Chapter 19

Question 1

What is the mole fraction?

Answer 1

the mole fraction is the fraction of the total number of moles that each chemical in a reaction is responsible for

Question 2

What is partial pressure?

Answer 2

Partial pressure is the part of the total pressure that each chemical in a reaction is responsible for.

Its also the pressure a gas exerts in a mixture of gases in a container.

Question 3

What is the symbol for partial pressure?

Answer 3

p

Question 4

What is the total pressure?

Answer 4

The sum of the partial pressures.

Question 5

How can you work out the partial pressure of a gas?

Answer 5

partial pressure = total pressure x mole fraction

Question 6

How do you work out the mole fraction of a gas?

Answer 6

Number of moles of that gas / total number of moles of gas

Question 7

How do you calculate equilibrium quantities? (using ICE technique)

Answer 7

initial concentration
change in concentration
equilibrium concentration

Use the stoichiometric equation and fill in the gaps with the information you are given.

Question 8

What is the equilibrium constant?

Answer 8

Kc

Question 9

What are the units for Kc + how do we work them out?

Answer 9

To work out the units of Kc you cancel common units from the products/reactants.

if all cancel out there are no units

if after cancelling you get mol dm-3/1 then you will get the units moldm-3

if after cancelling you get 1/moldm-3 then you get the units dm3mol-1

Question 10

How do you write the Kc expression?

Answer 10

[products]/[reactants]

Question 11

What does homogeneous equilibria mean?

Answer 11

Contains equilibrium species of all the same state/phase.

Question 12

What does heterozygous equilibria mean?

Answer 12

Contains equilibrium species of different states/phases.

Question 13

How do you write the Kp expression?

Answer 13

Kp = p(A) p(B) / p(C) p(D)

Question 14

What state must reactants be in to be in the kp expression?

Answer 14

Only gases are included

Question 15

When does Kp change?

Answer 15

When the temperature changes

Question 16

How do you work out Kp (step by step)?

Answer 16

find out the mole fractions of the gases present
their partial pressures
the Kp expression
the value of Kp

Question 17

What are the units for Kp?

Answer 17

Kpa-1/Pa-1/atm-1

Question 18

How does Le Chateliers Principle work?

Answer 18

if a change is made to a system in dynamic equilibrium, the position of the equilibrium moves to counteract this change

Question 19

How does equilibrium shift with temperature change?

Answer 19

increase in temp- Moves in the endothermic direction
decrease in temp- Moves in the exothermic direction

Question 20

What is the effect of changing temperature on the value of Kc?

Answer 20

For a reaction that is exothermic in the forward direction, increasing the temperature pushes the equilibrium from right to left. (towards endothermic side)

Therefore, the value of Kc will decrease as the ratio of [ products ] to [ reactants ] decreases
Conversely, if the temperature is raised in an endothermic reaction, the value of Kc will increase

Question 21

What is the effect of changing temperature on the value of Kp?

Answer 21

For a reaction that is exothermic in the forward direction, increasing the temperature pushes the equilibrium from right to left (towards the endothermic side)

Therefore, the value of Kp will decrease as the ratio of [ products ] to [ reactants ] decreases
Conversely, if the temperature is raised in an endothermic reaction, the value of Kp will increase

Question 22

What is the effect of changing pressure on the value of Kc?

Answer 22

No effect as it doesn’t involve gases.

Question 23

How does equilibrium shift when pressure changes?

Answer 23

increase in pressure- Shifts in the direction with the fewer number of moles of gas to decrease pressure.
decrease in pressure- Shifts in the direction that has the larger amount of moles of gas to increase pressure.

Question 24

What is the effect of changing pressure on the value of Kp?

Answer 24

The value of Kp is not affected by any changes in pressure.

Changes in pressure cause a shift in the position of equilibrium to a new position which restores the value of Kp

this is the same for changes in concentration and the Kc value.

Question 25

How does a catalyst affect the value of Kc/Kp?

Answer 25

If all other conditions stay the same, they are not affected by a catalyst.

A catalyst speeds up both the forward and reverse reactions at the same rate so the ratio of [ products ] to [ reactants ] remains unchanged. They only cause a reaction to reach equilibrium faster.