Equilibria

Question 1

what does it mean by a non-reversible reaction ‘going to completion’

Answer 1

at this points all reactants will be used up and converted into product

Question 2

reversible reactions

Answer 2

• products form but they also break down into reactants.
• an 100% conversion to products can never be achieved and there will always be a mixture of reactants and products at the end

Question 3

what is a closed system

Answer 3

where reactant and product are not allowed to escape

Question 4

what does it mean by the ‘position of equilibrium’?

Answer 4

the extent to which the reaction has taken place
if EQM lies to right - more product than reactant
if EQM lies to left - more reactant than product
can be shown by size of arrows

Question 5

example of a reaction which lies to LHS

Answer 5

the dissociation of water molecules into the hydrogen and hydroxide ion.
very few (1 in 10 million) water molecules dissociate and so the mixture is mainly molecules

Question 6

At equilibrium…

Answer 6

the reactants change into products at exactly the same rate at which the products change back into reactants. This only occurs in a closed system.

Question 7

dynamic equilibrium

Answer 7

in a closed system, reversible reactions attain a state of dynamic equilibrium when the rates of the forward and the reverse reactions are equal.
REACTION HAS NOT STOPPED
Concentration of R and P are constant, rarely equal.

Question 8

equilibrium shown by rate graph

Answer 8

kinda like this: >-

where the rates of f and r reaction are equal at equilibrium point

Question 9

equilibrium shown by concentration graph

Answer 9

100% reactants at start
as reaction proceeds, reactants used up and products formed
concentration of r and p are constant (=)

Question 10

Le Chatelier’s Principle

Answer 10

If a system is subjected to any kind of change the system readjusts itself to counteract the applied change

Question 11

an equilibrium can be shifted by changing the reaction conditions:

Answer 11

1. temperature
2. concentration
3. pressure

Question 12

enthalpy change for exothermic

Answer 12

negative

Question 13

enthalpy change for endothermic

Answer 13

positive

Question 14

if we cool down a reaction mixture…

Answer 14

it will move towards the side which releases heat energy to bring the temperature back up.

Question 15

if we heat up a reaction mixture…

Answer 15

it will move towards the side which absorbs the heat energy to get rid of excess heat and cool back down

Question 16

increasing temperature favours the…

Answer 16

endothermic reaction (which absorbs heat)

Question 17

decreasing temperature favours the…

Answer 17

exothermic reaction (which gives out heat)

Question 18

Colour change

Answer 18

if the equilibrium shifts to the right, the reaction mixture turns the colour of the product
if the equilibrium shifts to the left, the reaction mixture turns the colour of the reactants.

Question 19

4 lines in table used for EQM questions

Answer 19

change made
to reverse change
side favoured
EQM shifts to

Question 20

concentration definition

Answer 20

a measure of the number of particles per volume

Question 21

Concentration: ways to manipulate EQM to favour products

Answer 21

1. increase in concentration of reactants

2. decrease concentration of products

Question 22

Concentration: ways to manipulate EQM to favour reactants

Answer 22

1. decrease in concentration of reactants

2. increase in concentration of products

Question 23

2 different question types for concentration

Answer 23

1. adding more of a substance already in equation
2. adding new substance (what will it react with)
   eg adding OH- will react with H+

Question 24

steps when adding new substance to the reaction

Answer 24

1. write out ionic formula
2. identify what it will react with
3. follow usual steps to work out EQM shift

Question 25

pressure definition

Answer 25

the number of gas particles hitting walls of a container over a set time frame
higher pressure when larger number of gas particles are present (more collisions with wall of container)

Question 26

Important notes for pressure EQM

Answer 26

1. pressure only affects the equilibrium of a system that involves gases.
2. a pressure change will alter equilibrium only if there are different numbers of moles of gas on each side

Question 27

effect of increasing pressure

Answer 27

will favour the side with the lowest number of moles in the gas state

Question 28

effect of decreasing pressure

Answer 28

will favour the side with the highest number of moles in the gas state.

Question 29

problems with using a high pressure

Answer 29

more expensive

Question 30

effect of adding a catalyst

Answer 30

rate of froward and reverse reaction is increased
as there is the same decrease in Ea for the forward and reverse reaction, a catalyst does NOT effect EQM, it just reaches it quicker.