Chemical equilibria

Question 1

dynamic equilibrium

Answer 1

at equilibrium, there is no change in the concentrations of the products and the reactants because the rate of the forwards and backwards reactions is equal but the reaction has not stopped

Question 2

reversible reaction

Answer 2

a chemical reaction in which the products can be converted back into original reactants

Question 3

conditions for equilibrium (2)

Answer 3

system must be closed (reactants and products cannot escape)

position of equilibrium will be the same for a particular reaction given that temperature and pressure stay the same

Question 4

Le Chatelier’s principle

Answer 4

when a change is applied to a system in equilibrium, the position of equilibrium shifts as the system tries to oppose the applied change

Question 5

factors affecting position of equilibrium

Answer 5

concentration, temperature, pressure

Question 6

how does changing concentration affect the position of equilibrium

Answer 6

Increasing concentration of reactant/ decreasing product would shift the equilibrium to the right and increase yield of the products

System opposes the change so attempts to decrease concentration of reactant

Question 7

how does changing temperature affect the position of equilibrium

Answer 7

If the forwards reaction is exothermic, temperature needs to decrease to increase the yield of the products
 Equilibrium shifts to the right to oppose the decrease and increase the temperature by producing more product

If the forwards reaction is endothermic, temperature needs to increase to increase the yield of the products as equilibrium shifts to oppose the change and lower temperatures

Question 8

how does changing pressure affect the position of equilibrium

Answer 8

More moles of the products than the reactants, pressure needs to decrease to increase yield so equilibrium shifts to the right to oppose the change

More moles of the reactants than the products, pressure needs to increase to increase yield so equilibrium shifts to the right oppose the change

Question 9

how does the addition of a catalyst affect the position of the equilibrium

Answer 9

no effect on yield

affects the rate of the forwards and backwards reaction equally

Question 10

yield

Answer 10

how much product is actually made in a chemical reaction

Question 11

reasons for not 100% yield (5)

Answer 11

Impure reactants
Unexpected reactants producing unexpected products
Reaction is reversible
Product is lost when transferring between pieces of apparatus
Product is lost during separation processes

Question 12

uses of ammonia (2) and reason for one of them

Answer 12

Ammonia is a source of nitrogen for plants so used for fertilisers and also household cleaners

Question 13

equation for the haber process

Answer 13

N2 (g) + 3H2 (g) -> 2NH3 (g) ΔH= -ve

Question 14

conditions for the haber process

Answer 14

450°C, 200atm, iron catalyst

Question 15

explain for the temperature haber process

Answer 15

• 450°C exothermic so low temperature for high yield but slow rate of reaction so compromise with intermediate temperature and iron catalyst

Question 16

explain for the pressure haber process

Answer 16

• 200atm more moles of the reactants than the products so high pressure but expensive equipment and high risk of explosion so compromise to intermediate pressure

Question 17

describe the haber process (4)

Answer 17

1. Converter: H2 and N2 heated to 450°C and compressed to 200atm
2. Reaction vessel: gases pass over iron catalyst
3. Separator: gases are cooled, ammonia is removed as a liquid (condenses)
4. Unreacted H2 and N2 are recycled back to the reaction vessel

Question 18

what affects Kc

Answer 18

Value of Kc is only affected by temperature (not pressure or catalyst) and will change depending on whether the reaction is exothermic or endothermic

Question 19

affects on Kc if reaction is exothermic and temperature increases

Answer 19

shifts equilibrium to the left so decreases Kc

Question 20

affects on Kc if reaction exothermic is and temperature decreases

Answer 20

shifts equilibrium to the right so increase in Kc

Question 21

affects on Kc if reaction is endothermic and temperature increases

Answer 21

shifts equilibrium to the right so increase in Kc

Question 22

affects on Kc if reaction is endothermic and temperature decreases

Answer 22

shifts equilibrium to the left so decrease in Kc

Question 23

what does a small Kc represent

Answer 23

a shift to the left

equilibrium favours reactants

Question 24

what does a large Kc represent

Answer 24

a shift to the right

greater amount of products