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

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

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

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 ΔS tot = total entropy of the system and its surroundings. Entropy

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

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

Because the temp. is constant;

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. Heat Engines

A reversible Carnot Cycle consist of 4 reversible stages: 1.Reversible isothermal expansion from A to B. dS = q h /T h. 2. Reversible adiabatic expansion from B to C. dS = Reversible isothermal compression from C to D. dS = q c /T c 4.Reversible adiabatic compression from D to A. dS = 0.

The total change in entropy around the cycle is;

The efficiency, ε of heat engine

The efficiency, ε of heat engine:

c) Thermodynamic Temperature Thermodynamic temperature scale is defined as;

Entropy Changes a)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;

b) The expansion of a perfect gas The change in entropy of a perfect gas that expands isothermally from V i to V f is;

c) The variation of entropy with temperature

Constant pressure heat capacity; Hence, at constant pressure;

d)The measurement of entropy If a substance melts at T f and boils at T b, then its entropy above its boiling temperature is;

The standard reaction entropy, Δ r S °

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; The Helmholtz and Gibss Energies

Criteria for spontaneity Consider heat transfer at constant volume, dq v = dU; At constant V and no additional work;

When heat is transferred at constant pressure, dq p = dH;

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

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

Criteria of spontaneous change;

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

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. &

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’.

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

c)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.

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

The standard entropies and enthalpies of reaction can be combined to obtain the standard Gibbs energy of reaction, Δ r G ° ; Standard Molar Gibbs Energies

The standard Gibbs energy of reaction: