3.1.6.1 Chemical equilibria and Le Chatelier's principle

Question 1

What is a reversible reaction?

Answer 1

A reaction where the products of a reaction can themselves react to produce the original reactants.

Question 2

How to represent reversible reaction?

Answer 2

Double arrow symbol

Question 3

Describe the energy changes in a reversible reaction

Answer 3

If the forward reaction is exothermic, the backward reaction is endothermic

Question 4

Can a reversible reaction ever stop?

Answer 4

It can reach dynamic equilibrium, but it never stops!

Question 5

What is dynamic equilibrium?

Answer 5

Can only happen in a closed system at a constant temperature.
Where the rate of the forward reaction = rate of the backward reaction and the concentrations of reactants and products are constant

Question 6

Why do all reversible reactions reach dynamic equilibrium?

Answer 6

As the reactants get used up, the forward reaction slows down, and as more product is made, the backward reaction speeds up. Eventually the forward reaction will happen at the same rate as the backward reaction

Question 7

What is Le Chatelier’s principle?

Answer 7

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

Question 8

What are the factors that affect the position of equilibrium?

Answer 8

Temperature
Pressure
Concentration

Question 9

What effect does temperature have on the position of equilibrium?

Answer 9

If temp. is increased, equilibrium shifts to oppose this and move in the endothermic direction to try to reduce the temperature by absorbing heat.
Vice-versa

Question 10

What effect does changing pressure have on the position of equilibrium?

Answer 10

Increasing pressure causes the equilibrium to shift to the side with fewer moles of gaseous molecules to oppose the change and reduce pressure.
Vice-versa

Question 11

What effect does changing concentration have on the position of equilibrium?

Answer 11

If you increase the concentration of a reactant, equilibrium shifts right to oppose the change by removing and decreasing the concentration of the reactant so the yield of the product increases.
Vice-versa

Question 12

What is the effect of catalyst on the time taken to reach equilibrium?

Answer 12

Speeds up rate of reaction so reduces time taken to reach equilibrium

Question 13

What is the effect of a catalyst on the position of equilibrium?

Answer 13

No effect because it speeds up the rate of the forward and backward reactions by the same amount

Question 14

What are the things to be considered in industrial processes?

Answer 14

Cost, reaction rate, yield of desired products (and safety)

Question 15

What are the advantages of using a high pressure industrially?

Answer 15

Higher yield of product (depends),
Faster rate of reaction

Question 16

What are the disadvantages of using a high pressure industrially?

Answer 16

Expensive; need electrical energy to provide high energy/ pump the gasses.
High pressure also leads to unwanted polymerisation of ethene to poly(ethene)

Question 17

What are the advantages of using a high temperature industrially?

Answer 17

Rate of reaction is faster

Question 18

What are the advantages of using a lower temperature industrially?

Answer 18

Cheaper
Greater yield of ethanol