The_Second_Law

The_Second_Law - The Second Law of Thermodynamics The first...

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1 The Second Law of Thermodynamics The first law permits us to assess what changes are permissible? And the answer is : Only those that may occur for which the internal energy of an isolated system remains constant. The second law permits us to identify the spontaneous changes among those permissible changes. Is one state of the system accessible from another state of the system by a spontaneous change? – Introduces a state function, entropy, S The universe has a natural direction of change Spontaneous Change A chemical or physical change in the system can occur by itself without an input of energy from the surroundings, i.e., freezing of water at – 5 o C and 1 atm pressure Non - spontaneous Change The surroundings must support the system with a continuous input of energy Spontaneous instantaneous (Ripening, aging) If a change is spontaneous in one direction it is not spontaneous in the opposite direction. A chemical reaction proceeding towards equilibrium is an example of a spontaneous change. Can the sign of H predict spontaneous change? A few examples CH 4 ( g ) + 2 O 2 ( g ) CO 2 ( g ) + 2 H 2 O ( g ) H o rxn = - 802 kJ spontaneous + exothermic 2 Fe ( s ) + 3/2 O 2 ( g ) Fe 2 O 3 ( s ) H o rxn = - 826 kJ spontaneous + exothermic Na ( s ) + ½ Cl 2 ( g ) NaCl ( s ) H o rxn = - 411 kJ spontaneous + exothermic H 2 O ( l ) H 2 O ( s ) H o rxn = - 6.02 kJ spontaneous + exothermic at temperatures < 0 H 2 O ( s ) H 2 O ( l ) H o rxn = + 6.02 kJ spontaneous + endothermic at temperatures > 0
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2 H 2 O ( l ) H 2 O ( g ) H o rxn = + 44.0 kJ spontaneous + endothermic at P = 1 atm., T = 298K and dry air Recall that most water-soluble salts have a positive H o so ln and yet dissolve spontaneously NaCl ( s ) Na + ( aq ) + Cl ( aq ) H o so ln = + 3.9 kJ NH 4 NO 3 ( s ) NH + 4 ( aq ) + NO 3 ( aq ) H o so ln = + 25.7 kJ And some more endothermic processes also spontaneous N 2 O 5 ( s ) 2 NO 2 ( g ) + ½ O 2 ( g ) H o rxn = + 109.5 kJ Ba(OH) 2 8 H 2 O ( s ) + 2NH 4 NO 3 ( s ) Ba 2+ ( aq ) + 2NO 3 ( aq ) + 2NH 3 ( aq ) + 10H 2 O ( l ) H o rxn = + 62.3 kJ Conclusion: The sign of H cannot predict spontaneous change A new state property is needed to establish a criterion for spontaneity that is valid at all temperatures. This property is called Entropy . Carnot Cycle Sadi Carnot investigated the principles governing the transformation of thermal energy (heat) into mechanical energy (work). He based his discussion on a cycle of four reversible steps step 1: isothermal expansion step 2: adiabatic expansion step 3: isothermal compression step 4: adiabatic com pression The material composing the system is confined in a cylinder fitted with a piston. The cycle uses two heat reservoirs , one at a higher temperature, T h and one at a lower temperature, T c A heat reservoir is a system which has the same temperature everywhere within it and this
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This note was uploaded on 04/07/2012 for the course CHEM 340 taught by Professor Staff during the Spring '08 term at Ill. Chicago.

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The_Second_Law - The Second Law of Thermodynamics The first...

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