Entropy – Randomness & Disorder

Mr Nelson - 2010

Entropy

Entropy is defined as a state of disorder or randomness.

In general the universe tends to move toward release of energy and greater entropy.

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Entropy

Spontaneous chemical processes often result in a final state is more Disordered or Random than the original.

The Spontaneity of a chemical process is related to a change in randomness.

Entropy is a thermodynamic property related to the degree of randomness or disorder in a system.

Reaction of potassium Reaction of potassium metal with water. The metal with water. The products are more products are more randomly distributed randomly distributed than the reactantsthan the reactants

Entropy

• Entropy (S) is a term coined by Rudolph Clausius in the 19th century.

• Clausius was convinced of the significance of the ratio of heat delivered and the temperature at which it is delivered, .

qT

Second Law of Thermodynamics

States that the entropy of the universe increases for spontaneous processes, and the entropy of the universe does not change for reversible processes.

• Things will head toward higher levels of disorder.

Spontaneous Processes

Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.

Spontaneous Processes• Processes that are spontaneous at one temperature may

be nonspontaneous at other temperatures.

• Above 0 C it is spontaneous for ice to melt.• Below 0 C the reverse process is spontaneous.

Reversible Processes

In a reversible process the system changes in such a way that the system and surroundings can be put back in their original states by exactly reversing the process.

Entropy is Disorder

Disorder in a system can take many forms. Each of the following represent an increase in disorder and therefore in entropy:

1. Mixing different types of particles. i.e. dissolving salt in water.

2. A change is state where the distance between particles increases. Evaporation of water.

3. Increased movement of particles. Increase in temperature.

4. Increasing numbers of particles. Ex.

2 KClO3 2 KCl + 3O2

Entropy States

• The greatest increase in entropy is usually found when there is an increase of particles in the gaseous state.

• The symbol for the change in disorder or entropy is given by the symbol, S.

• The more disordered a system becomes the more positive the value for S will be.

• Systems that become more ordered have negative S values.

The entropy of a substance depends on its state: S (gases) > S (liquids) > S (solids)

Entropy, S

SSoo (J/K (J/K-1-1molmol-1-1))

HH22O (liquid)O (liquid) 69.9569.95

HH22O (gas)O (gas) 188.8188.8

SSoo (J/K (J/K-1-1molmol-1-1))

HH22O (liquid)O (liquid) 69.9569.95

HH22O (gas)O (gas) 188.8188.8

Entropy and States of Matter

S˚(BrS˚(Br22 liquid) < S˚(Br liquid) < S˚(Br22 gas) gas) S˚(HS˚(H22O solid) < S˚(HO solid) < S˚(H22O liquid)O liquid)

Entropy on the Molecular Scale• Ludwig Boltzmann described the concept of entropy on

the molecular level.• Temperature is a measure of the average kinetic energy

of the molecules in a sample.

Entropy on the Molecular Scale

• Each thermodynamic state has a specific number of microstates, W, associated with it.

• Entropy is

S = k lnW

where k is the Boltzmann constant, 1.38 1023 J/K.

Entropy on the Molecular Scale

• The number of microstates and, therefore, the entropy tends to increase with increases in

– Temperature

– Volume

– The number of independently moving molecules

Standard Entropies

• These are molar entropy values of substances in their standard states.

• Standard entropies tend to increase with increasing molar mass.

Gibbs Free Energy

At temperatures other than 25°C,

G° = H TS

Gibbs Free Energy

1. If G is negative, the forward reaction is spontaneous.

2. If G is 0, the system is at equilibrium.

3. If G is positive, the reaction is spontaneous in the reverse direction.

Free Energy and Temperature

• There are two parts to the free energy equation: H— the enthalpy term

– TS — the entropy term

• The temperature dependence of free energy, then comes from the entropy term.

• How does G change with temperature?

Free Energy and Temperature