Equilibria A2

Question 1

Define Kc.

Answer 1

Kc is the equilibrium constant for concentrations. It measures the ratio of product concentrations to reactant concentrations at equilibrium.

Question 2

How is Kc calculated?

Answer 2

Molar concentration of the products/reactants, with powers based on the number of moles of each present in the balanced equation

Question 3

How are expressions for Kc constructed for homogeneous equilibria?

Answer 3

For homogeneous equilibria (same phase), the Kc expression includes concentrations of substances in that phase.

Question 3

What is omitted in the Kc expression for heterogeneous equilibria?

Answer 3

For heterogeneous equilibria (different phases), solids and liquids are not included in the Kc expression.

Question 3

What happens to Kc when the temperature increases in an endothermic reaction?

Answer 3

In an endothermic reaction, an increase in temperature shifts the equilibrium position to favor the products, increasing Kc.

Question 4

What happens to Kc when the temperature increases in an exothermic reaction?

Answer 4

In an exothermic reaction, an increase in temperature shifts the equilibrium position to favor the reactants, decreasing Kc.

Question 4

How does an increase in reactant concentration affect Kc?

Answer 4

An increase in reactant concentration shifts the equilibrium towards the product side, increasing Kc.

Question 5

How does an increase in pressure (for gases) affect Kc?

Answer 5

An increase in pressure shifts the equilibrium toward the side with fewer moles of gas, changing Kc accordingly (for gas-phase reactions).

Question 6

What is reaction rate?

Answer 6

The change in concentration of a reactant or product in a given time.

Question 7

What makes an effective collision?

Answer 7

Particles must have sufficient energy

Particles must collide in correct orientation

Question 8

Which factors can change the rate of a chemical reaction?

Answer 8

Concentration
Temperature
Use of a catalyst
Surface area of solid reactants

Question 9

What effect does increasing temperature have on reaction rate?

Answer 9

Reaction rate increases

Question 10

Explain why increasing temperature has its effect on reaction rate

Answer 10

Particles have more energy
Higher speed
Collide more frequently
More successful collisions
Rate increases

Question 11

What is the rule of thumb for reaction rate and temperature?

Answer 11

A 10C increase in temperature doubles the rate.

Question 12

What effect does increasing concentration/pressure have on reaction rate?

Answer 12

Reaction rate increases

Question 13

Explain why increasing concentration/pressure has its effect on reaction rate

Answer 13

Concentration increases
More particles per unit volume
More frequent collisions
More successful collisions
Rate increases

Question 14

How can progress of a chemical reaction be followed?

Answer 14

Monitoring the removal of a reactant.

Following the formation of a product.

Question 15

What are the two methods of determining reaction rate, when a gas is produced?

Answer 15

Monitoring the volume of gas produced

Monitoring the loss of mass of reactants

Question 16

What does a catalyst do?

Answer 16

A substance that changes the rate of a chemical reaction without undergoing any permanent change itself.

Question 17

What are the main properties of a catalyst?

Answer 17

Not used up in the reaction.
May react with a reactant to form an intermediate or provide a surface for the reaction to take place.
The catalyst is regenerated at the end of the reaction.

Question 17

What is a homogeneous catalyst?

Answer 17

Has the same physical state as the reactants.

Question 18

How does a homogeneous catalyst interact with the reactants?

Answer 18

Forms an intermediate

This intermediate breaks down to give the product and regenerates the catalyst

Question 19

Give two examples of homogeneous catalysts

Answer 19

Sulphuric acid in the production of esters

Chlorine radicals in ozone depletion

Question 19

How are the units of Kc worked out?

Answer 19

Substitute units into the expression

Cancel common units

Question 20

In a heterogeneous equilibrium, what is discounted in the expression for Kc?

Answer 20

Any species that are not a gas or in solution (aqueous) .

so solids and liquids are ommited

Question 21

What is a mole fraction?

Answer 21

The proportion of the volume of a gas to the total volume of gases in the mixture.

Question 22

What is the partial pressure?

Answer 22

The contribution that the gas makes towards the total pressure P.

Question 23

mole fraction equation?

Answer 23

Mole fraction χ(A) = n(A)/n(total)

Question 24

How is partial pressure calculated?

Answer 24

p(A) = mole fraction x total pressure P
p(A) = χ(A) x P

Question 25

What is the way to check the partial pressures are correct?

Answer 25

Sum of the partial pressures equals the total pressure.

Question 26

what is Kp?

Answer 26

Kp is the equilibrium constant using gases.

Question 27

With the equilibrium H(2) + I(2) = 2HI, what is the expression for Kp?

Answer 27

Kp = p(HI)^2 / p(H(2)) x p(I(2))

Question 28

What does the magnitude of an equilibrium constant represent?

Answer 28

The extent of an equilibrium

Question 29

What would K=1, K=100 and K=0.01 represent?

Answer 29

K=1: equilibrium halfway between reactants and products
K=100: equilibrium in favour of products
K=0.01: equilibrium in favour of reactants

Question 30

What is the effect on K when the temperature is increased and the forward reaction is exothermic?

Answer 30

Equilibrium constant decreases

Equilibrium shifts to reactant side

Question 31

What is the effect on K when the temperature is increased and the forward reaction is endothermic?

Answer 31

Equilibrium constant increases

Equilibrium shifts to product side

Question 32

What effect does increasing pressure have on a reaction with fewer moles of gaseous products than reactants?

Answer 32

Products increase
so Reactants decrease
so Equilibrium shifts to right

Question 33

What effect does increasing pressure have on a reaction with more moles of gaseous products than reactants?

Answer 33

Products decrease
Reactants increase
Equilibrium shifts to left

Question 34

What effect does increasing pressure have on a reaction with the same number of moles of gaseous products and reactants?

Answer 34

No change

Equilibrium stays the same

Question 35

What is the effect of a catalyst on the equilibrium constant?

Answer 35

Catalysts affect the rate but not the equilibrium position.

Equilibrium is reached faster but position does not change.