TOPIC 17 TOPIC 17 EQUILIBRIUM EQUILIBRIUM

TOPIC 17 TOPIC 17

EQUILIBRIUM EQUILIBRIUM

17.1THE EQUILIBRIUM LAW

By: Merinda Sautel Alameda Int’l Jr/Sr High

School Lakewood, CO [email protected]

ESSENTIAL IDEA

The position of equilibrium can be quantified by the equilibrium law. The equilibrium constant for

a particular reaction only depends on the temperature.

NATURE OF SCIENCE (1.8 and 1.9)

Employing quantitative reasoning – experimentally determined rate expressions for forward and backward reactions can be deduced directly from the stoichiometric equations and allow Le Chatelier’s principle to be applied.

THEORY OF KNOWLEDGE

The equilibrium law can be deduced by assuming that the order of the forward reaction and backward reaction matches the coefficients in the chemical equation. What

is the role of deductive reasoning in science?

We can use mathematics successfully to model equilibrium systems. Is this because we create

mathematics to mirror reality or because the reality is intrinsically mathematical?

Many problems in science can only be solved when assumptions are made which simplify the mathematics.

What is the role of intuition in problem solving?

APPLICATION/SKILLS

Solution of homogeneous

equilibrium problems using the

expression for K

.

A homogeneous system is one in which

all substances are in the same phase for

example all gases or all solutions.

Heterogeneous Equilibria

The position of a heterogeneous equilibrium

does not depend on the amounts of pure solids or liquids present. Liquids and solids are

never a part of the K expression.

Write the equilibrium expression for the reaction:

PCl₅(s)  PCl₃(l) + Cl₂(g)

Pure

solid Pure

liquid

 K  [ C l

]

Remember the Equilibrium Constant K

[ ] [ ] [ ] [ ]

l m

j k

K C D

 A B

Where

Where K K

is the equilibrium constant, is the equilibrium constant, and is unitless.

and is unitless. The only thing that The only thing that can change K

can change K

for a reaction is a for a reaction is a change in temperature.

change in temperature.

jA + kB  lC + mD

Solving for Equilibrium Concentration

Consider this reaction at some temperature:

H₂O(g) + CO(g)  H₂(g) + CO₂(g) K = 2.0 Assume you start with 8 moles of H₂O and 6 moles of CO in a 1 dm³ container. How many moles of H₂O, CO, H₂, and CO₂ are present at equilibrium?

Here, we learn about “ICE” – the most “ICE”

important problem solving technique in solving all equilibrium problems.

ICE TABLES

Only molar concentrations are used in ICE tables.

ICE stands for

“Initial” concentrations,

“Change” in concentrations, and

“Equilibrium” concentrations.

Solving for Equilibrium Concentration

H₂O(g) + CO(g)  H₂(g) + CO₂(g) K = 2.0 Step #1: We write the K expression for

the reaction. Always use concentrations in ICE tables.

2 2

2

[ ][ ]

2.0 [ ][ ]

H CO H O CO



Solving for Equilibrium Concentration

H₂O(g) + CO(g)  H₂(g) + CO₂(g) Initial:I

Change:

Change:

Equilibrium:

Equilibrium:

Step #2: We make an ICE table using the initial concentrations. The “change” comes from the coefficients.

8 6 0 0

-x -x +x +x

8-x 6-x x x

Solving for Equilibrium Concentration

Equilibrium:

Equilibrium: 8-x8-x 6-x6-x xx xx Step #3: We plug equilibrium concentrations into our equilibrium expression, and solve for x

H₂O(g) + CO(g)  H₂(g) + CO₂(g)

( )( )

2.0 (8 )(6 )

x x

x x

  

4

x 

ADDITIONAL COMMENTS

• Many equilibrium ICE tables have to be solved using the quadratic equation.

• You will not be expected to use calculations involving the quadratic equation.

• When K

is very small (10

or smaller), you will be making an assumption that the

change in concentration of the reactants is

basically zero.

UNDERSTANDING/KEY IDEA 17.1.A

Le Chatelier’s principle for changes in concentration can be explained

by the equilibrium law.

LeChatelier’s Principle

When a system at equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress.

Translated:

Translated: The system undergoes a temporary The system undergoes a temporary shift in order to restore equilibrium.

shift in order to restore equilibrium.

1. When you add a substance or heat, the 1. When you add a substance or heat, the

system shifts to the opposite side.

system shifts to the opposite side.

2. When you take out a substance or heat, the 2. When you take out a substance or heat, the

system shifts to the side of the take out.

system shifts to the side of the take out.

3.3. When you increase pressure, the system When you increase pressure, the system

shifts to the side with the least number of shifts to the side with the least number of

gaseous molecules gaseous molecules..

KINETICS AND EQUILIBRIUM

• The equilibrium constant is the ratio of the rate constants of the forward and backward

reactions.

• If “k” for the forward reaction is greater than “k”

for the backward reaction, then the reaction is product favored and proceeds toward

completion. K is large.

• If “k” for the backward reaction is greater than

“k” for the forward reaction, then the reaction

has barely taken place. K is small.

• Using the relationship between k, the rate constant, and K, the equilibrium constant, we can add to our interpretation of how

equilibrium responds to changing

conditions.

CHANGING CONCENTRATION

• If you increase the concentration of the

reactant(s), you will increase the rate of the forward reaction thus shifting equilibrium to the right.

• If you increase the concentration of the

product(s), you will increase the rate of the backward reaction thus shifting equilibrium to the left.

• The value of the equilibrium constant, K,

stays constant since only temperature affects

K.

ADDING A CATALYST

• Adding a catalyst increases the rate constants, k, of both the forward and

reverse reactions by the same amount so

the equilibrium constant is not affected.

CHANGING TEMPERATURE

• We know from kinetics and our studies of the Arrhenius equation, k = Ae

, that as

temperature increases, the rate constant “k”

increases.

• From the potential energy diagrams, we know

that the activation energies of the forward and

backward reactions are different so their rate

constants would be different and are affected

differently by temperature.

• The ratio of the rate constants of the forward and backward reactions is temperature dependent.

• For an endothermic reaction, where E

of the forward reaction is greater than that of the backward reaction, the increase in

temperature has a greater effect on

increasing “k” for the forward reaction so K

increases as temperature increases.

HOW DO YOU PROVE IT?

• Take the time with your partner and prove the previous statement using the potential energy diagrams and the

Arrhenius equation k = Ae

.

UNDERSTANDING/KEY IDEA 17.1.B

The position of equilibrium

corresponds to a maximum value

of entropy and a minimum value of

the Gibbs free energy.

• Different reactions have very different K

values reflecting a wide range of the

directions and extents of these reactions.

• Some reactions go almost all the way to completion and others barely react at all.

• Gibbs free energy can help determine

these differences in reactions.

• ΔG

= negative

– Spontaneous

– Reaction proceeds in the forward direction

• ΔG

= positive

– Non-spontaneous

– Reaction proceeds in the backward direction

• ΔG

= zero

– Reaction is at equilibrium

• A reaction with a value of ΔG

that is large and negative appears to occur

spontaneously and has an equilibrium

mixture with a high proportion of products.

• A reaction with a value of ΔG

that is large and positive appears to be non-

spontaneous and has an equilibrium

mixture with a high proportion of reactants.

pixshark.com

ΔG neg ΔG neg

ΔG pos

Non-

spontaneous

ΔG pos

Non-

spontaneous

The position of equilibrium corresponds to a maximum value of entropy for the system.

In other words, any system has the highest

possible value of entropy when free energy

is at a minimum (equilibrium).

UNDERSTANDING/KEY IDEA 17.1.C

The Gibb’s free energy change of a

reaction and the equilibrium constant can both be used to measure the

position of an equilibrium reaction and are related by the equation,

ΔG

= -RTlnK.

APPLICATION/SKILLS

Understand the relationship

between ΔG and the equilibrium

constant.

APPLICATION/SKILLS

Be able to do calculations with the

equation ΔG

= -RTlnK.

K

AND THERMO DATA

• We now have 2 terms which relate to the position of equilibrium.

– K_c, the equilibrium constant

– ΔG^◦, the change in free energy

• From the equation, ΔG

= -RTlnK, we can deduce relationships between the two

values, K

and ΔG

.

ΔG^◦ negative ln K positive K>1 mostly products ΔG^◦ positive ln K negative K<1 mostly reactants ΔG^◦ = 0 ln K = 0 K=1 mixture of both

The equation ΔG^◦ = -RTlnK can be used to solve for either Gibb’s free energy or the equilibrium constant.

It is useful in situations where the K is difficult to measure

directly such as if the reaction is too slow to reach equilibrium or when the component amounts are too small to measure.

The value of Gibb’s free energy is the reason that different reactions have such different values for K.

Remember that Gibb’s free energy is also dependent upon the change in enthalpy and change in entropy ΔG^◦ = ΔH – TΔS.

Citations

International Baccalaureate Organization. Chemistry Guide, First assessment 2016. Updated 2015.

Brown, Catrin, and Mike Ford. Higher Level Chemistry. 2nd ed.

N.p.: Pearson Baccalaureate, 2014. Print.

ISBN 978 1 447 95975 5 eBook 978 1 447 95976 2

Most of the information found in this power point comes directly from this textbook.

The power point has been made to directly complement the Higher Level Chemistry textbook by Brown and Ford and is used for direct instructional purposes only.