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Chapter 15 Principles of Chemical Equilibrium

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Chapter 15Principles of Chemical Equilibrium

Contents in Chapter 15

15-1Dynamic Equilibrium

15-2The Equilibrium Constant Expression

15-3Relationships Involving Equilibrium Constants

15-4The Magnitude of an Equilibrium Constant

15-5The Reaction Quotient, Q: Predicting the Direction of Net Change

15-6Altering Equilibrium Conditions: Le Châtelier’s Principle

15-7Equilibrium Calculations: Some Illustrative Examples

15-1Dynamic Equilibrium

- Dynamic equilibrium: A condition where forward and reverse processes proceed at equal rates and no further net change occurs.
- Chemical equilibrium: At the condition that the concentrations of reactants and products no longer changes with time.
- Physical equilibrium: The equilibrium between two phases, that is according to physical processes.

AgI(s) Ag+(aq) + I–(aq)

- Example of Chemical equilibrium

Radioactive

(I-131)

Radioactive

(I-131)

Radioactive

(I-131)

I2(H2O) I2(CCl4)

- Example of physical equilibrium

H2O top layer

CCl4 bottom layer

15-2The Equilibrium Constant Expression

- Established empirically (trial and error)

(Continuous)

15-2(Continuous)

- Derived theoretically (kinetic view of equilibrium)
- In chemical kinetics view, the equilibrium constant of a reaction can be expressed as a ratio of the rate constants of “forward” and “reverse” reactions:
Kc = (forward rate)/(reverse rate) = kf/kr

- The expression of chemical equilibrium according to the coefficients of the balance equation, independent to the kinetic mechanism.

- In chemical kinetics view, the equilibrium constant of a reaction can be expressed as a ratio of the rate constants of “forward” and “reverse” reactions:

Example: Proof a net reaction A + 2B AB2 has same equilibrium constant expression although the reaction mechanisms are different:

kf'

- If the mechanism with single elementary step:
- A + 2B AB2K (equilibrium constant)

kr'

(B)If the mechanism with multi-elementary step, such as:

Step 1 2B B2K' (equilibrium constant)

Step 2A + B2 AB2K'' (equilibrium constant)

NetA + 2B AB2K (equilibrium constant)

kf''

kr''

kf

kr

Solution:

(A)ratef = kf[A][B]2; rater = kr[AB2]

at equilibrium, ratef = rater

(B)

15-2(Continuous)

- The Equilibrium Constant and Activities
- Activity: The effective concentration for obtaining the (true) thermodynamic constants, the activity of a species is obtained as the product of an activity coefficient and the ratio of the stoichiometic concentration (or pressure) to that of a reference state.

X: activity coefficient, [X]: actual concentration,

Co: concentration of reference state.

15-2(Continuous)

- A General Expression for K

15-3Relationships Involving Equilibrium Constants

- Relationship of K to the Balanced Chemical Equation
- Reverse an equation, invert the value of K.
- Multiply the coefficients in a balanced equation by a common factor, raise the equilibrium constant to the corresponding power.
- The balanced chemical equation must be written when citing a value for K.

(Continuous)

- Combining Equilibrium Constant Expressions
- Individual equations are combined (that is, added), their equilibrium constants are multiplied to obtain the equilibrium constant for the overall

(Continuous)

- Equilibria Involving Gases

- Concentration equilibrium constant (KC) using molar concentrations ([ ]), unit in M:

- Pressure equilibrium constant (KP), using partial pressure (p), unit in bar:

- Conversion between KC and KP:
KP = KC(RT)Δn(R=0.08314 bar L K–1 mol–1)

Δn = mol of gaseous Products － mol of gaseous Reactants

In this case: Δn = (g+h) – (a+b)

* P = (n/V)RT

(Continuous)

- Equilibria Involving Pure Liquids and Solids
- The equilibrium constant expression does not include terms of pure solid and liquid phases.

Example:

C(s) + H2O(g) CO(g) + H2(g)

CaCO3(s) CaO(s) + CO2(g)

Kc = [CO2] KP = pCO2 = KC(RT)

H2O(l) H2O(g)

Kc = [H2O(g)] KP = pH2O = KC(RT)

15-4The Magnitude of an Equilibrium Constant

- A very large numerical value of K, if K > 1010, signifies that a reaction goes to completion.
- A very small numerical value of K, if K < 10–10, signifies that the forward reaction occurs only to a very slight extent.
- Although a reaction may be thermodynamically favored, it may kinetically controlled.
- Thermodynamic equilibrium constant expression uses dimensionless unit known as activities.

15-5The Reaction Quotient, Q: Predicting the Direction of Net Change

- For an instantaneous conditions, the expression having the same form as K is called the reaction quotient, Q.
- The Q value is not constant for a reaction, but is useful for predicting the direction in which a net change must occur to establish equilibrium.
- If Q < K, a net change occurs in the forward direction, that is, from left to right.
- If Q > K, a net change proceeds in the reverse direction, that is, from right to left.
- If Q = K, the equilibrium is reached.

15-6Altering Equilibrium Conditions: Le Châtelier’s Principle

- Le Châtelier’s Principle: When a change (concentration, temperature, pressure, or volume) is imposed on a system at equilibrium, the system responds by attaining a new equilibrium condition that offsets the impact of the change.

(Continuous)

- Effect of Changing the Amounts of Reacting Species on Equilibrium
- At equilibrium, Q = K.
- If one of the reactants is added or one of the products is removed, resulted in Q < K, the concentrations of all species must change to make Q once again = K.
- If Q < K, the reaction is pushed (shift) to the right.

(Continuous)

- Effect of Changes in Pressure or Volume on Equilibrium
- Add an inert gas to the constant-volume reaction mixture: total pressure increased, but the partial pressure of the reactants and products do not change, no effect on the equilibrium condition.
- Change the pressure by changing the volume of the system: For example, if the volume is reduced, i.e., concentration increased, an equilibrium shifts in the direction producing the smaller number of moles of gas. (If no change in the number of moles of gas in a reaction, no effect on an equilibrium).

(Continuous)

- Effect of Temperature on Equilibrium

- The equilibrium constant (K) depend on temperature:

- For endothermic reaction (△Ho > 0), increase T, increase K, shift right for restoring the equilibrium.
- For exothermic reaction (△Ho < 0), increase T, decrease K, shift left for restoring the equilibrium.

(Continuous)

- Effect of a Catalyst on Equilibrium
- The role of a catalyst is to change the mechanism of a reaction to lower activation energy.
- Catalyst dosenot effectthe equilibrium.

15-7Equilibrium Calculations: Some Illustrative Examples

(Continuous)

?

(Continuous)

(Continuous)

(Continuous)

End of Chapter 15