Chapter 15 Notes- Chemical Equilibrium
15.1 The Concept of Equilibrium [p.627]
1. Chemical equilibrium – opposing reactions are proceeding at equal rates
2. Equilibrium State – mixture of reactants and products whose concentrations do not change
a. Equilibrium Mixture- pt at which concentrations do not change
b. Reversible –reaction can go forward and backwards
3. At equilibrium the RATE of the forward reaction = the RATE of the reverse reaction
4. To achieve equilibrium in which the rate of the reactions and the concentrations remain constant
the system must be closed.
15.2 The Equilibrium Constant [p.630]
1. General Information
a. Law of Mass Action – expresses the relationship between the concentrations of the
reactants and products at equilibrium
b. Equilibrium- constant expression speed of chemical reaction
i. Depends only on the stoichiometry of the reaction, not the reaction mechanism
ii. Products are in the numerator and reactants are in the denominator
For the reaction aA + bB  dD + eE
[ D] d [ E ]e
Kc 
[ A] a [ B]b
c. Equilibrium constant (Kc) – numerical value
2. Evaluating Kc
a. Always constant (at least in terms of experimental error)
b. Initial concentrations of product or reactants does not effect the value
c. Common practice to write constant without units
3. Equilibrium Constants in Terms of Pressure, Kp
a. Only works with reactants and products that are gases
b. Kp is found when partial pressures of the reactants and products are used in the
equilibrium-constant expression (same set-up)
c. Kp and Kc are usually different numbers
d. Possible to calculate one from the other using the combined gas law.
K p  K c ( RT ) n
Where Δn = the no. of moles of products – the no. of moles of the reactant
4. Equilibrium Constants and Units
a. Activities do not have units
b. Thermodynamic equilibrium constants do not have units
15.3 Interpreting and Working with Equilibrium Constants [p. 635]
1. The Magnitude of Equilibrium Constants
a. If K>> 1 (Large K): Equilibrium lies to the right; products predominate
b. If K<< 1 (small K): Equilibrium lies to the left; Reactants predominate
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Chapter 15 Notes- Chemical Equilibrium
2. The Direction of the Chemical Reaction and K
a. The equilibrium-constant expression for a reaction written in one direction is the
reciprocal of the one for the reaction written in the reverse direction
b. Must indicate the direction of the reaction to give understanding K
3. Relating Chemical Equations and Equilibrium Constants
a. The equilibrium constant of a reaction in the reverse direction is the inverse of the Kc in
the forward reaction
b. Kc of a reaction that has been multiplied by a is the Kc raised to a power equal to the
number
c. Kc for a net reaction made of two or more steps is the product of the Kc for the
individual steps.
15.4 Heterogeneous Equilibria [p.585]
a. Homogeneous equilibria –substances all in the same phase
b. Heterogeneous equilibria- substances are in more than one phase
i. Whenever a pure solid or pure liquid is involved in a heterogeneous equilibrium,
its concentration is not included in the equilibrium-constant expression for the
reaction
ii. Solvents are also omitted from the equilibrium-constant expression
15.5 Calculating Equilibrium Constants [p.641]
1. Tabulate all known initial and equilibrium concentrations of species that appear in equilibriumconstant expression
2. for species in which both initial and equilibrium concentrations are known, calculate the change
in the concentration that occurs as the system reaches equilibrium
3. Use stoichiometry of reaction to calculate changes in all concentration for all the species in the
equilibrium
4. From initial concentrations and changes in concentration, calculate equilibrium constants. Use
these to evaluate the equilibrium constant
15.6 Applications of Equilibrium Constants [p. 644]
1. Predicting the Direction of the Reaction
a. Reaction quotient, Q- is a number obtained by substituting reactant and product
concentrations or partial pressures at any point during a reaction into the equilibriumconstant expression
b. Q= K: the reaction quotient will equal the Kc only if the system is at equilibrium
c. Q> K: concentration of products is too large and reactant’s concentration is too small.
The reaction will proceed to the right.
d. Q< K: concentration of the products is small and the reactant’s concentrations are too
large. The reaction will proceed to left to establish equilibrium
2. Calculating Equilibrium Constants
a.
15.7 Le Chatelier’s Principle[p. 605]
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Chapter 15 Notes- Chemical Equilibrium
1. Le Chatelier’s Principle = 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 equilibrium
position so as to counteract the effect of the disturbance
2. Change in Reactant or Product Concentrations
a. Increase in concentration of a component will shift the reaction in the direction that will
reduce the increase
i. Increase a reactant, shift toward product
ii. Increase a product, shirt toward the reactant
b. Decrease in concentration a component, shifts reaction in the direction of the decrease
3. Effects of Volume and Pressure Changes (following is true as long as temperature is constant)
a. At constant temperature, reducing volume of a gaseous equilibrium mixture causes the
system to shift in direction that reduces number of moles of gas
b. Increasing Volume, shifts in direction of more gas molecules
c. If the number of moles is the same on both sides a change in volume (pressure) will not
shift the reaction
d. K will not change as long a s temperature is constant
e. Changes in volume will affect the pressure, if Qp is not equal to Kp, the direction of the
reaction needed to reestablish equilibrium
4. Effect of Temperature Changes
a. Changes in temperature typically changes the Kp
b. Changes in heat are treated the same as changes in concentration
c. Endothermic: Increasing T results in an increase in K
d. Exothermic : Increasing T results in a decrease in K
e. Opposite effect for cooling
5. Effects of Catalysts
a. Kp is not affected by a catalyst
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