Chapter 13: Chemical Equilibrium

Transcription

1 Chapter 13: Chemical Equilibrium May 5 2:04 PM 13.1 The Equilibrium Condition When you finish this section you will be able to list some characteristics of reactions at equilibrium. Chemical equilibrium is the state where the concentrations of all reactants and products remain constant with time. Equilibrium is not static but a highlydynamic situation A double arrow ( direction. ) is used to show that a reaction can occur in either The Changes in concentrations with time for the reaction H2O(g) + CO(g) H 2(g) + CO 2(g) The Changes with time in the rates of forward and reverse reactions for H2O(g) + CO(g) H 2(g) + CO 2(g) A concentration profile for the reaction N2(g) + 3H 2(g) 2NH 3(g) Apr 23 9:55 AM 1

2 Example 13.1 Background Questions 1. What is dynamic equilibrium? 2. What is true about the initial rate of forward and reverse reactions in a system where only reactants are present? 3. What is true about the rates forward and reverse reactions at equilibrium? 4. Why does equilibrium occur? 13.2 The Equilibrium Constant When you finish this section you will be able to write a mass action expression for a given balanced chemical equation. Law of Mass Action for a reaction na + mb pc + qd the equilibrium constant K is given by K = [C] p [D] q [A] n [B] m One way to determine the significance of K is to assume that the forward reaction involves na and mb in its rate determining step, and the reverse reaction involves pc and qd in its rate determining step. Apr 24 1:01 PM rate foward = k f [A] n [B] m rate reverse = k r [C] p [D] q At equilibrium, rate forward = rate reverse, or k f [A] n [B] m = k r [C] p [D] q. Rearranging: k f = K = [C] p [D] q k r [A] n [B] m K is thus the ratio of the forward to reverse rate constants (not the rate)! As with the kinetic rate determining step, reactant or product coefficients become exponents when put in the mass action expression, or equilibrium expression Example 13.2 A Equilibrium Expressions Write the equilibrium expression for each of the following reactions: a. PCl 5 (g) PCl 3 (g) + Cl 2 (g) b. S 8 (g) 8S(g) c. Cl 2 O 7 (g) + 8H 2 (g) 2HCl(g) + 7H 2 O(g) Apr 24 1:13 PM 2

3 13.4 Heterogeneous Equilibria Equilibrium expressions involve concentrations (or pressures) of substances that change from initial to equilibrium conditions. A pure substance such as water changes amount, but not concentration. The concentrations of pure solids and liquids remain constant, their concentrations are NOT included in the equilibrium expression for the reaction. CaCO 3 (s) CaO(s) + CO 2 (g) K = [CO 2 ] The position of the equilibrium does not depend on the amounts of CaCO 3 and CaO present. Apr 27 11:57 PM 13.5 Application of the Equilibrium Constant When you finish this section you will be able to: Use the reaction quotient (Q) to predict the direction of chemical reactions toward equilibrium. Use the reaction quotient to aid in solving simple equilibrium problems. What K can/cannot indicate: K does not reflect how fast a reaction goes. A large value of K means that mostly products will be present at equilibrium. A small value K means that mostly reactants will be present at equilibrium. Reaction quotient, Q, predicts the direction that the reaction will go to reach equilibrium. It is calculated by using the law of mass action on the initial, not equilibrium, concentrations (or pressures) of the reaction substances. If Q is equal to K, the system is at equilibrium. If Q is greater than K, mathematically, there is too much product present. The system will shift to the left to reach equilibrium. If Q is less than K, there is too much reactant present. The system will shift to the right to reach equilibrium. Apr 28 12:13 AM 3

4 Example 13.5A Predicting the Direction of Equilibrium The reaction between hydrogen gas and iodine gas is: H 2 (g) + I 2 (g) 2HI(g) K = 7.1 x 10 2 at 25 o C Predict the direction that the system will shift in order to reach equilibrium given each of the following initial conditions: a. Q = 427 b. Q = 1522 c. [H 2 ] = 0.81 M [I 2 ] = 0.44 M [HI] = 0.58 M d. [H 2 ] = M [I 2 ] = M [HI] = 1.35 M e. [H 2 ] = M [I 2 ] = M [HI] = 1.50 M Apr 28 1:17 PM Apr 23 11:23 PM 4

5 Example 13.6 A Equilibrium Calculations Small Value For K The reaction between nitrogen and oxygen to form nitric oxide has a value for the equilibrium constant at 2000 K of K= 4.1 x10 4. If 0.50 moles of N 2 and 0.86 mole of O 2 are put into a 2.0 L container at 2000 K, what would be the equilibrium concentrations of all species? Assumption Because K is small, let us assume that x is negligible compared to 0.25 and Testing our Assumption We must test our assumption and check the answer. Is x less than 5% of 0.25 M? Yes. Is it less than 5% of 0.464? Yes. Double Check We can check our math by solving for K using our equilibrium concentrations. Apr 29 7:57 PM 13.7 Le Châtelier s Principle When you finish this section you will be able to predict the response of a system to stresses placed on it. Le Châtelier s Principle: If a change is imposed on a system at equilibrium the position of the equilibrium will shift in a direction that tends to reduce that change. Haber process: N 2 (g) + 3H 2 (g) 2NH 3 (g) + 92 kj Consider the following effects on the position of equilibrium: Concentration if a component (reactant or product) is added to a reaction system at equilibrium (at constant T and P or constant T and V), the equilibrium position will shift in the direction that lowers the concentration of that component. If a component is removed, the opposite effect occurs. Pressure Add or remove a gaseous reactant or product. Add an inert gas. It increases the total pressure but has no affect on the concentration or partial of the reactants or products. Temperature the value of K changes with temperature. Look over the effects of each on the direction of equilibrium Apr 29 8:01 PM 5

6 Example 13.7A Le Châtelier s Principle Nitrogen gas and oxygen gas combine at 25ºC in a closed container to form nitric oxide as follows: N 2 (g) + O 2 (g) 2NO(g) ΔH = kj K p = 3.3 x What would be the effect on the direction of equilibrium (i.e. would it shift to the left, right, or not at all) if the following changes were made to the system? a. N 2 is added b. He is added c. The container is made larger d. The system is cooled Example 13.7B Practice with Le Châtelier s Principle The combination of hydrogen gas and oxygen gas to give water vapor can be expressed by 2H 2 (g) + O 2 (g) 2H 2 O(g) ΔH = 484 kj Predict the effect of each of the following changes to the system on the direction of equilibrium. a. H 2 O is removed as it is being generated b. H 2 is added c. The system is cooled Apr 29 8:23 PM 6