Chapter 12 Chemical Equilibrium

Chapter 12 ChemicalEquilibrium

Publishing as Benjamin Cummings

Collision Theory of Reactions

A chemical reaction occurs when:

•Collisions between molecules have sufficient energyto break the bonds in the reactants.

•Molecules collide with the proper orientation.

•Bonds between atoms of the reactants (N2 and O2)are broken and new bonds (NO) can form.

Publishing as Benjamin Cummings

Collision Theory of Reactions

A chemical reaction does not take place if the:

•Collisions between molecules do not have sufficientenergy to break the bonds in the reactants.

•Molecules are not properly aligned.

Publishing as Benjamin Cummings

N2 O2

Activation Energy

•The activation energyis the minimumenergy needed for areaction to takeplace.

•When a collisionprovides energyequal to or greaterthan the activationenergy, product canform.

Reaction Rate and Temperature

Reaction rate,

•Is the speed at which reactant is used up.

•Is the speed at which product forms.

•Increases when temperature rises becausereacting molecules move faster providing morecolliding molecules with energy of activation.

Reaction Rate and Concentration

Reaction rate,

•Is affected by theconcentration of thereactants.

•Is increased by addingmore reactants, whichincreases the number ofcollisions.

Reaction Rate and Catalysts

A catalyst:

•Speeds up the rate ofa reaction.

•Lowers the energy ofactivation.

•Is not used up duringthe reaction.

Summary of Factors and ReactionRate

Learning Check

Indicate the effect of each factor listed on the rate of the

following reaction as: 1) increases 2) decreases

3) none

2CO(g) + O2(g) 2CO2 (g)

A. Raising the temperature

B. Adding O2

C. Adding a catalyst

D. Lowering the temperature

Reversible Reactions

In a reversible reaction, there is both a forward and a

reverse reaction.

•Suppose SO2 and O2 are present initially. As theycollide, the forward reaction begins.

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

•As SO3 molecules form, they also collide in thereverse reaction that forms reactants. The reversiblereaction is written with a double arrow.

forward

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

reverse

Chemical Equilibrium

In a system at equilibrium

•The rate of the forwardreaction becomes equalto the rate of the reversereaction.

•The forward and reversereactions continue atequal rates in bothdirections.

•There is no furtherchange in the amountsof reactant and product.

Equilibrium

A reaction of N2 and O2 forms NO, which reacts in the

reverse direction. At equilibrium, the amounts of N2,

O2, and NO remain constant.

N2(g) + O2(g) 2NO(g)

Learning Check

Complete with

1. equal 2. not equal 3. forward

4. reverse 5. changes 6. does not change

A. Reactants form products in the ________ reaction.

B. At equilibrium, the reactant concentration _______.

C. When products form reactants, it is the _______ reaction.

D. At equilibrium, the rate of the forward reaction is ______ to the

rate of the reverse reaction.

E. If the forward reaction is faster than the reverse, the amount of

products ________.

Equilibrium Constants

For the following reaction:

aA bB

•The equilibrium constant expression, Kc, gives theconcentrations of the reactants and products atequilibrium.

Kc = [B]b

[A]a

•The square brackets indicate the moles/liter of eachsubstance.

•The coefficients b and a are written as superscriptsthat raise the moles/liter to a specific power.

Writing a Kc Expression

•In the Kc expression for the following reaction atequilibrium:

2CO(g) + O2(g) 2CO2(g) STEP 1

•The products are shown in the numerator and thereactants are shown in the denominator.

Kc= [CO2] [products] STEP 2

[CO] [O2] [reactants]

•The coefficients are written as superscripts. STEP 3

Kc= [CO2]2

[CO]2 [O2]

Learning Check

The Kc expression for the following reaction is:

N2(g) +3Cl2(g) 2NCl3(g)

1) [NCl3]2) [N2][Cl2]3

[N2][Cl2] [NCl3]2

3) [NCl3]24) [NCl3]2

[N2]3 [Cl2] [N2][Cl2]3

Heterogeneous Equilibrium

In heterogeneous equilibrium,

•Solid and/or liquid states may be part of a reaction.

2NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(g)

•The concentration of solids and/or liquids is constant.

•The Kc expression is written with only the gases.

Kc = [CO2][H2O]

Calculating EquilibriumConstants

When the following reaction,

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

reaches equilibrium, the concentrations are:

H2 1.2 mol/L, I2 1.2 mol/L, and HI 0.35 mol/L.

What is the Kc?

STEP 1 Write the Kc expression

Kc = [HI]2

[H2][I2]

STEP 2 Substitute equilibrium concentrations.

= (0.35)2 = 8.5 x 10-2

(1.2)(1.2)

Reaching Chemical Equilibrium

A container filled with SO2 and O2 or only SO3,

•Eventually contains mostly SO3 and small amountsof O2 and SO3.

•Reaches equilibrium in both situations.

Equilibrium Can Favor Product

If equilibrium is reached after most of the forward

reaction has occurred,

•The system favors the product.

Equilibrium with a Large Kc

At equilibrium,

•A reaction with a large Kc produces a large amount ofproduct. Very little of the reactants remain.

Kc=[NCl3]2 = 3.2 x 1011

[N2][Cl2]3

•A large Kc favors the products.

N2(g) + 3Cl2(g) 2NCl3(g)

When this reaction reaches equilibrium, it will essentiallyconsist of the product NCl3.

Equilibrium Can Favor Reactant

•If equilibrium is reached when very little of theforward reaction has occurred,

•The reaction favors the reactants.

Equilibrium with a Small Kc

At equilibrium,

•A reaction that produces only a small amount of producthas a small Kc.

Kc = [NO]2 =2.3 x 10-9

[N2] [O2]

•A small Kc favors the reactants.

N2(g) + O2(g) 2NO(g)

Learning Check

For each Kc, indicate whether the reaction at equilibrium

contains mostly: (1) reactants or (2) products

__A. H2(g) + F2(g) 2HF(g)

Kc = 1 x 1095

__B. 3O2(g) 2O3(g)

Kc = 1.8 x 10-7

Using Kc to Solve for EquilibriumConcentration

At equilibrium, the reaction PCl5(g) PCl3(g) + Cl2(g)

has a Kc of 4.2 x 10-2 and contains [PCl3] = [Cl2] = 0.10 M.

What is the equilibrium concentration of PCl5?

LeChâtelier’s Principle

LeChâtelier’s principle states that:

•Any change in equilibrium conditionsupsets the equilibrium of the system.

•A system at equilibrium under stress willshift to relieve the stress.

•There will be a change in the rate of theforward or reverse reaction to returnthe system to equilibrium.

Summary of Changes onEquilibrium

Condition Changes andEquilibrium

Learning Check

Indicate if each change on a reaction at equilibrium shifts

2NO2( g) + heat 2NO(g) + O2(g)

1) toward products2) toward reactants

3) no change

A. Adding NO

B. Adding N2

C. Raising the temperature

D. Removing O2

E. Increasing the volume

Saturated Solution

A saturated solution:

•Contains the maximum amount of dissolvedsolute.

•Contains solid solute.

•Is an equilibrium system.

rate of dissolving = rate of recrystallization

solidions in solution

Solubility Product Constant

The solubility product for a saturated solution:

•Gives the ion concentrations, which are constant atconstant temperature.

•Is expressed as a Ksp.

•Does not include the solid, which is constant.

•With Fe(OH)2(s) Fe2+(aq) + 2OH−(aq)

IsKsp = [Fe2+] [OH−]2

Learning Check

Write the Ksp expression for each of thefollowing:

A. FeS(s)

B. Ag2CO3(s)

Calculating Solubility ProductConstant

Calculate the Ksp of PbSO4 with a solubility of 1.4 x 10-4 M.

STEP 1

PbSO4(s) Pb2+(aq) + SO42−(aq)

STEP 2

Ksp = [Pb2+][SO42−]

STEP 3

= (1.4 x 10-4) x (1.4 x 10-4)

= 2.0 x 10-8

Solubility Product Constant