Chemical equilibrium

Question 1

In a spontaneous reaction at constant temperature and pressure is ΔrG positive or negative.

Answer 1

ΔrG < 0

Question 2

What is ΔrG?

Answer 2

ΔrG < 0 = the difference in molar Gibbs energy between the product and reactant.

Question 3

What occurs at the lowest Gibbs energy in the reaction?

Answer 3

The lowest Gibbs energy is achieved when the reactants are completely converted to products.

Question 4

What is a dynamic equilibrium?

Answer 4

when the overall proportion of reactants and products doesn’t change, individual molecules may still react—but, for every molecule that undergoes the forward reaction, another molecule will undergo the reverse reaction. There is a dynamic equilibrium.

At dynamic equilibrium, the reaction rate of the forward reaction is equal to the reaction rate of the backward reaction.

Question 5

Basic definition of equilibrium constant, K.

Answer 5

The composition of a reaction mixture in terms of equilibrium

Question 6

What are the three types of equilibrium constant?

Answer 6

Kp - equilibrium constant in terms of pressures.
Kc - equilibrium constant in terms of concentrations
K - equilibrium constant in terms of activities

Question 7

How is Kp determined?

Answer 7

For gas reactions, an equilibrium constant, Kp, can be calculated in terms of equilibrium partial pressures, Peqm.

Question 8

How is Kc determined?

Answer 8

For reactions in solution, Kc, calculated in terms of the molar concentrations is more useful.

Question 9

How do you define the thermodynamic equilibrium constant, K?

Answer 9

K is defined as the ratio of the thermodynamic activities of the products at equilibrium to those of the reactants

Question 10

What is Activity?

Answer 10

Activity is a measure of the effective concentration of a species under non-ideal (e.g., concentrated) conditions

Can be used to calculate equilibrium constants and reaction rates.

Question 11

The activity of an dissolved species may be determined from what expression?

Answer 11

[A]/[A]° = [A]/1 mol dm^−3

Question 12

the activity of a gaseous species may be determined from what expression?

Answer 12

p/p° = p/1 bar.

Question 13

What units do activities have?

Answer 13

No units

Question 14

reaction quotient, Q: definition:

Answer 14

The reaction quotient, Q, is a measure of the relative amounts of products and reactants present in a reaction at a given time.

Question 15

How can you investigate how G changes?

Answer 15

You need a measure of the composition of the reaction mixture as it changes during the reaction.

Use the reaction quotient.

Question 16

Properties of the reaction quotient:

Answer 16

Similar expression to the thermodynamic equilibrium constant.

The activities are not at equilibrium.

Question 17

What happens to K and Q when the system reaches equilibrium?

Answer 17

When the system reaches equilibrium, the value of Q equals the value of K.

Question 18

How can you use the reaction quotient to predict the direction of the reaction?

Answer 18

If ΔrG < 0 or Q < K, the forward reaction proceeds.
If ΔrG > 0 or Q > K, the reverse reaction proceeds.
If ΔrG = 0 or Q = K, the system is at equilibrium.

Question 19

When does a reaction reach equilibrium?

Answer 19

A reaction reaches equilibrium when it reaches its minimum Gibbs energy at constant temperature and pressure.

Question 20

What’s the expression used to show the quantitative relationship between Gibbs energy and K:

Answer 20

ΔrG° = -RT lnK

ΔrG° =Standard Gibbs energy change
R = gas constant = 8.314 J/K/mol
T = temperature in K
lnK= natural log of the thermodynamic equilibrium constant

Question 21

What does it mean for the equilibrium mixture if K is large (K&raquo_space; 1)?

Answer 21

If K is large (K&raquo_space; 1) then the equilibrium mixture will contain mainly products.

Question 22

What does it mean for the equilibrium mixture if K is small (K &laquo_space;1)?

Answer 22

If K is small (K &laquo_space;1) then the reaction does not proceed vary far and little product forms.

Question 23

What does it mean for the equilibrium mixture if K ≈ 1?

Answer 23

the equilibrium mixture will contain substantial amounts of both product and reactant.

Question 24

How can you know if a reaction is spontaneous?

Answer 24

Around RT, say at 298 K, if the value of ΔrG for the reaction is negative, the reaction is spontaneous.

Question 25

What is the threshold value of ΔrG?

Answer 25

22.8 kJ / mol

Question 26

What is Le Chateliers’ principle?

Answer 26

When a change is made to a system in dynamic equilibrium, the system responds to minimize the effect of the change.

Question 27

Why doesn’t the equilibrium constant vary with pressure at constant temperature?

Answer 27

The equilibrium constant K depends on ΔrG°, the standard Gibbs energy change for a reaction at 1 bar pressure:

ΔrG= −RT ln K

ΔrG is the value at 1 bar of pressure – so by definition, it does not change with changing pressure.

Therefore K is a thermodynamic constant at constant temperature so that:

The equilibrium constant doesn’t vary with pressure at constant temperature.

Question 28

partial pressure equation:

Answer 28

P(A) = X(A) x P(total)

X(A) = mole fraction of component A.

P(total) = total pressure of the system.

Question 29

What does adding or removing one of the components from a reaction effect?

Answer 29

Adding or removing one of the components from a reaction will affect the concentration (or pressure) of the others in the equilibrium mixture.

Question 30

for an exothermic reaction, what happens the the equilibrium position if the temperature is increased?

Answer 30

the equilibrium position for an exothermic reaction will move to favour the reactants if the temperature is increased.

Question 31

for an endothermic reaction, what happens the the equilibrium position if the temperature is increased?

Answer 31

the equilibrium position for an endothermic reaction will move to favour the products if the temperature is increased.

Question 32

What is the van ‘t Hoff equation?

Answer 32

lnK = ( −ΔrH° / R )( 1 / T ) + ( ΔrS° / R)

Question 33

What is a catalyst?

Answer 33

A catalyst is a substance that increases the rate of a reaction without being used up.

Question 34

What does a catalyst affect in a reaction?

Answer 34

Catalysts affect rate, not thermodynamics, the presence of a catalyst will affect the rate of both forward and back reactions but the position of the equilibrium will be unchanged.