equilibria [P1] PAPER 1+2

Question 1

equilibrium constant (Kc) equation?

Answer 1

ensure everything is according to CONCENTRATIONS (⚠️ volume needs to be in dm3) before calculating

i.e. [A], [B], [C] and [D] are:
• equilibrium moles of A / volume of container
• equilibrium moles of B / volume of container
etc.

Question 2

Answer 2

concentration of water is effectively constant since it is so much larger than the other concentrations

Question 3

Does the addition of a catalyst affect the equilibrium of a reaction? Explain why.

Answer 3

• no
• catalysts increase the rates of the forward and backwards reactions by the same amount

Question 4

What affects the value of Kc?

Answer 4

Changing the temperature of the system

Question 5

Is the value of Kc affected by changes in pressure/concentration?

Answer 5

No

Question 6

What is meant by dynamic equilibrium?

Answer 6

• The forwards and backwards reaction are taking place at the same time, and at the same rates, in a closed system
• Concentrations of all reactants and products are constant

Question 7

what is a closed system?

Answer 7

nothing can get in or out

Question 8

Why are compromise temperatures and pressures used in industry?

Answer 8

• to maximise yield
• reduces costs

Question 9

According to Le Chatelier’s Principle, what happens to the position of the equilibrium of a reaction when the temperature of the system increases?

Answer 9

• the forwards/backwards reaction is endothermic
• equilibrium position moves to the right/left in the endothermic direction to oppose the increase in temperature
• increases equilibrium yield of [endothermic reaction products]

[for temperature decrease - shift in the exo direction instead of endo]

Question 10

According to Le Chatelier’s Principle, what happens to the position of the equilibrium of a reaction when the pressure of the system increases?

Answer 10

• equilibrium position moves to oppose the increase in pressure by moving to the [side with fewer moles] as the right/left side has fewer moles
• increases/decreases equilibrium yield of…

e.g.

Question 11

According to Le Chatelier’s Principle, what happens to the position of the equilibrium of a reaction when the concentration of a reactant increases?

Answer 11

• equilibrium position moves to oppose the increase in [reactant] by moving to the right to use it up
• equilibrium yield of [product] increases

Question 12

According to Le Chatelier’s Principle, what happens to the position of the equilibrium of a reaction when the concentration of a reactant decreases?

Answer 12

• equilibrium position moves to oppose the decrease in [reactant] by moving to the left to make more of it
• decreases equilibrium yield of [product]

Question 13

working out equilibrium moles?

Answer 13

Initial moles
Change in moles
Equilibrium moles - ICE

See what Q has given you - calculate the change in moles and use it to calculate whatever they’re asking for
⚠️ make sure you take into account the molar ratios (from the equation)

Question 14

Working out units of Kc?

Answer 14

• ratio of mol:dm-3 should be 1 : -3

• pretend each variable in the general Kc equation is moldm-3
• simplify the fraction - don’t be stupid with indices ⚠️
• there are no units if the moles of products are equal to the moles of reactants

e.g.

Question 15

Why may a high pressure be used for a reaction in industry?

Answer 15

• more particles in a given volume, so more collisions in a given time
• increases the rate of reaction
• the reaction gets to equilibrium faster

Question 16

For a reaction, the total number of moles on the left side is equal to the number of moles on the right side.

State the effect, if any, on the equilibrium yield of the forwards reaction products when pressure is increased.

Answer 16

no effect

Question 17

Methanol is made by the reaction of carbon monoxide with hydrogen.

CO+2H2 ⇌CH3OH
∆H=–91kJmol–1

The reaction uses a copper-based catalyst, a pressure of 10 MPa and a temperature of 550 K.

These conditions are used to provide a balance between equilibrium yield, reaction rate and cost.

Describe how the use of a catalyst, and changes in pressure and temperature, each affect equilibrium yield, reaction rate and cost.

[6 marks]

Answer 17

Use of a catalyst…
• doesn’t affect equilibrium yield
• gives a faster rate
• lowers costs

Increasing pressure…
• gives a higher equilibrium yield for this reaction
• gives a faster rate
• the higher the pressure, the higher the cost

Temperature:
• higher temperature gives a faster rate
• lower temperature gives a higher equilibrium yield
• the higher the temperature, the higher the cost

Question 18

The Kc of a the forwards reaction of a reversible reaction is ‘x’. What is the Kc of the backwards reaction?

Answer 18

1/x (1/units too if there are units)

e.g. if the Kc of the forwards reaction is 5 moldm-3, then the Kc of the backwards reaction 0.2mol-1dm3

Question 19

Reaction 2 is exothermic. A typical compromise temperature of 200 °C is used industrially for this reaction.

Explain the effect of a change of temperature on both the position of equilibrium and the rate of reaction, and justify why a compromise temperature is used industrially.

[6 marks]

Answer 19

Question 20

One reason why HIGH pressures aren’t used in industry?

Answer 20

expensive equipment costs

Question 21

State le Chatelier’s Principle.

Answer 21

If any factor is changed which affects an equilibrium, the position of equilibrium will shift to oppose the change

Question 22

2A + B ⇌ 3C + D (A, B, C and D are solutions)

Why would adding more water to the equilibrium mixture lower the amount of A in the mixture?

Answer 22

• All concentrations (of A, B, C and D) fall
• Equilibrium moves to the right, to the side with more moles, to oppose the decrease in concentration

Question 23

Justify the statement that adding more water to the equilibrium mixture will lower the amount of A in the mixture.

Answer 23