Spontaneous events occur without having any energy supplied to them from the surroundings
Many spontaneous reactions are exothermic, this is as the products reach a lower energy (more stable) state
Endothermic reactions can also be spontaneous however they result in a higher energy product so energy and enthalpy cannot explain this alone, entropy must also be considered...
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Entropy
Entropy shows how disordered something is
Most spontaneous reactions are accompanied by an increase in disorder
The degree of disorder of a substance is measured by its entropy
The symbol for entropy is S
The greater the disorder the higher the entropy
Solids have lower entropy than liquids which have lower entropy than gases
Complex substances have higher entropy values than simple ones
Entropy increases with temperature
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Entropy Values
Values of entropy are measured in JK-1mol-1
Under standard conditions (298K and 100kPa) there is a standard entropy for a reaction
A change in entropy (/\S) for a reaction is calculated by:
/\S = sum of products S - sum of reactants S
for an increase in entropy /\S is positive
for an decrease in entropy /\S is negative
/\ = delta
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Free Gibbs Energy
Free energy is related to the enthalpy and entropy (and temperature) of a chemical reaction or a change in state
The symbol is /\G
The equation is /\G = /\H - T/\S
The units used in the equation are: /\G = kJmol-1 /\H = kJmol-1 T = K /\S = kJK-1mol-1 (JK-1mol-1/1000)
A reaction is said to be spontaneous/feasible when /\G is equal or less than 0.
When /\G = 0 then T = /\H//\S this is the minimum temperature the reaction is feasible
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Graphs of /\G = /\H - T/\S
When /\H is -ve and /\S is +ve as T increases /\G decreases
When /\H is -ve and /\S is -ve as T increases /\G increases
When /\H is +ve and /\S is +ve as T increases /\G decreases
When /\H is +ve and /\S is -ve as T increases /\G increases
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