Chemical Equilibria (Kc) and Kp

Question 1

Where will equilibrium shift if reactant is added?

Answer 1

To the RIGHT.

Question 2

Where will equilibrium shift if product is added?

Answer 2

Shift to the LEFT.

Question 3

Where will equilibrium shift if reactants are reduced?

Answer 3

Shift to the LEFT.

Question 4

Where will equilibrium shift if products are reduced?

Answer 4

Shift to the RIGHT.

Question 5

Where will equilibrium shift if Temperature is increased?

Answer 5

Moves in the Endothermic direction.

Question 6

Where will equilibrium shift if Temperature is decreased?

Answer 6

Moves in the Exothermic direction.

Question 7

Where will equilibrium shift if Pressure is increased?

Answer 7

Moves to side with fewer molecules.

Question 8

Where will equilibrium move if Pressure is decreased?

Answer 8

Moves to the side with more molecules.

Question 9

Where will equilibrium shift if Volume is increased?

Answer 9

• Pressure decreases

- Moves to side with more molecules

Question 10

Where will equilibrium shift if Volume is decreased?

Answer 10

• Pressure increases

- Moves to the side with fewer molecules

Question 11

State where equilibrium is reached on an equilibrium graph.

Answer 11

• where two lines meet and plateau.

Question 12

What is the definition of dynamic equilibrium?

Answer 12

• The concentrations of the reactants and products remain constant
• The rate of the forward reaction = rate of the reverse reaction

Question 13

How is a dynamic equilibrium reached? Use reaction rates in your answer.

Answer 13

• At the start of the reaction the forward reaction is fast and the backward reaction is slow
• The backward reaction then speeds up as the forward reaction slows down
• The rate of the forward reaction becomes the same as the rate of the backward reaction

Question 14

State a condition for a reaction in equilibrium

Answer 14

· Closed system

Question 15

State Le Chatelier’s principle?

Answer 15

The position of equilibrium will shift to oppose the change.

Question 16

How to answer Qs if one of the following conditions are changed:

• the TEMPERATURE
• the PRESSURE or VOLUME of the container
• the CONCENTRATION of a reagent

Answer 16

1) STATE THE ANSWER (Yield of X increases/Kp decreases)
2) MAKE A STATEMENT CONCERNING THE CONDITION WHICH HAS BEEN CHANGED (e.g. The forward reaction is endothermic/ There are fewer moles of gas on the RHS)
(1) STATE THAT THE POSITION OF EQUILIBRIUM SHIFTS TO THE RIGHT OR LEFT……..TO OPPOSE THE CHANGE IN CONDITION

Question 17

What happens to equilibrium when Pressure is changed but there are equal amounts of molecules on either side?

Answer 17

If there were equal number of moles of product and reactant, then changing the pressure would not affect the position of equilibrium.

Question 18

A catalyst is a substance that speeds up the rate of reaction by providing an alternative reaction pathway with a lower activation energy.

When a catalyst is added to a reversible reaction….

Answer 18

…..the rate of both the forward and reverse reactions increases equally

· there is no change to the position of the equilibrium

· dynamic equilibrium is reached faster.

Question 19

Why does a catalyst not affect the position of equilibrium?

Answer 19

Catalysts increase the rate of the forward and backward reactions equally.

Question 20

Why are catalysts added to the Haber process, which is a reversible reaction that produces ammonia?

Answer 20

To reduce the time taken to reach dynamic equilibrium

Question 21

Compromise conditions are necessary in Haber process because….

Answer 21

…. higher pressures would be too expensive

· 450’C gives a good yield in a fast time

Question 22

What is the expression for Kc?

Answer 22

Kc = [products]/[reactants]

Question 23

How can the units for Kc be worked out?

Answer 23

Total molecules of products - Total molecules of reactants

Multiply that number by mol dm-3

Question 24

How can a known Kc be used to find out one of equilibrium concentration?

Answer 24

The Kc equation can be rearranged.

Question 25

If Kc is greater than 1…

Answer 25

• then there are more products than reactants at equilibrium and the equilibrium is shifted to the right.

Question 26

If Kc is less than 1…

Answer 26

• there are more reactants than products at equilibrium and the equilibrium has shifted to the left.

Question 27

If Kc is exactly 1…

Answer 27

• then the concentration of products is equal to the concentration of reactants.

Question 28

What is the only condition that can change value of Kc?

Answer 28

Temperature.

Question 29

You can find Kc from the concentrations of reactants and products at equilibrium using the following process…

Answer 29

1- Initial amounts of each product or reactant

2- Change in each reactant

3- Equilibrium number of moles for each product and reactant

4- Equilibrium concentration of each reactant (mol dm-3)

5- Kc expression (and calculation)

6- Units

Question 30

Kp expression

Answer 30

Partial pressure of Products/Partial Pressure of Reactants

Question 31

If Kp is greater than 1…

Answer 31

• it means the equilibrium is towards the right hand side (i.e. more products than reactants).

Question 32

If Kp is less than 1…

Answer 32

• it means the equilibrium is towards the left hand side (i.e. more reactants than products).

Question 33

If Kp is exactly 1…

Answer 33

it means the equilibrium is in the middle (i.e. equal amounts of products and reactants).

Question 34

Which is the only condition that can cause a change in Kp?

Answer 34

Temperature.

Question 35

A range of different calculations which involve the Kp expression. These include:

Answer 35

Initial mol

Change

Equilibrium moles

Mol Fraction

Partial Pressure

Question 36

How can you calculate Partial Pressure?

Answer 36

Mole Fraction x Total Pressure

Question 37

How can you calculate Mole Fraction

Answer 37

No. of moles of Gas A in the mixture/ Total No. of moles of all gases

Question 38

What is meant by Compromise Temperature?

Answer 38

Balance between rate and yield.