Chapter 16 Chemical Equilibrium Reversible reactions Reversible reaction

Chapter 16 Chemical Equilibrium

Reversible reactions • Reversible reaction – a reaction which can also convert products back to reactants • Reaction rate for the forward and reverse reactions dependent on concentration

Dynamic equilibrium • When the rate of the forward reaction equals the rate of the reverse reaction, the reaction is in a dynamic equilibrium • rateforward = ratebackwards • Rates are equal, but concentration is not

Recap When a reaction is at equilibrium, what can be said about the rate of the forward reaction and the rate of the reverse reaction? Are the concentrations of the reactants and products the same at equilibrium?

• At equilibrium, the concentration of the reactants and products can be related through the equilibrium constant K – Remember: k = rate constant, K = eq. constant The equilibrium constant expression - K

Writing out the K expression

Questions

• When K >> 1 – Rxn is product favored – [P]’s higher than [R]’s @ equilibrium Large K value

• When K << 1 – Rxn is reactant favored – [R]’s is higher than [P]’s @ equilibrium Small K value

Medium K value? ? ? • When K ≈ 1 – Neither direction is favored – Rxn proceeds about ½ way – Calculations must be done to figure out whether the [R]’s or the [P]’s is higher

• K remains the same for a reaction at a specific temperature, independent of initial concentrations

Heterogeneous equilibrium • Pure solids and liquids excluded in the equilibrium constant expression

Practice • Write the equilibrium constant expression for the following reactions • C(s) + 2 H 2(g) CH 4(g) • HF(aq) + H 2 O(l) • 2 KCl. O 3(s) H 3 O+(aq) + F-(aq) 2 KCl(s) + 3 O 2(g)

Reaction Quotient (Q) • Similar to K, but it can be at anytime during a reaction (not just equilibrium) • Can determine direction of reaction when compared to the K value, heading towards products or heading towards reactants

Summary of meaning Q • When Q = K, reaction at equilibrium • When Q > K, reacts toward reactants, to the left • When Q < K, reacts towards products, to the right

Practice • Right before a reaction starts, and if there are no products yet, what is the Q value? • When the Q value is lower than K, which way will the reaction proceed, toward products on the right, or toward reactants on the left? • If a reaction is proceeding to the left, is Q larger or smaller than K?

Problems • • At a certain temperature, K = 8. 5 x 10 -3. A reaction mixture contains [NH 3] 0. 166 M and [H 2 S] 0. 166 M. What is the value of Q? If the reaction proceeds to equilibrium, will it produce more reactants or products?

ICE Tables • Method to determine concentrations of reactants and products at equilibrium if given initial concentrations and K • Initial, Change, and Equilibrium (ICE) [A] [B] • Example reaction Initial 1. 0 0. 00 –A 2 B Change -x +2 x Equilibrium 1. 0 - x 2 x Solve for x, May need to use quadratic formula

ICE example • H 2 (g) + I 2(g) 2 HI (g) • What are the equilibrium concentrations of the reactants and products if you start with 1. 50 M H 2 and 1. 50 M I 2 and have an equilibrium constant of 1. 68? • K = 1. 68 [H 2] [I 2] [HI] Initial 1. 50 0. 00 Change -x -x +2 x Equilibrium 1. 50 - x 1. 50 -x 2 x

This is the change in concentration. Use correct x value [H 2] at equilibrium = 1. 50 – x 1. 50 -(0. 589) = 0. 911 M possible = answer 1. 50 -(-2. 762) = 4. 262 M not possible [HI] at equilibrium = 2 x 2(0. 589) = 1. 178 M possible = answer 2(-2. 762) =-5. 524 M not possible

ICE table Review • Set up general ICE table and fill in Initial concentrations of products and reactants. • Fill in the Change with the term –x for reactant and +x for product, then multiply it by the coefficient of the molecule • Fill in the Equilibrium values in terms of the Initial concentrations plus the Change term • Then write the K expression. Then plug in the Equilibrium terms into the K expression. • Solve for x. May need to use quadratic formula.

Practice • A rxn mixture that originally contains 11 mol A and 0 mol B in a 1. 0 L container is allowed to reach equilibrium. What is [A] and [B] at equilibrium? • A(g) B(g) K = 10 • A rxn mixture that originally contains [X] 5. 0 M and [Y] 6. 0 M and [Z] 0. 0 M. What is [X] , [Y] , [Z] at equilibrium? • X(g) + Y(g) 2 Z(g) K=2

Le Châtelier’s Principle

Le Châtelier’s Principle • Le Châtelier's Principle - if a system at equilibrium is disturbed, the position of equilibrium will shift to minimize the disturbance – Concentration – Pressure/Volume – Temperature – Concentrations of all the chemicals will change until equilibrium is re-established – The new concentrations will be different, but the equilibrium constant (K) will be the same

Le Châtelier’s: Concentration

Le Châtelier’s: Pressure/volume

Le Châtelier’s: Temperature • If the temp of a system at equilibrium is changed, the system will shift in a direction to counter that change • A new K is established at the new temp • Consider whether an endothermic or an exothermic reaction

Endothermic reaction

Exothermic reaction

Le Châtelier’s: Catalysts • Catalysts provide an alternative, more efficient mechanism • Speed up reaction by lowering the Ea • Result in the same ratios of products and reactants at equilibrium • Do not affect the position of equilibrium • K stays the same

Questions • The reaction 2 SO 2(g) + O 2(g) Û 2 SO 3(g) is exothermic. How will each of the following changes affect the equilibrium concentrations of each gas once equilibrium is re-established? 1) Adding more O 2 to the container 2) Removing SO 3 3) Compressing the gases 4) Cooling the container 5) Doubling the volume of the container 6) Warming the mixture 7) Adding SO 3 8) Adding a catalyst to the mixture

Reversible reactions go to equilibrium Chapter 16 Summary Equilibrium constant K and how to write the expression Heterogeneous equilibrium Reaction quotient Q, direction of a reaction ICE tables, how to calculate Eq concentrations Le Châtelier’s principle, disturbing the Eq