Chapter 13Chemical Equilibrium

Chapter 13Chemical Equilibrium

Many reactions do not go to completion instead of reaching an equilibrium state

Chemical Equilibrium:

-The state reached when the concentrations of reactants and products remain constant over time.

-The rate forward and reverse have become equal

Equilibrium is a dynamic process . The conversions of reactants to products and products to reactants are still going on, although there is no net change in the number ofreactant and product molecules.

N2O4(g) 2NO2(g)

13.2 The Equilibrium Constant, Kc For a reaction aA + bB cC + dD

The Equilibrium Constant Kc,

Law of mass action - The value of the equilibrium constant expression, Kc, is constant for a

given reaction at equilibrium and at a constant temperature.

⇒ The equilibrium concentrations of reactants and products may vary, but the value for Kc remains the same.

Other Characteristics of Kc

1) Equilibrium can be approached from either direction.

2) Kc does not depend on the initial concentrations of reactants and products.

3) Kc does depend on temperature.

4) Kc value is written without units

Using the Equilibrium Constant

When we know the numerical value of the equilibrium constant, we can make certain judgments about the extent of the chemical reaction

Writing Equilibrium Expression

Simply write the chemical formula products over the chemical formula of the reactants

 For homogenous equilibrium

◦Reactants and products are in the same phases

◦E.gCH4(g) + H2O(g) CO(g) + 3 H2(g)

For heterogeneousequilibrium

◦Reactants and products are in different phases

E.gCaCO3(s) CaO(s) + CO2(g)Kc = [CO2]

13.3 The Equilibrium Constant Kp

When writing an equilibrium expression for a gaseous reaction in terms of partially pressure, we call it equilibrium constant, Kp

N2O4(g) 2NO2(g)

Relating the Equilibrium Constant Kp and Kc

R = gas constant (0.08206 L atm/K mol)

T = Temperature (in K)

∆n = is the number of moles of gaseous products minus the number of moles of gaseous reactants.

Writing K’c and K’p

The reverse equilibrium constant, K’c, is the ratio of the equilibrium concentrations of reactants over the equilibrium concentrations of products each raised to the power of their stoichiometric coefficients

a.2 SO2(g) + O2(g) 2 SO3(g)

b.CO(g)+ 2H2(g) CH3OH(g)

Example

Does Kc = Kp for

(a) H2(g) + F2(g) 2HF(g)? (b) 2SO2(g) + O2(g) 2SO3(g)?

For the reaction, 2SO2(g) + O2(g) 2SO3(g)

 (a) write the equilibrium constant expression, Kp.

(b) What is the value for Kp if Kc = 2.8x102 at 1000 K?

Examples

Consider the following unbalanced reaction

(NH4)2S(s) 2NH3(g) + H2S(g)

An equilibrium mixture of this mixture at a certain temperature was found to have [NH3] = 0.278 M and [H2S] = 0.355 M. What is the value of the equilibrium constant (Kc) at this temperature?

Predicting the Direction of Reaction

If Q > K, the reaction will go to the left.

• The ratio of products over reactants is too large & the reaction will move toward equilibrium

by forming more reactants.

2. If Q < K, the reaction will go to the right.

• The ratio of products over reactants is too small & the reaction will move toward

equilibrium by forming more products.

3. If Q = K, the reaction mixture is already at equilibrium, so no shift occurs.

Example: For the reaction, B 2A, Kc = 2. Suppose 3.0 moles of A and 3.0 moles of B are introduced into a 2.00 L flask. (a) In which direction will the reaction proceed to attain equilibrium?

(b) Will the concentration of B increase, decrease or remain the same as the system moves towards equilibrium?

Finding Equilibrium concentrations from Initial Concentrations

Steps to follow in calculating equilibrium concentrations from initial concentration

Write a balance equation for the reaction

Make an ICE (Initial, Change, Equilibrium) table, involves

The initial concentrations

The change in concentration on going to equilibrium, defined as x

The equilibrium concentration

Substitute the equilibrium concentrations into the equilibrium equation for the reaction and solve for x

Calculate the equilibrium concentrations form the calculated value of x

Check your answers

Calculating Equilibrium Concentrations

Sometimes you must use quadratic equation to solve for x, choose the mathematical solution that makes chemical sense

Quadratic equationax2 + bx + c = 0

For homogeneous equilibrium

aA(g) bB(g) + cC(g)

Initial concentration (M)[A]initial [B] initial [C] initial

Change in concentration(unknown M ) - ax+ bx+ cx

Equilibrium[A] initial - ax[B] initial+ bx[C] initial + cx

Forheterogeneousequilibrium

aA(g) bB(g) + cC(s)

Initial concentration (M)[A]initial [B] initial …….

Change in concentration(unknown M ) - ax+ bx……….

Equilibrium[A] initial - ax[B] initial+ bx…………

Examples

The value of Kc for the reaction is 3.0 x 10-2. Determine the equilibrium concentration if the initial concentration of water is 8.75 M

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

Examples

Consider the following reaction

I2(g) + Cl2(g) 2ICl(g) Kp = 81.9 (at 25oC)

A reaction mixture at 25oC initially contains PI2 = 0.100 atm, PCl2 = 0.100 atm, and PICl = 0.100 atm. Find the equilibrium pressure of I2, Cl2 and ICl at this temperature.

In which direction does the reaction favored?

Example

At 700 K, 0.500 mol of HI is added to a 2.00 L container and allowed to come to equilibrium. Calculate the equilibrium concentrations of H2, I2, and HI . Kc is 57.0 at 700 K.

H2(g) + I2(g) 2HI(g)

A flask initially contained hydrogen sulfide at a pressure of 5.00 atm at 313 K. When the reaction reached equilibrium, the partial pressure of sulfur vapor was found to be 0.15 atm. What is the Kp for the reaction?

2H2S(g) 2H2(g) + S2(g)

13.6 Le Châtelier’s Principle

Le Châtelier’s Principle: If a stress is applied to a reaction mixture at equilibrium, net reaction occurs in the direction that relieves the stress.

13.7Altering an Equilibrium Mixture: Concentration

N2(g) + 3H2(g) 2NH3(g)

Example: An equilibrium mixture of 0.50 M N2, 3.00 M H2, and 1.98 M NH3 is disturbed by increasing the N2 concentration to 1.50 M. Which direction will the net reaction shift to re-establish the equilibrium?

Example

The reaction of iron (III) oxide with carbon monoxide occurs in a blast furnace when iron ore is reduced to iron metal:

Fe2O3(s) + 3 CO(g) 2 Fe(l) + 3CO2(g)

Use Le Chatellier’s principle to predict the direction of net reaction when an equilibrium mixture is disturbed by:

a.adding Fe2O3

b.Removing CO2

c.Removing CO; also account for the change using the reaction quotient Qc

13.8 Altering an Equilibrium Mixture: Changes in Pressure and Volume

N2(g) + 3H2(g) 2NH3(g)

when an equilibrium is disturbed by a change in volume which results in a corresponding change in pressure, Le Châtelier’s principle predicts that

•an increasein pressure by reducing the volume will bring about net reaction in the direction that decreases the number of moles of gas.

•a decreasein pressure by enlarging the volume will bring about net reaction in the direction that increasesthe number of moles of gas.

Example

Consider the following reaction at chemical equilibrium

2KClO3(s) 2 KCl(s) + 3O2(g)

a. What is the effect of decreasing the volume of the reaction mixture?

b.Increasing the volume of the reaction mixture?

c. Adding inert gas at constant volume?

Examples

Does the number moles of products increases, decreases or remain the same when each of the following equilibrium is subjected to a increase in pressure by decreasing the volume?

◦PCl5(g) PCl3(g) + Cl2(g)

◦3 Fe(s) + 4H2O(g) Fe3O4(s) + 4 H2(g)

Altering an Equilibrium Mixture: Temperature

Heat can be considered a reactant in an endothermic rxn and a product in an exothermic rxn.

Endothermic (ΔH > 0) R + Heat  Products

Exothermic (ΔH < 0) R  Products + Heat

When an equilibrium is disturbed by a change in temperature, Le Châtelier’s principle predicts that

•the equilibrium constant for an exothermicreaction (negative DH°) decreasesas the temperature increases.

•Contains more reactant than product

•Kc decreases with increasing temperature

•the equilibrium constant for an endothermicreaction (positive DH°) increasesas the temperature increases.

•Contains more product than reactant

•Kc increases with increasing temperature

Example

The following reaction is endothermic

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

a.What is the effect of increasing the temperature of the reaction mixture?

b.Decreasing the temperature?

Examples

In the first step of Ostwald process for the synthesis of nitric acid, ammonia is oxidized to nitric oxide by the reaction:

2 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(l) ΔHo = -905 kJ

How does the temperature amount of NO vary with an increases in temperature?

Does Kc become larger or smaller

The Effect of a Catalyst on Equilibrium

Catalyst increases the rate of a chemical reaction

◦Provide a new, lower energy pathway

◦Forward and reverse reactions pass through the same transition state

◦Rate for forward and reverse reactions increase by the same factor

◦Does not affect the composition of the equilibrium mixture

◦Does not appear in the balance chemical equation

◦Can influence choice of optimum condition for a reaction

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