Equilibria Flashcards
what does it mean by a non-reversible reaction ‘going to completion’
at this points all reactants will be used up and converted into product
reversible reactions
- 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
what is a closed system
where reactant and product are not allowed to escape
what does it mean by the ‘position of equilibrium’?
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
example of a reaction which lies to LHS
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
At equilibrium…
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.
dynamic equilibrium
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.
equilibrium shown by rate graph
kinda like this: >-
where the rates of f and r reaction are equal at equilibrium point
equilibrium shown by concentration graph
100% reactants at start
as reaction proceeds, reactants used up and products formed
concentration of r and p are constant (=)
Le Chatelier’s Principle
If a system is subjected to any kind of change the system readjusts itself to counteract the applied change
an equilibrium can be shifted by changing the reaction conditions:
- temperature
- concentration
- pressure
enthalpy change for exothermic
negative
enthalpy change for endothermic
positive
if we cool down a reaction mixture…
it will move towards the side which releases heat energy to bring the temperature back up.
if we heat up a reaction mixture…
it will move towards the side which absorbs the heat energy to get rid of excess heat and cool back down
increasing temperature favours the…
endothermic reaction (which absorbs heat)
decreasing temperature favours the…
exothermic reaction (which gives out heat)
Colour change
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.
4 lines in table used for EQM questions
change made
to reverse change
side favoured
EQM shifts to
concentration definition
a measure of the number of particles per volume
Concentration: ways to manipulate EQM to favour products
- increase in concentration of reactants
2. decrease concentration of products
Concentration: ways to manipulate EQM to favour reactants
- decrease in concentration of reactants
2. increase in concentration of products
2 different question types for concentration
- adding more of a substance already in equation
- adding new substance (what will it react with)
eg adding OH- will react with H+
steps when adding new substance to the reaction
- write out ionic formula
- identify what it will react with
- follow usual steps to work out EQM shift
pressure definition
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)
Important notes for pressure EQM
- pressure only affects the equilibrium of a system that involves gases.
- a pressure change will alter equilibrium only if there are different numbers of moles of gas on each side
effect of increasing pressure
will favour the side with the lowest number of moles in the gas state
effect of decreasing pressure
will favour the side with the highest number of moles in the gas state.
problems with using a high pressure
more expensive
effect of adding a catalyst
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.
