Chapter 7 - Chemical Equilibria

Question 1

Define irreversible reactions.

Answer 1

A complete reaction where reactants are converted into products until the limiting reactant is used up. The products have little or no tendency to re-form reactants.

Question 2

Define reversible reactions.

Answer 2

A reaction that can occur in both directions at the same time, where a mixture of reactants and products are obtained.

Question 3

Define dynamic equilibrium.

Answer 3

It is when the rate of forward reaction equals the rate of the reverse reaction. There will be no overall net change from reactant to product and vice versa. (reactant concentration and product concentration remain constant)

Question 4

What are 3 characteristics of a system in dynamic equilibrium?

Answer 4

1) Dynamic equilibrium is not static
2) Can only be achieved in a closed system (no loss or gain of materials to or from the surrounding)
3) Dynamic equilibrium can be achieved from “either direction” (beginning with only materials on either side of the equation

Question 5

What is homogenous equilibria?

Answer 5

Participating substances are in one phase only (reactants and products all in same state)

Question 6

What is heterogenous equilibria?

Answer 6

Participating substances are present in different phases.

Question 7

What is the position of equilibrium?

Answer 7

Relative proportion of products to reactants in an equilibrium mixture.

Question 8

How does the position of equilibrium tell you about the relative proportion of products to reactants?

Answer 8

At equilibrium,
if [reactant] < [product], it means that the position of equilibrium lies towards the right. (favouring products)
if [reactant] > [product], it means that the position of equilibrium lies towards the left. (favouring reactants)

Question 9

Define Le Chatelier’s Principle.

Answer 9

Le Chatelier’s Principle states that when a system in equilibrium is subjected to a change in conditions which disturbs the equilibrium, the position of equilibrium will shift in a way so as to reduce that change.

Question 10

What are 3 factors that can affect the state of equilibrium?

Answer 10

1) Change in concentration/partial pressure
2) Change in total pressure (applicable for gaseous systems only)
3) Change in temperature

Question 11

How does a change in concentration/partial pressure affect the state of equilibrium? (4)

Answer 11

if a reactant is added to an equilibrium mixture,

1) concentration of that reactant increases
2) by LCP, position of equilibrium shifts to the right, favouring products, so as to decreases concentration of that reactant
3) concentration of other reactants decrease
4) concentration of products increase

Question 12

How does a change in total pressure affect the state of equilibrium? (2 scenarios)

Answer 12

1) when total pressure is increased (by reducing volume), by LCP, position of equilibrium shifts to a direction that favours the production of fewer number of moles of gases, thus decreasing pressure.
2) when total pressure is decreased (by increasing volume, by LCP, position of equilibrium shifts to a direction that favours the production of more number of moles of gases, thus increasing pressure.
* state of equilibrium is only affected when there is an unequal number of moles of gaseous on each side of the equation.

Question 13

How does a change in temperature affect the state of equilibrium? (2 scenarios)

Answer 13

1) When a system in equilibrium is subjected to an increase in temperature, the system will favour the endothermic process which will absorb the additional heat.
2) Alternatively, decrease in temperature will favour the exothermic process which will generate additional heat.

Question 14

How does the presence of a catalyst affect the state of equilibrium?

Answer 14

When a catalyst is added to an equilibrium system, it increases both the forward and reverse reaction rates by the same extent, since the activation of both forward and backward reactions are lowered to the same extent. Thus, a catalyst shortens the time needed to attain the same final equilibrium concentrations. It does not affect the position of equilibrium and equilibrium composition.

Question 15

What is the Equilibrium Law?

Answer 15

The Equilibrium Law states that if a reversible reaction is allowed to reach equilibrium, the product of the concentration of each product (powers raised to their mole fraction) divided by the product of the concentration of each reactant (powers raised to their mole fraction) has a constant value known as the equilibrium constant (K), at a constant temperature.

Question 16

What does the magnitude of K (Kc or Kp) indicate? (2)

Answer 16

It is a useful indication of the extent of a reaction (how far the forward reaction occurs).

1) if Kc < 10^-3, position of equilibrium will lie more to the left (favouring reactants), and equilibrium composition will consist of largely unreacted reactants.
2) if Kc > 10^3, position of equilibrium will lie more to the right. Reaction has a higher tendency to form products. (Higher Kc would lead to higher yield)

Question 17

What is the factor that affects values of Kc or Kp?

Answer 17

The value of Kc and Kp is constant at a given temperature; the value changes only if the temperature is changed.
If forward reaction is favoured, K increases (product increases). If backward reaction is favoured, K decreases (reactant increases).

Question 18

What is the reactant quotient (Qc/Qp)?

Answer 18

The reaction quotient is a measure of the relative amounts of products and reactants present in a reaction at a given time (instantaneous), At a given temperature, Qc/Qp can take on any value until the reversible reaction reaches a state in which the reactant quotient becomes constant and numerically equivalent to the equilibrium constant.

Question 19

How can the reaction quotient be used to predict the direction of a reaction? (3 scenarios)

Answer 19

1) Qc < Kc: the ratio of initial concentrations of products to reactants is too small. To reach equilibrium, more products must be formed. Reaction proceeds forward (from left to right) to reach equilibrium.
2) Qc = Kc: the initial concentrations are the same as the concentrations at equilibrium. The system is already at equilibrium.
3) Qc > Kc: the ratio of initial concentrations of products to reactants is too large. To reach equilibrium, more reactants must be formed. Reaction proceeds backward (from right to left) to reach equilibrium.

Question 20

What are 2 major concerns in the design of industrial chemical processes?

Answer 20

Convert reactants into products

1) as quickly as possible (kinetics) - maximise rate of product formation
2) as completely as possible (equilibrium) - maximise proportion of product (yield)

Question 21

What are the 3 optimal conditions used in the Haber process?

Answer 21

1) Temperature: 450C
2) Pressure: 250 atm
3) Catalyst: finely divided iron catalyst

Question 22

How is the maximum yield of NH3 at equilibrium produced during the Haber process? (3)

Answer 22

1) Low temperature: because the rate of forward reaction is exothermic. However, at low temperature, the rate of reaction will be too slow, making the process uneconomical.
2) High pressure: because the forward reaction involves a decrease in number of moles of gases. However, the higher the pressure, the greater the cost and maintenance of equipment.
3) Continuously removing the ammonia product as it forms, shifting the position of equilibrium forward.

Question 23

In the Haber process, how is the rate of ammonia production increased? (2)

Answer 23

1) increasing concentrations of nitrogen and hydrogen

2) using finely divided iron as catalyst

Question 24

How does ΔG measure the spontaneity of a reversible reaction? (3 scenarios)

Answer 24

1) If ΔG is negative, the forward reaction is favoured.
2) if ΔG is positive, the reverse reaction is favoured.
3) if ΔG=0, the reaction is at equilibrium.

Question 25

How does ΔGº and K provide information on the dominant species (which is more) at equilibrium? (3)

Answer 25

Since ΔGºrxn = -RTlnK,

1) when K > 1, ΔG < 0, thus the product is dominant
2) when K < 1, ΔG > 0, thus the reactant is dominant
3) when K = 1, ΔG = 0, thus there are equal proportions of reactant and product