1.6 Equilibria

Question 1

State le Chatelier’s principle (1)

Answer 1

• (If any factor is changed which affects an equilibrium), the (position of) equilibrium will shift / move so as to oppose / counteract the change.

Question 2

State the effect, if any, of a catalyst on the time taken to reach equilibrium (1)

Answer 2

• (Time is) less / shorter / decreases / reduces

Question 3

State the effect, if any, of a catalyst on the position of an equilibrium (1)

Answer 3

• None

Question 4

Define Dynamic Equilibrium

Answer 4

• Dynamic equilibrium occurs when forward and backward reactions are occurring at equal rates
  The concentrations of reactants and products stays constant and the reaction is continuous

Question 5

Define a Closed System

Answer 5

One in which none of the reactants or products escape from the reaction mixture

Question 6

Define an Open System

Answer 6

matter and energy can be lost to the surroundings

Question 7

Define a homogenous system

Answer 7

system where all the chemicals are in the same phase (state of matter)

Question 8

What does the position of equilibrium refer to ?

Answer 8

• The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture
• When the position of equilibrium shifts to the left, it means the concentration of reactants increases
• When the position of equilibrium shifts to the right, it means the concentration of products increases

Question 9

What are the effects of changing the temperature on the position of equilibrium ?

Answer 9

Question 10

What are the effects of changing the concentration on the position of equilibrium ?

Answer 10

Question 11

What are the effects of changing the pressure on the position of equilibrium ?

Answer 11

If the number of moles of gas is the same on both sides of the equation then changing pressure will have no effect on the position of equilibrium

Question 12

Dynamic Equilibrium graphs:

Answer 12

Question 13

Why are compromise temperatures used in industry ?

Answer 13

• A compromise is made as a low temp gives good yield but the reaction would occur at a slow rate
• A high temperature would have a faster rate of reaction
• A compromise is made so the temperature is high enough for a fast rate but is also low to give a good yield of the desired product.

Question 14

Why are very high pressures not always used in industry ?

Answer 14

A high pressure would be ideal as equilibrium would shift to the product side however, it is expensive

Question 15

Answer 15

Question 16

Answer 16

Question 17

What are the effects on catalysts on the equilibrium ?

Answer 17

• A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the equilibrium is achieved
• It does not effect the position of equilibrium because it speeds up the rates of the forward and backward reactions by the same amount

Catalysts speeds up the rate allowing lower temperatures to be used (and hence lower energy costs) but have no effect on equilibrium

Question 18

What are the environmental benefits of catalysts ?

Answer 18

• Catalysed reactions can occur at lower temperature so less fuel needed and fewer emissions from fuels
• Catalysed reaction enables use of an alternative process with higher atom economy so meaning fewer raw materials needed and less waste products are produced

Question 19

What happens in a reversible reaction at equilibrium ?

Answer 19

• The forward and reverse reactions proceed at equal rates
• The concentrations of reactants and products remain constant

Question 20

What is the equilibrium constant Kc ?

Answer 20

The equilibrium constant is a value that expresses the relationship between the concentration of reactants and products present at equilibrium in a reversible reaction.

• f Kc is large/ greater than 1 - equilibrium lies to the right as product concentration is greater than reactant concentration.
• If Kc is small/ less than 1 - equilibrium lies to the left as reactant concentration is greater than product concentration.
• If Kc is greater than 10^10 reaction is going to completion.
• Only affected by temperature

Question 21

What is the formula for Kc ?

Answer 21

Question 22

Answer 22

Question 23

Answer 23

• 3 minutes
• (At equilibrium, ratefwd = rateback so) concentrations (of O2 and SO3) remain constant
• Allow (after this point) gradient is zero / curve flattens out

Question 24

Answer 24

• Decrease
• Increasing pressure moves equilibrium to the side of least moles i.e. backward reaction
• To oppose the increase in pressure or to decrease the pressure

Question 25

Explain why the equilibrium yield of hydrogen is unchanged if a catalyst is used in the reaction (2)

Answer 25

* A catalyst speeds up the rate of the forward and backward reaction * By the same amount

Question 26

Give two features of a reaction at equilibrium (2)

Answer 26

* **Concentrations** of reactants and products remain constant * Forward **rate** = Reverse / backward **rate**

Question 27

Explain how Figure 1 shows that the forward reaction in this equilibrium is exothermic (2)

Answer 27

* Lower yield at higher temperature * So, equilibrium has shifted backwards/left/in the endothermic direction to oppose the increase in temperature

Question 28

State whether the forward reaction in this equilibrium results in an increase, decreaseor no change in the amount, in moles, of gas (3)

Answer 28

* Increase * Lower yield at higher pressure * So, equilibrium has shifted backwards/to the side with fewest number of moles to oppose the increase in pressure

Question 29

The temperature of the equilibrium mixture is decreased. Predict the effect, if any, on the value of Kc Give a reason for your prediction (3)

Answer 29

* greater / increases * the equilibrium shifts in the **exothermic** direction * to increase the temperature / to oppose the temperature decrease

Question 30

How do we find the Kc for the reverse direction if the Kc for the forward direction has been found ?

Answer 30

(1) / (answer)