Entropy, S, is the tendency towards randomness or disorder in a system
– an ordered arrangement of particles has a lower entropy/disorder than the same number of particles in random arrangements
Entropy becomes more disordered with:
1.) ^ Temperature – more chaotic motion
2.) ^ Volume of gases
3.) change in phys. State
A change in entropy is calculated by subtracting the entropy of the reactants from the entropy of the products
DS = Sproducts – Sreactants
If DS>0, Sproducts > Sreactants and DS has increased (+)
If DS<0, Sproducts < Sreactants and DS has decreased (-)
Physical Changes where an increase in S is found:
– change in state (s>l, s>g, l>g) or dissolving
– water falling (i.e. rain)
– solutes moving from high to low conc. until equil. is met
Chemical Changes where is increase in S is found:
– more moles of products than reactants
– complex molecule broken into simpler ones
– change of state due to chemical change
Spontaneous Reactions
-spontaneous rxns are always accompanied by an increase in S
– exothermic rxns are spontaneous at all T and have an increase in S
– endothermic rxns will only be spontaneous if there is an increase in S
Second Law of Thermodynamics: All changes, either directly or indirectly, increases the entropy of the universe. Mathematically, this means
DSuniverse = DSsystem + DSsurroundings
The system can decrease its entropy as long as there’s a larger increase in entropy of the surroundings
Predicting Spontaneity
Gibbs Free Energy: energy available to do work
Reactions/changes are spontaneous (occur with no input of outside E) if accompanied by a decrease of Gibbs Free Energy
DG must be LESS THAN 0 (negative value) for a reaction to be spontaneous.
A rxn must release free energy to be spontaneous. (lose free energy, lose the ability to do work)
DG = DH – TDS Called the Gibbs-Helmholtz free energy equation
When DG is -, forward rxn is favourable
When DG is +, reverse rxn is favourable
When DG is 0, the rxn is at equil.
Exothermic reactions are favourable because DH is –
Most rxns are more favourable at higher T, as this increases the value of
-TDS, this makes DG –
| DH | DS | -TDS | Comment |
| – | + | – | DH and -TDS are (-) therefore DG = – therefore spont. |
| + | – | + | DH +, -TDS + therefore DG = + therefore non. spont |
| – | – | + | DH – and -TDS + therefore DG = – or +
– if T is low than DG = – |
| + | + | – | DH + and -TDS is – therefore DG = – or +
– if T is high than DG = – |
Equilibrium occurs in a system when it has reached its maximum entropy. A maximum state of disorder has been reached.
