Equilibria

Question 1

What is a reversible reaction?

Answer 1

Can go forwards or backwards

Question 2

What would ∆H be in the backwards reaction?
Forwards = N₂ + 3H₂ ⇌ 2NH₃
∆H = -92 KJ/mol
Backwards = 2NH₃ ⇌ N₂ + 3H₂

Answer 2

+92KJ/mol

Question 3

What are examples of reversible reactions?

Answer 3

Decomposition of NH₄Cl (ammonium chloride)

Dehydration of CuSO₄ * 5H₂O (copper sulphate crystals)

Question 4

Explain what happens during the decomposition of ammonium chloride
NH₄Cl ⇌ NH₃ + HCl

Answer 4

NH₄Cl (white solid) is heated in testube (promotes forwards reaction)
NH₃ + HCl cools down (promotes backwards reaction)
White solid forms at top of test tube

Question 5

Explain what happens during Dehydration of copper sulphate crystals
CuSO₄ * 5H₂O ⇌ CuSO₄ + 5H₂O

Answer 5

CuSO₄ * 5H₂O (blue) is heated to promote forwards reaction
H₂O added to CuSO₄ (white) to promote backwards reaction

Question 6

What are reversible reactions used for?

Answer 6

Haber process: how to make ammonia, used in fertilisers

Question 7

What is equilibrium?

Answer 7

in a reversible reaction, both forwards and backwards reaction occurs at the same time. It is dynamic

Question 8

What is dynamic equilibrium?

Answer 8

forwards and backwards reaction occurring at same rate

Question 9

What happens to the composition of the system?

Answer 9

will not change
(no observational change)

Question 10

Why is a closed system needed?

Answer 10

stops energy (or reactant/product) from being transferred to/from system

Question 11

What is Le Chatelier’s principle?

Answer 11

whenever a change in conditions is introduced to a system at equilibrium, the position of equilibrium shifts to try and counter the change
(a system at equilibrium will oppose any change made to it)

Question 12

What is equilibrium position?

Answer 12

gives an idea of how ratio of reactant:product compares

Question 13

reactants ⇌ products
what happens if equilibrium moves to left?

Answer 13

more reactant

Question 14

reactant ⇌ products
what happens if equilibrium moves to right?

Answer 14

more product

Question 15

What happens if you increase amount of reactant?

Answer 15

equilibrium moves to right as uses up extra reactant, increasing product

Question 16

What happens if you reduce amount of reactant?

Answer 16

equilibrium moves to left to create more reactant, increasing reactant

Question 17

What happens if you increase amount of product?

Answer 17

equilibrium moves to left as uses up extra product, increasing reactant

Question 18

What happens if you reduce amount of product?

Answer 18

equilibrium right as creates more product, increasing product

Question 19

What happens if you increase pressure?

Answer 19

equilibrium moves to side with less moles of gas, increasing side with less moles of gas

Question 20

What happens if you decrease pressure?

Answer 20

equilibrium moves to side with more moles of gas, increasing side with more moles of gas

Question 21

What happens if you increase temperature?

Answer 21

promotes endothermic reaction, equilibrium moves to side which is endothermic
e.g. backwards (left) = endothermic then moves to left side

Question 22

What happens if you decrease temperature?

Answer 22

promotes exothermic reaction, equilibrium moves to side which is exothermic (to release energy)

Question 23

What happens if you add a catalyst?

Answer 23

no effect on equilibrium
will achieve equilibrium quicker

Question 24

N₂O₄ ⇌ 2NO₂
∆H = +58 KJ/mol
forwards = endothermic
What happens if you increase temperature?
What happens if you increase pressure?

Answer 24

increase temp will promote endothermic reaction. Forwards reaction is endothermic so position of equilibrium moves to right, increasing product

increasing pressure moves position of equilibrium to side with less moles of gas which is left, increasing reactant

Question 25

How can more reactant be made?

Answer 25

reduce amount of reactant / increase product

Question 26

How can more product be made?

Answer 26

reduce amount of product / increase amount of reactant