6 - Chemical equilibria and Le Chatelier’s principle

Question 1

What is a reversible reaction?

Answer 1

Reactions which take place in forward and backward directions

Question 2

What is meant by dynamic equilibrium?

Answer 2

Both forwards and backward reactions occur simultaneously

- All reversible reactions reach dynamic equilibrium

Question 3

State two features of dynamic equilibrium

Answer 3

• Forwards and backward reactions occur at equal rates
• Concentration of reactants and products remain constant

Question 4

Why do the concentrations of reactants and products remain the same in dynamic equilibrium?

Answer 4

Reactants are becoming products as fast as products are becoming reactants

Question 5

How is equilibrium reached?

Using example A + B ⇌ C + D

Answer 5

• A + B react and initially forward reaction is fast as there is much of A and B, no reverse reaction as of yet
• As concentrations of C + D increase, the rate of reverse reaction increases, and forwards reaction slows as there is less of A + B
• We then get to a point where A + B are being produced at the same rate as C + D, therefore equilibrium is reached

Question 6

What are the conditions for equilibrium?

Answer 6

• Can only be reached in a closed system
• Can be approached in either directions as long as conditions remain constant

Question 7

What is Le Chatelier’s principle?

Answer 7

If an external condition is changed the equilibrium will shift to oppose the change and try to reverse it

- Only applies to homogenous systems

Question 8

What happens to the position of equilibrium when temperature is increased?

Answer 8

Equilibrium shifts to the endothermic direction to oppose the change
- This reduces temperature by absorbing heat

Question 9

What happens to the position of equilibrium when temperature is decreased?

Answer 9

Equilibrium shifts to the exothermic direction to oppose the change
- This increases temperature by giving out heat

Question 10

Example exam question

What effect would increasing temperature have on the yield of ammonia?

N₂ + 3H ⇌ 2NH₃
ΔH = -ve exo

Answer 10

If temperature is increased the equilibrium will shift to oppose this and move in the endothermic, backwards direction to try to decrease temperature. The position of equilibrium will shift towards the left, giving a lower yield of ammonia.

Question 11

What happens to the position of equilibrium when pressure is increased?

Answer 11

Equilibrium will shift towards the side with fewer moles of gas to oppose the change
- This reduces the pressure

Question 12

What happens to the position of equilibrium when pressure is decreased?

Answer 12

Equilibrium will shift towards the side with more moles of gas to oppose the change
- This increases the pressure

Question 13

If there are the same number of moles on both sides of the equation, what effect will pressure changes have on the position of equilibrium?

Answer 13

No effect

Question 14

Example exam question

What effect would increasing pressure have on the yield of methanol?

CO _(g) + 2H_{2 (g)} ⇌ CH₃OH _(g)

Answer 14

If pressure is increased the equilibrium will shift to oppose this and move towards the side with fewer moles of gas to try to reduce the pressure. The position of equilibrium will shift towards the right because there are 3 moles of gas on the left but only 1 mole of gas on the right, giving a higher yield of methanol.

Question 15

What happens to the position of equilibrium when concentration of reactants is increased?

Answer 15

Equilibrium shifts to the right (products) to oppose the change
- This decreases the concentration of the reactants

Question 16

What happens to the position of equilibrium when concentration of products is increased?

Answer 16

Equilibrium shifts to the left (reactants) to oppose the change
- This decreases the concentration of the products

Question 17

What happens to the position of equilibrium when a catalyst is added?

Answer 17

Has no effect on equilibrium however it allows equilibrium to be reached quicker

Question 18

Why might a compromise temperature be used in industry reactions?

Answer 18

Lower temperatures can give a higher yield but will result in a slow rate of reaction so a compromise temperature is used that gives a reasonable yield and rate

Question 19

Why might a compromise pressure be used in industry reactions?

Answer 19

Higher pressures can give a higher yield and will produce at a faster rate however high pressures are expensive to produce and contain so a compromise pressure will be used

Question 20

How are catalysts important for the industry?

Answer 20

Catalysts speed up the rate, allowing for a lower temperature to be used which therefore lower energy costs