2.2 Equilibrium and Yield

Question 1

Open chemical system

Answer 1

Both chemical matter and energy can be transferred in or out

Question 2

Closed chemical system

Answer 2

Only energy can be transferred in or out, and so there is no net gain or loss of chemical matter

Question 3

Non-reversible reaction

Answer 3

Proceeds to completion. So, does not proceed in the backward direction.

Question 4

Reversible reaction

Answer 4

Reactants combine to form products, and products combine to form reactants.

Question 5

Forward reaction

Answer 5

Reactants -> Products

Question 6

Backward reaction

Answer 6

Reactants <- Products

Question 7

State of chemical equilibrium

Answer 7

Reached for reversible reactions in closed chemical systems at fixed temperature.

When rate of forward reaction is equivalent to rate of backward reaction, so the net concentrations of reactants and products do not change with time.

Often, some reactants are unconsumed. Does not mean reaction has stopped.

Question 8

Graphical indication of chemical equilibrium

Answer 8

Parallel lines and gradient = 0

Question 9

Law of Chemical Equilibrium

Answer 9

At a given temperature, a chemical system may reach a state in which the particular ratio of reactant and product concentrations has a constant value.

Question 10

Equilibrium constant Kc

Answer 10

Indicates position of chemical equilibrium at a given temperature. Essentially, ratio of product concentrations to reactant concentrations.
Does not have units as it is a constant.

Question 11

Formula for Kc

Answer 11

Kc=[products]/[reactants]

So, if aA+bB <-> cC+dD:
Kc = ([C]^c x [D]^d)/([A]^a x [B]^b)

Question 12

Kc > 1

Answer 12

Concentration of products > Concentration of reactants

Forward reaction favoured.

Position of equilibrium lies to the right.

Question 13

Kc < 1

Answer 13

Concentration of reactants > Concentration of products

Backward reaction favoured.

Position of equilibrium lies to the left.

Question 14

Large Kc value

Answer 14

Reaction proceeds almost to completion.

Question 15

Small Kc value

Answer 15

Very little product in equilibrium mixture when state of chemical equilibrium is reached.

Question 16

Homogeneous equilibrium

Answer 16

All reactants and products are in same physical state.

Question 17

Heterogeneous equilibrium

Answer 17

Reactants and products are present in more than one physical state.

Question 18

Le Chatelier’s Principle

Answer 18

If a dynamic equilibrium is disturbed by changing the conditions, the position of the equilibrium moves to counteract the change.

Question 19

Nature of change in concentration of one substance when system is in state of equilibrium

Answer 19

Sharp increase/decrease in concentration of one substance

Question 20

Nature of change in overall pressure of gaseous mixture when system is in state of equilibrium

Answer 20

Sharp increase/decrease in concentration of all gaseous species

Question 21

Nature of change in overall temperature when system is in state of equilibrium

Answer 21

Gradual increase/decrease in concentration of all species

Question 22

Effect on equilibrium position if concentration of one reactant is increased

Answer 22

System will counteract by attempting to reduce concentration of specific reactant.
Forward reaction favoured.
Equilibrium shifts to the right.
Formation of more products.

Question 23

Effect on equilibrium position if concentration of one reactant is decreased

Answer 23

System will counteract by attempting to increase concentration of specific reactant.
Backward reaction favoured.
Equilibrium shifts to the left.
Formation of more reactants.

Question 24

Effect on equilibrium position if concentration of one product is increased

Answer 24

System will counteract by attempting to reduce concentration of specific product.
Backward reaction favoured.
Equilibrium shifts to the left.
Formation of more reactants.

Question 25

Effect on equilibrium position if concentration of one product is decreased

Answer 25

System will counteract by attempting to increase the concentration of specific product. Forward reaction is favoured. Equilibrium shifts to the right. Formation of more products.

Question 26

Effect on equilibrium position if pressure of gaseous mixture is increased

Answer 26

System will counteract by attempting to reduce pressure of gaseous mixture. Equilibrium shifted to the side of the equation which results in formation of least moles of gaseous particles. NOTE: if number of moles of gaseous particles equivalent on both sides of equation, no change to equilibrium position.

Question 27

Effect on equilibrium position if pressure of gaseous mixture is decreased

Answer 27

System will counteract by attempting to increase pressure of gaseous mixture. Equilibrium shifted to the side of the equation which results in formation of most moles of gaseous particles. NOTE: if number of moles of gaseous particles equivalent on both sides of equation, no change to equilibrium position.

Question 28

ΔH

Answer 28

Change in enthalpy of a system; specifically the change in heat content (thermal energy) at constant pressure. Always in reference to forward reaction.

Question 29

Effect on equilibrium position if temperature is increased

Answer 29

System will counteract by attempting to absorb additional thermal energy. Position of equilibrium shifts in the endothermic (ΔH is positive) direction.

Question 30

Effect on equilibrium position if temperature is decreased

Answer 30

System will counteract by attempting to release thermal energy. Position of equilibrium shifts in the exothermic (ΔH is negative) direction.

Question 31

Effect of concentration change on Kc

Answer 31

No effect; Kc remains the same.

Question 32

Effect of pressure change on Kc

Answer 32

No effect; Kc remains the same.

Question 33

Effect of temperature increase on Kc

Answer 33

Endothermic reaction: Kc increases Exothermic reaction: Kc decreases

Question 34

Effect of catalyst on position of equilibrium or Kc

Answer 34

No effect. Increases rate of forward and backward reactions to same extent; therefore, the equilibrium is not disturbed. Speeds up attainment of equilibrium by reducing Ea necessary.