equilibrium chapter 12. equilibrium chemical equilibrium is a dynamic condition in which...

EquilibriumChapter 12

Equilibrium� Chemical equilibrium is a dynamic condition

in which concentrations do not change and the rates of the forward and reverse reactions are equal.

� Meaning…� The reactants are making products at the same

rate that products are making reactants.

Equilibrium� Does this

graph look familiar?

� According to this Concentration v. Time graph, where is the equilibrium?

The equilibriumbeginshere.

Equilibrium� Hydrogen and nitrogen combine to form

ammonia at the same time ammonia is decomposing into hydrogen and nitrogen.

� The concentrations of hydrogen, nitrogen, and ammonia remain constant as both the forward and reverse reactions proceed at the same rate.

Equilibrium� Only reactions that meet these three

conditions can reach equilibrium:

• The reaction must occur in a closed container.

• The reaction must be a reversible reaction.• The reaction must have plenty of chemicals

to begin with.

Equilibrium� How is equilibrium measured?

with an Equilibrium Constant (Keq)

Equilibrium� For a reversible reaction (one that proceeds in

both the forward and reverse directions), the equilibrium constant (Keq) is a number showing the relationship between the mathematical product of the concentrations of the products divided by the mathematical products of the concentrations of the reactants, each raised to the power of its coefficient in a balanced equation.

Equilibrium� The equation to calculate this relationship is called an

equilibrium expression. � To determine the expression for any reaction:

� **Concentration (molarity) is represented by putting a [bracket] around a formula. [H+] means “the concentration of hydrogen ions”

Equilibrium� EXAMPLE:

3H2 (g) + N2 (g) 2NH3 (g) Write the equilibrium expression for the reaction

above:

• In the above Keq expression, [NH3] is raised to the power of 2 because its coefficient in the balanced equation is 2. [H2] is raised to the power of 3 because its coefficient is 3, and the power of [N2] is 1 because its coefficient is 1.

Equilibrium� Only use [GASES (g)] and [AQUEOUS

(aq)] compounds in Keq expressions because [SOLIDS (s)] and [LIQUIDS (l)] have no concentrations, so they do not need to be included.

Equilibrium� EXAMPLE:

C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l) Write the equilibrium expression for the reaction above:

(the [H2O] is not used because it’s a liquid)

Equilibrium� What does the calculated value for Keq tell

us?� Keq = 1 means that AT EQUILIBRIUM the

[products] = [reactants]� Keq > 1 means that AT EQUILIBRIUM the

[products] > [reactants]� Keq < 1 means that AT EQUILIBRIUM the

[products] < [reactants]

EquilibriumEXAMPLE:

A mixture of plenty of gaseous H2O and solid C are placed in a large container and allowed to come to equilibrium according to the following reaction:

C(s) + H2O(g) CO(g) + H2(g)The equilibrium concentrations of the species are as follows: [H2O] = 0.56 M, [CO] = 0.33 M, and [H2] = 0.040 M. What is the value of the equilibrium constant for this reaction?

EquilibriumEXAMPLE:

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

Does this equilibrium constant show that there are more products, more reactants, or equal amounts of the products and reactants? AND, how do you know?

There are more reactants; the value of Keq is smaller than 1.

EquilibriumEXAMPLE:

C(s) + H2O(g) CO(g) + H2(g)Let us assume that you run this reaction again in a new container with fresh water and carbon. This time, you are able to measure the carbon monoxide and hydrogen gas, only, finding [CO] = 0.15 M and [H2] = 0.045 M. What is the equilibrium concentration of water in this scenario?

You know the value of Keq… Keq 0.024You know the expression of Keq… Keq CO H2

H2O 0.024 0.15 0.045

x 0.024 x 0.15 0.045 0.024 x 0.00675

x 0.28125

So, [H2O] = 0.28 M

You know the expression of Keq… Keq CO H2

H2O

EquilibriumEXAMPLE:

4 HCl(aq) + MnO2(s) Cl2(g) + 2 H2O(l) + MnCl2(aq)

The equilibrium concentrations for this reaction are found to be: [HCl] = 1.20 M, [Cl2] = 0.78 M, and [MnCl2] = 0.015 M. What is the value of the equilibrium constant for this reaction?

Keq Cl2 MnCl2

HCl 4 0.78 0.015

1.20 4 0.005642

EquilibriumEXAMPLE:


Does this equilibrium constant show that there are more products, more reactants, or equal amounts of the products and reactants? AND, how do you know?

There are more reactants; the value of Keq is smaller than 1.

EquilibriumEXAMPLE:


Let us assume that you run this reaction again in a new container with fresh chemicals. At equilibrium, you measure [HCl] = 0.15 M and [Cl2] = 0.045 M. What is the equilibrium concentration of manganese(II) chloride in this reaction?

You know the value of Keq… Keq 0.005642You know the expression of Keq… Keq Cl2 MnCl2

HCl 4

0.005642 0.045 x

0.15 4

0.005642 0.15 4 0.045 x 0.000002856 0.045 x

x 0.0000634725

So, [MnCl2 ] = 0.0000635 M

Le Chatelier’s Principle

� When a system (reaction) that is at equilibrium is STRESSED, the rate of either the forward or reverse reaction must change to accommodate the new change in conditions. Therefore, the system will shift its equilibrium point in order to relieve the stress, and the concentrations of reactants and products will change.

Le Chatelier’s Principle� Think of a teeter totter….� If it is in balance, it can teeter and

totter back and forth with a nice smooth, constant motion.

� If weight is added to one side, that side goes down towards the ground and the other side flies upwards.

� In order to get back in balance, one person may have to push harder or faster, or change position.

� Eventually, the teeter totter will balance out and go up and down smoothly again, but each of the people on the teeter totter has had to make adjustments to get back into balance.


� Each stress alters the collisions, which alters the rate, which shifts the reaction. Stresses that may affect a chemical reaction:� concentration changes� energy changes (usually temperature changes)� volume changes


� Concentration Changes:� Changing the concentration of a reactant or

product:• If you add N2, the system will have to use it up by

making more NH3. Therefore the system will SHIFT towards the products, or to the RIGHT

• If you add H2, the system will SHIFT RIGHT• If you use up N2, the system will have to make more so

it will SHIFT towards the reactants, or SHIFT LEFT


� Concentration Changes (cont.):� Changing the concentration of a reactant or

product:• If you use up H2, the system will SHIFT LEFT• If you add NH3, the system will SHIFT LEFT to use

the excess NH3 up• If you use up NH3, the system will SHIFT RIGHT to

produce more


� Concentration Changes (cont.):� Introducing a substance not involved in

the reaction:• If you add O2 to the system… even though it is

not part of the reaction, O2 will react with H2 to form water. This will use up some H2, so the system will SHIFT LEFT to make up for that loss.


� Energy Changes� If HEAT is a PRODUCT:

• If you increase the temperature of the system, it will SHIFT LEFT to use up the excess heat.

• If you decrease the temperature of the system, it will SHIFT RIGHT to produce more heat


� Energy Changes (cont.)� If HEAT is a REACTANT:

• If you increase temperature, it will SHIFT RIGHT to use up excess heat

• If you decrease temperature, it will SHIFT LEFT to produce more heat


� Pressure changes� Pressure changes affect ONLY GASES!! Recall

that Boyles’ Law states that if you increase the pressure on a gas, it’s volume will decrease. If you decrease the pressure on a gas, it’s volume will increase.


� Pressure changes (cont.)� You must have a balanced equation before

determining equilibrium shifts based on pressure.


� Pressure changes (cont.)� Increase the pressure on the above system and it

will SHIFT RIGHT � WHY?� because there are fewer moles of gas (less

volume) on the right side. (there are 2 moles of gas on the right vs. 4 moles of gas on the left)


� Pressure changes (cont.)� Decrease the pressure on the above system and it

will SHIFT LEFT � WHY?� because that is were the greatest volume of gas

exists.

You Try…� Determine what will happen to the

equilibrium system below as each stress is added. (Describe how the rates will change.)� More NH3(aq) is added to the system.

Cu(NH3)42+(aq)

Blue-purple

Cu2+(aq) +Pale blue

4 NH3(aq)

colorless

More productis added so thereaction shifts

LEFT.

You Try…� Determine what will happen to the

equilibrium system below as each stress is added. (Describe how the rates will change.)� Solid Cu(NO3)2 is added to the system.

Cu(NH3)42+(aq)

Blue-purple

Cu2+(aq) +Pale blue

4 NH3(aq)

colorless

More product (Cu2+)is added so thereaction shifts

LEFT.

You Try…� Determine what will happen to the equilibrium

system below as each stress is added. (Describe how the rates will change.)� Solid NaOH is added to the system.

� [Cu2+ + 2 OH– Cu(OH)2]

Cu(NH3)42+(aq)

Blue-purple

Cu2+(aq) +Pale blue

4 NH3(aq)

colorless

Since Cu2+ reacts with OH– to create Cu(OH)2…

Less product is present so the reaction shifts RIGHT.

equilibrium chapter 12. equilibrium chemical equilibrium is a dynamic condition in which...

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