1.6 & 1.10 Chemical Equilibria, Kc & Kp

Question 1

What is a closed system?

Answer 1

No products or reactants can escape

Nothing can enter

Question 2

What is dynamic equilibrium?

Answer 2

When the forward and backward reaction in a reversible reaction occur at the same rate
And
The concentrations of reactants and products remain unchanged

(Happens in a closed system)

Question 3

What is Le Chatelier’s principal?

Answer 3

When a change is applied to a system in dynamic equilibrium, the system reacts in such away to oppose the effect of the change

Question 4

How will equilibrium be affected by increasing temperature?

Answer 4

Endothermic direction

It will try to cool the surroundings therefore take in the heat which opposes the change

Question 5

How will equilibrium be affected by decreasing temperature?

Answer 5

Exothermic direction

It will try to heat the surroundings therefore give out the heat which opposes the change

Question 6

How will equilibrium be affected by increasing pressure?

Answer 6

Move to the side with fewer gas particles (big number)

As fewer gas particles in a given volume causes a lower pressure which opposes the change

Question 7

How will equilibrium be affected by decreasing pressure?

Answer 7

Move to the side with more gas particles (big number)

As more gas particles in a given volume causes a higher pressure which opposes the change

Question 8

How will equilibrium be affected by increasing reactant concentration?

Answer 8

Equilibrium moves to the products

Right

Question 9

How will equilibrium be affected by decreasing reactant concentration?

Answer 9

Equilibrium moves to the reactants

Left

Question 10

How will equilibrium be affected by increasing product concentration?

Answer 10

Equilibrium moves to the reactants

Left

Question 11

How will equilibrium be affected by decreasing product concentration?

Answer 11

Equilibrium moves to the products

Right

Question 12

How will equilibrium be affected by removing C?

A+B ⇌ C+D

Concentrations after?

Answer 12

The equilibrium will move to the right

A+B - decreases
C - remains the same
D - increases

Question 13

In industry what is significant?

Answer 13

You never want to equilibrium to stop so you remove your desired product to keep the required reaction going

Question 14

What doesn’t affect equilibrium? Why?

Answer 14

Catalysts

It only increases the rate of reaction so speeds up the forward and backward reactions equally

Question 15

Why is high pressure often not used? What is used?

Answer 15

Dangerous
Expensive

Compromise conditions

Question 16

When deciding what to change in a system what should you consider?

Answer 16

If no state symbols are given don’t use pressure!!

Question 17

What is the equilibrium constant?

Answer 17

Kc

It is a ratio of concentration of reactants and products at equilibrium

Question 18

How do you work out Kc?

Answer 18

[products]
—————
[reactants]

If the moles are more than one you raise it as a power on the outside of the square brackets

Question 19

What do square brackets indicate?

Answer 19

Concentration

Question 20

What affects the equilibrium constant Kc and Kp?

Answer 20

Temperature

Nothing else!!

Question 21

What does Kc actually tell us?

Answer 21

If the equilibrium is more to the left or right

Less than 1 = left
1 = Equilibrium
More than 1 = right

Question 22

How do you calculate equilibrium moles?

Answer 22

From the two moles given of the same compound take one from the other and use the difference to work out the ‘moles’ of the rest (above compounds)

Then the ‘moles’ you worked out add or take from the starting equilibrium moles you were given in the question - depending on the reaction direction

Question 24

What must you do when putting numbers into the brackets for Kc?

Answer 24

You work out the equilibrium moles then divide by the volume to get the concentrations

Question 25

What does a high value of Kc indicate?

Answer 25

A high equilibrium yield of products (favours the right)

Question 26

What doesn’t affect the equilibrium constant?

Answer 26

Change in pressure
Change in concentrations of the reactants or products
A catalyst

Question 27

How do you calculate Kc units?

Answer 27

Each concentration = moldm^-3

Cancel them out in the fraction and move and change signs accordingly

If it has no units literally write no units

Question 28

Why could actual yield differ from equilibrium yield?

Answer 28

If the reaction isn’t allowed to reach equilibrium
Due to the desired product being constantly removed forcing the equilibrium to keep producing it
Then equilibrium yield is never reached

Question 29

What is Kp?

Answer 29

The equilibrium constant for a reversible reaction that takes place completely in the GAS PHASE

Question 29

What is partial pressure?

Answer 29

The pressure a gas would exert individually under the same conditions

Question 29

What is significant about partial pressures?

Answer 29

The sum of the partial pressures is equal to the total pressure caused by the gas reaction

Question 30

How do you work out partial pressures?

Answer 30

Equilibrium moles
Divided by
Total equilibrium moles

All multiplied by the total pressure

(Do this for each compound)

Question 31

How to work out Kp?

Answer 31

(partial pressures of products)
——–——————–—————
(partial pressures of reactants)

If the moles are more than one you raise it as a power on the outside of the brackets
(Put a p just inside the bracket to indicate that it’s a partial pressure)

Question 32

How do you work out Kp units?

Answer 32

Each partial pressure = KPa

Cancel them out in the fraction and move and change signs accordingly

If it has no units literally write no units

Question 33

What would you use as the partial pressure if a compound was a solid?

Answer 33

1

As this is a constant