OCR AS Chemistry F332: Equilibria

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Preview of OCR AS Chemistry F332: Equilibria

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Equilibrium is a state of balance in which nothing changes.
However, the system is dynamic, i.e. in constant motion.
As fast as the reactants are converted into products, the products are being converted back
into reactants.
The forward reaction is the same as the back reaction.
A chemical system is in dynamic equilibrium when:
The concentrations of the reactants and the products remain the same
The rate of the forward reaction is the same as the rate of the reverse reaction
If the backward reaction is significant then the reaction is reversible.
Position of Equilibrium
There are many equilibrium mixtures possible for a given reaction system, depending on the
concentrations of the substances you mix and the conditions such as temperature and
We often use the term position of equilibrium to describe one set of equilibrium
concentrations for a reaction.
If one of the concentrations is changed then the system is no longer in equilibrium, and the
concentrations of all the substances will change until a new position of equilibrium is
If most of the reactants become products before the reverse reaction increases sufficiently
to establish equilibrium, we say that the position of equilibrium lies to the right ­ the
If little of the reactants have changed to products when the reverse reaction becomes equal
to the rate of the forward reaction, we say that the position of equilibrium lies to the left.

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Shifting the Position of Equilibrium
The position of an equilibrium can be altered by changing:
The concentrations of reacting substances if dealing with solutions
The pressure of reacting gases
The temperature
Le Chatelier's Principal
If a system is at equilibrium and a change is made in any of the conditions, then the system
responds to counteract the change as much as possible.
Using Catalysts
Using a catalyst does not shift the position of equilibrium.
A catalyst lowers the activation enthalpy of a reaction.…read more

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Changing the Concentration
Concentration change Equilibrium shift What it does
Increasing reactants To the right Decreases reactants
Increasing products To the left Decreases products
Decreasing reactants To the left Increases reactants
Decreasing products To the right Increases products
Changing the Pressure
Pressure change Equilibrium shift Example
Increases To the side with fewer gas CO (g) + 2H2 (g) = CH3OH (g)
molecules 3 molecules 1 molecule
Moves to the right
Decreases To the side with more gas CH4 (g) + H2O (g) = CO (g)…read more

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Linking Equilibria together
In many systems, particularly naturally occurring systems, two or more equilibria are linked
together so that the product of one equilibrium is the reactant in the second equilibrium .
This is the case for the two reversible reactions involving carbon dioxide.
CO2 (g) = CO2 (aq)
CO2 (aq) + H2O (l) = HCO3- (aq) + H+ (aq)
The product of the first equilibrium is a reactant in the second, so that the dissolved carbon
dioxide is involved in both equilibria.…read more

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Steady State Systems
A chemical equilibrium can only be established in a closed system which is sealed off from
its surroundings.
In an open system, a series of reactions can reach a steady state, where the concentrations
of reactants and products remain constant.…read more


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