ERT 108/3 PHYSICAL CHEMISTRY SECOND LAW OF THERMODYNAMICS

1. ERT 108/3PHYSICAL CHEMISTRYSECOND LAW OF THERMODYNAMICS Prepared by: Pn. Hairul Nazirah Abdul Halim

2. About This Chapter • To explain the origin of the spontaneity of physical and chemical change. • Heat engines • Entropy • Calculation of entropy changes • Helmholtz and Gibbs energies

3. The direction of spontaneous change • What is the direction of spontaneous change of the following reaction? ½ N2 + 3/2 H2 NH3 • Answer to this question is obtained by calculating the entropy change in the system and the surroundings.

4. Entropy • Second Law of Thermodynamics use entropy, S to identify the spontaneous changes. • The entropy of an isolated system increases in the course of a spontaneous change: • Where ΔStot = total entropy of the system and its surroundings.

5. Thermodynamic definition of entropy Unit of entropy: J K-1 Unit of molar entropy: J K-1 mol-1

6. Entropy change for the isothermal expansion of a perfect gas Isothermal, dT = 0. Hence, ΔU = 0.

7. Because the temp. is constant;

8. Heat Engines Automobile engine • Automobile engine operates in a cyclic process of fuel intake, compression, ignition and expansion, and exhaust. • Occurs several thousand times per minutes. • Is used to perform work on the surroundings. • The system consists of piston & cylinder assembly with diathermal walls. • The expansion and contraction of the gas caused by changes in its temperature drives the piston in or out of cylinder.

9. A reversible Carnot Cycle consist of 4 reversible stages: • Reversible isothermal expansion from A to B. dS = qh/Th. 2. Reversible adiabatic expansion from B to C. dS = 0. 3. Reversible isothermal compression from C to D. dS = qc/Tc • Reversible adiabatic compression from D to A. dS = 0.

11. The total change in entropy around the cycle is;

12. The efficiency, εof heat engine

13. The efficiency, εof heat engine:

14. c) Thermodynamic Temperature Thermodynamic temperature scale is defined as;

15. Entropy Changes • Entropy change of phase transition at the transition temp. • Phase transition such as solid melts to liquid, liquid phase turns into a gas. • At constant pressure; • The change in molar entropy of the system;

18. b) The expansion of a perfect gas The change in entropy of a perfect gas that expands isothermally from Vi to Vf is;

19. c) The variation of entropy with temperature

20. Constant pressure heat capacity; Hence, at constant pressure;

21. d) The measurement of entropy If a substance melts at Tf and boils at Tb, then its entropy above its boiling temperature is;

22. The standard reaction entropy, ΔrS°

23. The Helmholtz and Gibss Energies • Consider a system in thermal equilibrium with its surroundings at a temperature T. • When a change in the system occurs and there is a transfer of energy as heat between system and its surrounding;

24. Criteria for spontaneity Consider heat transfer at constant volume, dqv = dU; At constant V and no additional work;

25. When heat is transferred at constant pressure, dqp = dH;

26. Helmholtz energy, A is defined as; • When the states of the system changes at constant temp.;

27. Gibbs energy, G is defined as; • When the states of the system changes at constant temp.;

28. Criteria of spontaneous change;

29. b) Some remarks on the Helmholz energy • A change in system at constant temp. and volume is spontaneous if . • The criterion of equilibrium,

30. & Both of the above equations are interpreted as follows: • –ve value of dA is favoured by –ve value of dU and +ve value of TdS.

31. c)Maximum work The change in the Helmholtz function is equal to the maximum work accompanying a process; A sometimes called the ‘maximum work function’ or ‘work function’.

32. When a macroscopic isothermal change takes place in the system; • With;

33. Some remarks on the Gibbs energy • Gibbs energy = free energy • At constant pressure and temperature, chemical reactions are spontaneous in the direction of decreasing Gibbs energy. • If G decreases as the reaction proceeds: spontaneous tendency to convert reactants to products. • If G increases, the reverse reaction is spontaneous.

34. e) Maximum additional (non-expansion) work Wad,max = Maximum additional (non-expansion) work, is given by the change in Gibbs energy;

35. Standard Molar Gibbs Energies • The standard entropies and enthalpies of reaction can be combined to obtain the standard Gibbs energy of reaction, ΔrG°;

36. The standard Gibbs energy of reaction: