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 = S_{products} – S_{reactants}

If DS>0, S_{products} > S_{reactants} and DS has increased (+)

If DS<0, S_{products} < S_{reactants} 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

DS_{universe} = DS_{system} + DS_{surroundings}

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.

*SchoolWorkHelper*, 2021, https://schoolworkhelper.net/entropy/.