Chapter 15: Chemical Equilibrium
15.1 The Concept of Equilibrium
15.2 The Equilibrium Constant
LO 6.1 The student is able to, given a set of experimental observations regarding physical, chemical, biological, or environmental processes that are reversible, construct an
explanation that connects the observations to the reversibility of the underlying chemical reactions or processes.
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
General equilibrium equation
aA + bB ⇌ dD + eE
The Equilibrium Constant in term of concentration
Kc =
Sample Exercise 15.1
Sample Exercise 15.1 Writing Equilibrium-Constant ExpressionsWriting Equilibrium-Constant Expressions (a)(a) 2 O3(g) ⇌ 3 O2(g)
(b)(b) 2 NO(g) + Cl2(g) ⇌ 2 NOCl(g)
(c)(c) Ag+(aq) + 2 NH3(aq) ⇌ Ag(NH3)2+(aq)
Evaluating Kc
The Equilibrium constant in terms of pressure
aA + bB ⇌ dD + eE
Kp =
K
p= K
c(RT)
∆nR is .0821
T must be in Kelvin When would Kp = Kc?
Sample Exercise 15.2
Sample Exercise 15.2 Converting between Converting between KKcc and and KKpp
In the synthesis of ammonia from hydrogen and nitrogen N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
Kc = 9.60 at 300oC, Calculate Kp for this reaction at this temperature.
15.3 Understanding and Working with Equilibrium Constants
LO 6.7 The student is able, for a reversible reaction that has a large or small K, to determine which chemical species will have very large versus very small concentrations at equilibrium.
The magnitude of equilibrium constants.
Sample Exercise 15.3
Sample Exercise 15.3 Interpreting the Magnitude of an Equilibrium ConstantInterpreting the Magnitude of an Equilibrium Constant The following diagrams represent three systems at equilibrium, all in the same-size containers. (a) (a) Without doing any calculations, rank the systems in order of increasing Kc. (b)(b) If the volume of the containers is 1.0 L and each sphere represents 0.10 mol, calculate Kc for each system.
Practice Exercise 2 Practice Exercise 2
For the reaction H2 (g) + I2(g) ⇌ 2HI (g) , Kp = 794 at 289K and Kp = 54 at 700K. Is the formation of HI favored more at the higher or lower temperature?
LO 6.2 The student can, given a manipulation of a chemical reaction or set of reactions (e.g., reversal of reaction or addition of two reactions), determine the effects of that manipulation on Q or K.
The direction of the chemical equation and K
N2O4 (g) ⇌ 2NO2 (g)
Kc = = .212
What about
2NO2 (g) ⇌ N2O4 (g)
Relating Chemical Equation Stoichiometry and Equilibrium Constants What if N2O4 (g) ⇌ 2NO2 (g) were multiplied by 2??
Equations can also be added together like they were in the kinetics chapters...The Keq of the SUMSUM of the equations is the PRODUCTPRODUCT of the Keqs
Given that, at 700K, Kp= 54.0 for the reaction H2(g) + I2(g) ⇌2HI (g) , and Kp = 1.04x10-4 for the reaction N2 (g) + 2H2 (g) ⇌ 2NH3 (g), determine the value of Kp
for the reaction 2 NH3 (g) + 3I2 (g) ⇌6HI + N2 (g) at 700K
15.4 Heterogeneous Equilibria
Decomposition of calcium carbonate into calcium oxide and carbon dioxide gas.
Sample Exercise 15.5
Sample Exercise 15.5 Writing Equilibrium-Constant Expressions forWriting Equilibrium-Constant Expressions for Heterogeneous Reactions
Heterogeneous Reactions
Write the equilibrium-constant expression Kc for (a)(a) CO2(g) + H2(g)⇌CO(g) + H2O(l)
(b)(b) SnO2(s) + 2 CO(g)⇌Sn(s) + 2 CO2(g)
15.5 Calculating equilibrium Constants
LO 6.5 The student can, given data (tabular, graphical, etc.) from which the state of a system at equilibrium can be obtained, calculate the equilibrium constant, K.
A closed system initially containing 1.000x10-3 M H2 and 2.000 x10-3 M I2 at 448oC is allowed to reach equilibrium. Analysis of the equilibrium mixture shows that the concentration of HI is 1.87x10-3 M. Calculate Kc at 448oC for the reaction taking place
H2 (g) + I2 (g) ⇌ 2HI (g)
[H2] [I2] [HI]
IInitial CChange
EEquilibrium
Sulfur trioxide decomposes at high temperature in a sealed container:
2SO3 (g) ⇌ 2SO2 (g) + O2 (g)
Initially, the vessel is charged at 1000K with SO3 (g) at a partial pressure of 0.500 atm. At equilibrium, the SO3 partial pressure is .200 atm. Calculate the value of Kp at 1000K
[SO3 ] [SO2] [O2] IInitial
CChange
EEquilibrium
15.6 Applications of Equilibrium Constants
LO 6.4 The student can, given a set of initial conditions (concentrations or partial pressures) and the equilibrium constant, K, use the tendency of Q to approach K to predict and justify the prediction as to whether the reaction will proceed toward products or reactants as equilibrium is approached.
Predicting the direction of a reaction
Q = reaction quotient a number obtained from
substituting starting concentrations into the equilibrium constant
expression...
if Q>K
if Q<K
if Q=K
At 1000K, the value of Kp for the reaction 2SO3 (g) ⇌ 2SO2 (g) + O2 (g) is .338.
Calculate the value of Qp, and predict the direction in which the reaction will proceed toward equilibrium if the initial partial pressures are PSO3 = .16 atm, PSO2 =.41 atm, and PO2 = 2.5 atm.
Calculating equilibrium concentrations
LO 6.6 The student can, given a set of initial conditions (concentrations or partial
pressures) and the equilibrium constant, K, use stoichiometric relationships and the law of mass action (Q equals K at equilibrium) to determine qualitatively and/or quantitatively the conditions at equilibrium for a system involving a single reversible reaction.
AT 500K the reaction PCl5 (g) ⇌ PCl3 (g) + Cl2 (g) has Kp = .497. In an equilibrium mixture at 500K, the partial pressure of PCl5 is .860 atms and that of PCl3(g) is .350 atm. What is the partial pressure of Cl2 in the equilibrium mixture?
A 1.000 L flask is filled with 1.000 mol of H2 and 2.000 mol of I2 at 448oC. The value of the equilibrium constant Kc is 50.5. What are the concentration of H2, I2, and HI in the flask at equilibrium?
H2 (g) + I2 (g) ⇌ 2HI (g)
1. Set up ICE chart with initial and equilibrium concentration of all species.
H2 (g) + I2 (g) ↔ 2HI (g) Initial
Change Equilibrium
2. Write the change in equilibrium as “x”, account for stoichiometry.
3. Calculate the equilibrium concentrations
4. Plug equilibrium concentrations into equilibrium expression setting it equal to the equilibrium constant.
5. Make equal to 0 and use the quadratic formula to solve for “x”. One of these will not make sense.
6. Go back to the equilibrium line of the ICE chart to solve for the equilibrium concentrations.
For the equilibrium PCl5 (g) ⇌ PCl3 (g) + Cl2 (g) the equilibrium constant Kp has the value of .497 at 500 K. A gas cylinder at 500 K is charged with PCl5 (g) at an initial pressure of 1.66 atm. What are the equilibrium pressures of PCl5, PCl3, and Cl2 at this temperature?
Initial Change Equilibrium
Practice Free Response. (based on 2003B) After a 1.0 mole sample of HI(g) is placed into an evacuated 1.0 L container at 700. K, the reaction represented below occurs. 2HI(g) ⇌ H2(g) + I2(g) The concentration of HI(g) as a function of time is shown on the graph to the right.
(a) Write the expression for the equilibrium constant Kc, for the reaction. (1)
(b) What is [HI] at equilibrium? (1)
(c) Determine the equilibrium concentrations of H2(g) and I2(g). (1)
(d) On the graph above, make a sketch that shows how the concentration of H2(g) changes over time. (2)
(e) Calculate the value of Kc for the reaction at 700. K. (1)
(f) At 1000 K, the value of Kc for the reaction is 2.6 x 10-2 . In an experiment, 0.75 mole of HI(g), 0.10 mole of H2(g) and 0.50 mole of I2(g) are placed in a 1.0 L container and allowed to reach equilibrium at 1000 K. Determine whether the equilibrium concentration of HI(g) will be greater than, equal to, or less than the initial concentration of HI(g). Justify your answer. (2)
15.7 Le Chatelier's Principle
LO 6.3 The student can connect kinetics to equilibrium by using reasoning about
equilibrium, such as LeChatelier’s principle, to infer the relative rates of the forward and reverse reactions.
LO 6.8 The student is able to use Le Chatelier's principle to predict the direction of the shift resulting from various possible stresses on a system at chemical equilibrium.
LO 6.9 The student is able to use Le Chatelier's principle to design a set of conditions that will optimize a desired outcome, such as product yield.
LO 6.10 The student is able to connect LeChatelier’s principle to the comparison of Q to K by explaining the effects of the stress on Q and K.
If a system at equilibrium is disturbed by a change in temperature, pressure If a system at equilibrium is disturbed by a change in temperature, pressure or the concentration of one of the components, the system will shift its
or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance.
equilibrium position so as to counteract the effect of the disturbance.
Change in reactant or Product Concentration
SUMMARY -
Effects of Volume and Pressure Changes
SUMMARY -
Effect of Temperature Changes
SUMMARY -
Effect of a catalyst
Sample Exercise 15.12
Sample Exercise 15.12 Using Le Châtelier’s Principle to Predict Shifts inUsing Le Châtelier’s Principle to Predict Shifts in Equilibrium
Equilibrium
Consider the equilibrium
N2O4(g) ⇌ 2 NO2(g) ∆H° = 58.0 kJ
In which direction will the equilibrium shift when (a)(a) N2O4 is added,
(b)(b) NO2 is removed,
(c)(c) the pressure is increased by addition of N2(g),
(d)(d) the volume is increased,
(e)(e) the temperature is decreased?
Sample Exercise 15.13
Sample Exercise 15.13 Predicting the Effect of Temperature on Predicting the Effect of Temperature on KK (a)
(a) Using the standard heat of formation data in Appendix C, determine the standard enthalpy change for the reaction
N2(g) + 3 H2(g) ⇌ 2 NH3(g) (b)
(b) Determine how the equilibrium constant for this reaction should change with temperature.
a) delta H = -92.38 kJ
b)
Sample Integrative Exercise
Sample Integrative Exercise Putting Concepts TogetherPutting Concepts Together
At temperatures near 800 °C, steam passed over hot coke (a form of carbon obtained from coal) reacts to form CO and H2:
C(s) + H2O(g) ⇌ CO(g) + H2(g)
The mixture of gases that results is an important industrial fuel called water gas.
(a)(a) At 800 °C the equilibrium constant for this reaction is Kp = 14.1. What are the equilibrium partial pressures of H2O, CO, and H2 in the equilibrium mixture at this temperature if we start with solid carbon and 0.100 mol of H2O in a 1.00-L vessel?
(b)(b) What is the minimum amount of carbon required to achieve equilibrium under these conditions?
(c)(c) What is the total pressure in the vessel at equilibrium?
(d)(d) At 25 °C the value of Kp for this reaction is 1.7 × 10–21. Is the reaction exothermic or endothermic?
(e)(e) To produce the maximum amount of CO and H2 at equilibrium, should the pressure of the system be increased or decreased?
Practice Free Response.
Consider the reaction below that is allowed to reach equilibrium at a certain temperature.
CaCO3(s) ⇌ CaO(s) + CO2(g) ΔH = + 179 kJ/molrxn (a) Write the Kp expression for the reaction. (1)
(b) What are the optimum conditions of temperature and pressure, that would lead to a large % of CO2(g) in the equilibrium mixture? (2)
(b) In each case below, predict the shift in the equilibrium position that the applied stress would cause, and give an explanation in terms of Q and K to justify your choice. (9)
CaCO3(s) ⇌ CaO(s) + CO2(g) ΔH = + 179 kJ/molrxn Stress applied to equilibrium
system (assume all other variables remain
unchanged)
Check only ONE box in
each row Explanation in terms of Q and K
Shift to
LHS No
change Shift to RHS
A. CO2(g) is added to the system
B. The volume of the container is
decreased
C. CaO(s) is removed from the system