Chapter 13

Equilibrium

Chemical Equilibrium

• The state where the concentrations of all reactants and products remain constant with time– The concentration of each species does NOT

have to be the same

• The forward and reverse reactions happen at the same rate

• Relative amounts follow stoichiometry

Chemical Equilibrium Is…..

• Dynamic

– Reactions are always taking place

– Reactant turns into product

• Forward Reaction

– Product turns back into reactant

• Reverse Reaction

Progress Toward Equilibrium

• Reactions don’t start at equilibrium

• In the beginning there are (usually) only reactants– No reverse reaction

• As the reaction proceeds product molecules are produced and the reverse reaction begins

• Eventually equilibrium is reached

N2 + 2H2 2NH3 (Fig 13.5)

Equilibrium Constant Expressions

• A mathematical equation that relates the concentration of products to the concentration of reactants for a reaction at equilibrium

• The equilibrium constant is Keq• K for Konstant, eq for equilibrium• General form of the expression is

concentration of products over concentration of reactants– Pressure can also be used

The Equilibrium Constant Expression

• For the reaction

jA + kB lC + mD• Where j, k, l, & m are coefficients and • A, B, C, and D are chemicals• The equilibrium constant expression is

kj

ml

BA

DCKeq

][][

][][

Equilibrium Constant Expressions

• A species in brackets indicates concentration

• Only solutions and gases are included in equilibrium expressions

• If you have a solid or liquid leave it out– Just think of it as a 1 in the expression

The Equilibrium Constant

• A value that relates the position of a reaction at equilibrium

• Calculated by inserting values into the equilibrium constant expression

• Keq does NOT have units• Keq > 1

– Product favored

• Keq <1– Reactant favored

Things To Consider

• The equilibrium constant is only constant for a given temperature– If the temp changes so does the constant

• There are many sets of conditions that will satisfy the equilibrium constant at a given temperature– For example if K=4 = (4/1) = (16/4) = (1/.25)

Other Fun

• If a reaction is reversed, what happens to K?

• K’ = 1/K

• If a reaction is multiplied through by a coefficient what happens to K

• K’ = Kn

Given the reaction

2PBr3(g) + 3Cl2(g) 2PCl3(g) + 3Br2(g)

A) Write the equilibrium constant expression

B) At equilibrium the concentrations are

PBr3=2.0M Cl2=3.0M PCl3=.10M Br2=.15M

Calculate the equilibrium constant

C) Calculate the equilibrium constant if the above reaction is reversed and the concentrations are the same

D) Calculate the equilibrium constant if the above reaction is divided by 2.

Homework

• Page 645 #’s 15,17,19ab,21ab,24,26

Equilibrium Involving Pressure

• Pressure of gases can be used in equilibrium expressions too.

• Since PV=nRT

• Then n/V = Concentration

• So P = CRT Pressure of a gas is directly related to concentration

• When a gas pressure is used Keq is Kp

Kp Expression

• For the reaction

jA + kB lC + mD• Where j, k, l, & m are coefficients and • A, B, C, and D are chemicals in the gas phase• The equilibrium constant expression is

kB

jA

mD

lC

PP

PPKp

)()(

)()(

• Write the Kp and Kc expressions for the reaction

PCl5(s) PCl3(g) +Cl2(g)

Kp to Kc

• Kp and Kc can be determined from each other.

• Kp = Kc(RT)Δn

• Where n is (l+m) – (j+k)

• Sum of product coefficients minus sum of reactant coefficients

Is it equilibrium?• Sometimes there are lots of chemicals in a

container. Need to check if it is at equilibrium.

• Use reaction quotient, Q

• Found same way as K but has a different meaning

• Q = K Reaction at equilibrium

• Q < K Needs more product shifts right

• Q > K Needs more reactants shifts left

Solving Equilibrium Problems

• Write the equation

• Write equilibrium expression

• List what is given

• Determine what needs to be solved

• Set up a solution

• #40 page 647

2NO(g) + Br2(g) 2NOBr(g)

• #43 page 647

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

• #45 page 647

SO2(g) NO2(g) SO3(g) + NO(g)

Homework

• Page 646 #’s 28,30,35,39,44,46

• #50 page 647

N2O4(g) 2NO2(g) Kp=.25

Initial pressure of N2O4 is 4.5 atm

Initial pressure of NO2 is 0 atm

• #50 page 647

N2O4(g) 2NO2(g) Kc=4.0x10-7

Initial [N2O4] of is 0.10M

Initial [NO2] is 0M

My Equilibrium is Stressed

• A reaction at equilibrium can be stressed out by changing reaction conditions

– There are three stresses for reactions

• Concentration

• Pressure

• Temperature

LeChatlier’s Principle

• When a stress is applied to a reaction at equilibrium the reaction will shift to relieve the stress placed upon it.

Concentration

• A reaction is stressed when the concentration of a reactant or product is increased OR decreased– Add a chemical to increase– Remove a chemical to decrease

• Reactions shift away from increased concentrations

• Reactions shift toward decreased concentrations

Example• Consider the reaction

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

• How will the reaction shift with the following– Increase the amount of SO2

• Right– Increase the amount of SO3

• Left– Decrease the amount of O2

• Left

Pressure

• Pressure can be increased

• By decreasing volume

• Pressure can be decreased

• By increasing volume

• Increased pressure shifts reactions away from side with more gas molecules

• Decreased pressure shifts reactions toward side with more gas molecules

Example

• Consider the reaction

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

• How will the reaction shift with the following– Increase the pressure in the container

• Right– Decrease the pressure in the container

• Left

Temperature

• Direction of shift depends of whether the reaction is exothermic or endothermic

– How do you know

• Exothermic reactions release heat

–It is a product

• Endothermic reactions absorb heat

–It is a reactant

What Type of Thermic Is It?

• Exothermic

A + B C + 49kJ

A + B C ΔH = -49kJ

• ΔH means enthalpy (energy change)

• Endothermic

A + B + 49kJ C

A + B C ΔH = 49kJ

Temperature Change

• Think of the temp as a concentration of heat– Increasing the temp. will shift the reaction

away from the heat– Decreasing the temp. will shift the

reaction toward the heat.• Same as concentration shift

• Change in temp. is the only stress that changes the equilibrium constant.

Example

• Consider the reaction

2SO2(g) + O2(g) 2SO3(g) + 197kJ

• How will the reaction shift with the following

– Increase the temperature

• Left

– Decrease the temperature

• Right

Homework

• Page 647 #’s 47,49,54,59,62