.00 L container. At equilibrium, the ONCl had been 9.0 % dissociated (decomposed). Determine the Kc. 2ONCl(g) 2NO(g) + Cl2(g) P 508 11-15 and P 511 16-19
Small Equilibrium Constant Small K values mean that the reactants are favoured. You divide the smallest initial concentration by the K value. If the answer is larger than 500, you can make an approximation in your equilibrium value. The atmosphere contains large amounts of oxygen and nitrogen. The two gases do not react, however, at ordinary temperatures. They do react at high temperatures, such as the temperatures produced by a lightning flash or a running car engine. In fact, nitrogen oxides from exhaust gases are a serious pollution problem. A chemist is studying the following equilibrium reaction. N2(g) + O2(g) 2NO(g) A chemist puts 0.085 mol of nitrogen and 0.038 mol of oxygen in a 1.0 L rigid cylinder. At the temperature of the exhaust gases from a particular engine, the value of Kc is 4.2 x 10-8. What is the concentration of NO(g) in the mixture at equilibrium? P 515 25-29