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ERT 108/3 PHYSICAL CHEMISTRY SECOND LAW OF THERMODYNAMICS Prepared by: Pn. Hairul Nazirah Abdul Halim
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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
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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
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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
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Thermodynamic definition of entropy Unit of entropy: J K -1 Unit of molar entropy: J K -1 mol -1
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Entropy change for the isothermal expansion of a perfect gas Isothermal, dT = 0. Hence, ΔU = 0.
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Because the temp. is constant;
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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
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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 = 0. 3. Reversible isothermal compression from C to D. dS = q c /T c 4.Reversible adiabatic compression from D to A. dS = 0.
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The total change in entropy around the cycle is;
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The efficiency, ε of heat engine
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The efficiency, ε of heat engine:
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c) Thermodynamic Temperature Thermodynamic temperature scale is defined as;
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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;
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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;
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c) The variation of entropy with temperature
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Constant pressure heat capacity; Hence, at constant pressure;
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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;
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The standard reaction entropy, Δ r S °
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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
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Criteria for spontaneity Consider heat transfer at constant volume, dq v = dU; At constant V and no additional work;
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When heat is transferred at constant pressure, dq p = dH;
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Helmholtz energy, A is defined as; When the states of the system changes at constant temp.;
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Gibbs energy, G is defined as; When the states of the system changes at constant temp.;
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Criteria of spontaneous change;
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b)Some remarks on the Helmholz energy A change in system at constant temp. and volume is spontaneous if. The criterion of equilibrium,
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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. &
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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’.
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When a macroscopic isothermal change takes place in the system; With;
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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.
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e) Maximum additional (non-expansion) work W ad,max = Maximum additional (non-expansion) work, is given by the change in Gibbs energy;
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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
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The standard Gibbs energy of reaction:
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