A-Level/ Advanced Higher Chemistry course notes. Concise notes covering chemical equilibrium. Notes include a clear and easy to follow summary of the topic, highlighted key-points, and supporting examples where appropriate.
CHEMICAL EQUILIBRIUM
A chemical reaction is in equilibrium when the rate of the forward reaction is equal
to the rate of the reverse reaction
At equilibrium the concentrations the reactants and the products will be constant but
of
not equal The composition of the reactants and products in the equilibrium mixture
remains constant indefinitely
an equilibrium can
The position of equilibrium can be changed only beestablished
by in a closedsystem
concentration
altering the of a reactant or product
changing the pressure if there are different numbers of moles of gas on
both sides of the balanced equation
altering the temperature
A catalyst speeds up the rate at which the equilibrium is reached but has no effect
on the position of equilibrium
The Equilibrium Constant K
The equilibrium constant K characterises the equilibrium composition of the reaction
mixture
For the general reaction aA BB cc d the equilibrium expression is
d
CC
A B
A CBT Cc and D are the equilibrium concentrations of A B C and D
a b c and d are the stoichiometric coefficients in the balanced reaction equation
The concentration values are
usually measured in moth but for gaseous reactions
the partial pressures may be used Whatever the concentrations are measured in
the value calculated for K has no units
The value of an equilibrium constant indicates the position of equilibrium
Homozygous equilibria all species are in the same state
Heterozygous equilibria the species are in more than one state
, The concentrations of pure solids
and pure liquids at equilibrium are
taken as constant and given the
value of 1 in the equilibrium
expression
The numerical value of the equilibrium
constant depends on thereaction
temperature and is independent of
concentration and or pressure
catalyst alsohas no effect on the
A
numerical value the equilibriumconstant
of
The Effect of Temperature on the Equilibrium constant
The value of the equilibrium constant K is dependent on temperature
For an endothermic reaction an increase temperature favours the products and so the
in
ratio of products reactants increases and therefore K increases In general for endothermic
reactions a rise in temperature causes an increase in K and the yield of product is
increased
For an exothermic reaction an increase in temperature favours the reactants and so the
ratio of products reactants decreases and therefore k decreases In general for exothermic
reactions a rise in temperature causes a decrease in K and the yield of product is
decreased
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