Dynamic Equilibrium and Le Chatelier's Principle

Question 1

What is meant by the a reversible reaction?

Answer 1

A reaction made up of a forward reaction and a reverse reaction

Question 2

When is the forward reaction rate the fastest? Why is this?

Answer 2

At the very start of the reaction. This is because the reactants have the greatest concentration at the start and so the frequency of successful collisions is the greatest

Question 3

What is dynamic equilibrium?

Answer 3

When the rate of the forward reaction becomes equal to the rate of reverse reaction dynamic equilibrium is achieved

Question 4

Define dynamic equilibrium

Answer 4

Dynamic equilibrium exists in a closed system when the rate of forward reaction is equal to the rate of the reverse reaction

Question 5

What is a closed system?

Answer 5

No materials are added or taken away

Question 6

What are the characteristics of dynamic equilibrium?

Answer 6

1 - The rate of the forwards reaction is equal to the rate of the reverse reaction
2 - The system is closed
3 - The concentration of the reactants and products do not change
4 - The macroscopic properties do not change

Question 7

What is meant by the term macroscopic properties?

Answer 7

Temperature, pressure and concentration of reactants and products

Question 8

If the forward reaction had a pink colour and the reverse reaction had a blue colour (for example), how is it possible to detect dynamic equilibrium is reached?

Answer 8

• The colour of the reaction would be constant - does not change further

Question 9

In a reaction, 70% of the mixture contained the products of the forward reaction, what does this say about the equilibrium?

Answer 9

Indicates the equilibrium position lies to the right, as there is more product than reactant (more of the right side species’ are present)

Question 10

What is indicated when the equilibrium position lies to the left?

Answer 10

There is more of the left side materials present and there is less right side materials present in the mixture

Question 11

Define Le Chaterlier’s principle?

Answer 11

When a system in dynamic equilibrium is subjected to change, the equilibrium position will shift to minimise the change

Question 12

What are the 4 factors that affect a dynamic equilibrium?

Answer 12

• Concentration of reactants or products
• Pressure if the reaction involves gases
• Temperature
• Catalysts

Question 13

2HI H2 + I2

Explain what happens to the equilibrium position when the concentration of HI is increased

Answer 13

• The position of the equilibrium shifts to the right
• To minimise the increase in HI concentration
• More H2 and I2 will be produced
• Less HI produced (after the change)

Question 14

2SO2(g) + O2(g) 2SO3(g)

i) State and explain the effect of a decrease in SO3 concentration on the equilibrium position
ii) Predict the effect of the decreasing concentration of SO3 on the composition of the equilibrium mixture

Answer 14

i) The equilibrium position will shift to the right to minimise the decrease in concentration of SO3
ii) More SO3 will be produced and less SO2 + O2 will be produced after the change

Question 15

2NO2(g) N2O4(g)
brown colourless

For the reaction shown above, predict and explain the effect of increasing the concentration in NO2 on the appearance of the equilibrium mixture

Answer 15

The equilibrium position will shift to the right to minimise the increase in concentration of NO2. The appearance of the mixture will change to be a lighter brown. This is because less NO2 will be produced and more N2O4 will be produced

Question 16

What is the only thing that pressure considers in a system?

Answer 16

Gases

Question 17

N2(g) + 3H2(g) 2NH3(g)

Explain what happens to the equilibrium position when the total pressure of the system is increased?

Answer 17

• More moles of gas in left materials than right material ( 4 moles on left, 2 on right)
• The equilibrium position shifts to the right
• To minimise the increase in pressure
• Because there are less moles of gas on the right
• More NH3 and less N2 + H2 produced

Question 18

2SO2(g) + O2(g) 2SO3(g)

State and explain the effect on the equilibrium above of an increase in pressure on the equilibrium position

Answer 18

The equilibrium position shifts to the right to minimise the increase in pressure because there is less moles of gas on the right. This means more SO3 produced, less SO2 and O2

Question 19

Do catalysts affect equilibrium position?

Answer 19

They do not affect the equilibrium position, they only increase the rate of the forward reaction and reverse reaction by the same factor - equilibrium will be reached faster

Question 20

2HI H2 + I2 enthalpy change = -53kJ/mol

What affect will increasing the temperature have on the equilibrium? Explain

Answer 20

• The forwards reaction is exothermic, which means that it gives out heat energy to the surroundings
• The reverse reaction is endothermic, which means that it absorbs heat energy from the surroundings

If the temperature of the system was increased (by adding heat to the system):
- The equilibrium position will shift to the left to minimise the effect of a increasing temperature, by absorbing heat energy because the forward reaction is exothermic

Question 21

In industrial processes, it is very rare that a process will have an extremely high percentage yield of product. Where would the position of the equilibrium be to create an extremely high percentage yield and why is it rare to find a process like this?

Answer 21

• The equilibrium position will be shifted far to the right to give a really high percentage yield
• However, this is not always the case in industrial processes because this may lead to a very slow rate of reaction, so a compromise between yield and rate must be reached

Question 22

How does temperature affect rate of reaction?

Answer 22

A low temperature will gave a slow rate of reaction

Question 23

What is the only factor that can affect the equilibrium constant (Kc)?

Answer 23

Temperature

Question 24

What does the Kc do?

Answer 24

Gives a quantitative measure of the proportion of product to reactant in an equilibrium mixture hence indicates where the equilibrium position lies in an equilibrium

Question 25

What s the equation for Kc?

Answer 25

Kc = [products]/[reactants]

NOTE: the squared brackets is a shorthand way of writing concentration and these concentrations are brought to the power of the number of moles of each material

Question 26

If Kc is a large number, what does this indicate about the position of the equilibrium in a reaction?

Answer 26

Lies far to the right and a high percentage yield of product is achieved

Question 27

If Kc is a small number, what does this indicate about the equilibrium position?

Answer 27

Lies far to the left and a low percentage yield of product is achieved

Question 28

If Kc is approx. 1, what does this indicate about the position of the equilibrium?

Answer 28

Lies about halfway between reactants and products