Entropy – Randomness & Disorder

1. Entropy – Randomness & Disorder Mr Nelson - 2010

2. 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. 2

3. 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 metal with water. The products are more randomly distributed than the reactants

4. q T 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, .

5. 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.

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

7. 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.

8. 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.

9. Entropy is Disorder Disorder in a system can take many forms. Each of the following represent an increase in disorder and therefore in entropy: • Mixing different types of particles. i.e. dissolving salt in water. • A change is state where the distance between particles increases. Evaporation of water. • Increased movement of particles. Increase in temperature. • Increasing numbers of particles. Ex. 2 KClO3 2 KCl + 3O2

10. 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.

11. Entropy, S The entropy of a substance depends on its state: S (gases) > S (liquids) > S (solids) So (J/K-1mol-1) H2O (liquid) 69.95 H2O (gas) 188.8

12. Entropy and States of Matter S˚(Br2 liquid) < S˚(Br2 gas) S˚(H2O solid) < S˚(H2O liquid)

13. 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.

14. 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  1023 J/K.

15. 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

16. Standard Entropies • These are molar entropy values of substances in their standard states. • Standard entropies tend to increase with increasing molar mass.

17. Gibbs Free Energy At temperatures other than 25°C, DG° = DH  TS

18. Gibbs Free Energy • If DG is negative, the forward reaction is spontaneous. • If DG is 0, the system is at equilibrium. • If G is positive, the reaction is spontaneous in the reverse direction.

19. There are two parts to the free energy equation: H— the enthalpy term TS — 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

20. Free Energy and Temperature