Chapter 14 Chemical Equilibrium - …

1 Chapter 14 139 Chapter 14 Chemical Equilibrium Chemical Equilibrium is introduced in this Chapter . Much of this is a straightforward application of equations and principles, but unless students have developed a good mind picture of a Chemical reaction and its dynamic status at Equilibrium , they will often flounder in explaining or applying routine equations to new situations. Having learned in the kinetics Chapter that balanced equation coefficients may not be appropriate in rate laws, students may not understand that they are vital in Equilibrium constant expressions. An excellent source of materials on Equilibrium is in the section Special Focus: Chemical Equilibrium in the Professional Development Workshop Materials by the College Board.

2 This resource has seven chapters moving step by step through all types of Equilibrium . Animations included in this Chapter are Chemical Equilibrium and Le Ch telier s Principle. Major Concepts to Know The Chapter first emphasizes writing Equilibrium constant expressions, Kc and Kp, and converting between them. However, writing an expression does not mean the reaction is at Equilibrium . Only when numerical values are inserted and the ratio is identical to the Equilibrium constant do we know the system is at Equilibrium . Otherwise, the expression gives the Equilibrium quotient Q. The Equilibrium expression is a mass action ratio, where the products raised to the power of their coefficients is divided by the reactants raised to their coefficients.

3 The reverse reaction would be the inverse number. If the value of K is greater than one, the products term must be larger than the reactants term in the denominator, so at Equilibrium , much reaction has occurred. If K is less than one, the products must be less than the reactants, and at Equilibrium , not much reaction has occurred. Chapter 14 140 Chemical Equilibrium is achieved when the forward and backward rates are equal. Another common student misconception is that this means the amount of product equals the amount of reactant, which would only be true for a K = and with all substances of unit coefficients. This does happen if one reactant is changing to one product as in the phase change, H2O(l) H2O(g), then the number of molecules changing is the same.

4 In Equilibrium expressions, the students must remember to write ions with charges and not to include solids or pure liquids like water in the expression. The text points out that this is because the concentration terms should really be included as compound activities, which are relative to concentrations in standard states. For this reason, K (and Q) values are considered unitless, so this is the one part of the test where students do not have to write units. However, since the actual values of K depend on the units used in the reactant or product terms, they must be entered appropriately in the same units throughout. In Kc expressions, concentrations in molarity are generally used.

5 In the expression [A], the brackets stand for concentration. When writing Kp expressions, brackets are never used, and the symbol P is used for pressure, usually expressed in atmospheres, or mmHg. Note that only gases are put in Kp expressions. Students can handle gas molecule changes in gas reactions by using commonsense arguments or the expression: Kp = Kc(RT) n where n = moles of gas product moles of gas reactant. R is the gas constant K .1mol 1, and T is the temperature in Kelvin. When a reaction happens in several steps, each step has a K value. Multiplying the K values of each step together will give the overall K.

6 Chapter 14 141 If a reaction is not at Equilibrium , a calculation can be done to determine what way it will shift to reach Equilibrium . Since it is a division problem, the variable is called Q for quotient. If Q = K, then the reaction is at Equilibrium . If Q > K, then there is too much product, and the reaction will shift to the left to reach Equilibrium . (Make more reactant, so it divides to a smaller number). If Q < K, then there needs to be more product, and the reaction will shift to the right to reach Equilibrium . Reaction Direction and the Relative Sizes of K and Q Le Ch telier s principle will help students determine what will happen when a stress is put on a system.

7 Changes in any concentration will disturb the Equilibrium and cause a shift. For instance, whenever the system is stressed with the addition of more of a substance, the reaction will shift away from where it has been added. o Changes in volume and pressure affect gas reactions. When the volume is reduced, the pressure increases. This will cause a shift to the side with the fewest moles of gas. Chapter 14 142 It is very important that students understand that changing concentrations or even the way expressions are written does not change the value of the constant K that is determined for a particular reaction. The only change that can change a value of K is temperature.

8 K values are temperature-dependent since forward and reverse reactions have different H values and are affected differently by a change in temperature. An easy way for students to handle T changes is to think of heat as a reactant or product. In endothermic reactions, heat is a reactant, and in exothermic reactions, heat is a product. Using Le Ch telier s idea, an increase will cause a shift away from the stress; an increase of temperature in an endothermic reaction will cause a shift to the right, increasing the product concentrations and the value of K. Increasing the temperature in an exothermic reaction will cause a shift to the left, increasing the reactant concentrations and decreasing the value of K.

9 Catalysts do not affect equilibriums, but they do affect how fast a Chemical reaction reaches Equilibrium . The Haber process is a frequent topic of questions and should be thoroughly understood in all its aspects. Chapter 14 143 Vocabulary to Know Equilibrium constant Heterogeneous Equilibrium Homogeneous Equilibrium Law of mass action Le Ch telier s principle Physical Equilibrium Reaction quotient Math Skills to Know How to change Kc to Kp using Kp = Kc( ) n How to write an Equilibrium expression, substitute, and solve How to find an overall K using K cK c = Kc Solve for Q and predict the direction of the reaction Suggested Problems (* also electronic) Concept of Equilibrium and the Equilibrium constant: 3, 4 Equilibrium constant expressions.

10 6, 7*, 8*, 9*, 10, 11, 13, 14* 21*, 22, 23* 32* What does the Equilibrium constant tell us: 34, 37, 40*, 41*, 43*, 44*, 45*, 47, 48* Factors that affect Chemical Equilibrium : 49, 51, 52, 53* 58*, 61, 62* Additional questions: 67, 106, 107 Chapter 14 144 Suggested Demonstrations or Labs Jeffrey A. Paradis, Determining the Equilibrium Constant of a Complex, Hands On Chemistry Laboratory Manual (New York, NY: McGraw-Hill, 2006). Jeffrey A. Paradis, Le Ch telier s Principle: Stress Management, Hands On Chemistry Laboratory Manual (New York, NY: McGraw-Hill, 2006). Chapter 14 145 Questions 1. What is true at Equilibrium ?