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

Spontaneity and Thermodynamics

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

The Reaction Quotient

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.

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