1440Ch19trans - Chapter 19 Chemical Thermodynamics Chemical...

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Chemical Thermodynamics Chapter 19 Chemical Thermodynamics
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Chemical Thermodynamics First Law of Thermodynamics • You will recall from Chapter 5 that energy cannot be created nor destroyed. • Therefore, the total energy of the universe is a constant. • Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa.
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Chemical Thermodynamics SPONTANEOUS PROCESS SPONTANEOUS PROCESS A process that occurs without ongoing outside intervention. Examples Nails rusting outdoors
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Chemical Thermodynamics Ice melting at room temperature Expansion of gas into an evacuated space Formation of water from O 2 (g) and H 2 (g): 2H 2 (g) + O 2 (g) 2H 2 O(g)
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Chemical Thermodynamics Why are some processes spontaneous and others not? We know that temperature has an effect on the spontaneity of a process. e.g. T>0 o C ice melts spontaneous at this temp. H 2 O (s) H 2 O (l) T<0 o C water freezes spontaneous at this temp. H 2 O (l) H 2 O (s) T=0 o C water and ice in equilibrium H 2 O (l) H 2 O (s)
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Chemical Thermodynamics Exothermic processes tend to be spontaneous. Example Rusting of nail - SPONTANEOUS! 4Fe(s) + 3O 2 (g) Fe 2 O 3 (s) H = - 822.2 kJ.mol -1 Formation of water - SPONTANEOUS! 2H 2 (g) + O 2 (g) 2H 2 O(l) H = - 285.8 kJ.mol -1
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However , the dissolution of ammonium nitrate is also spontaneous , but it is also endothermic . NH 4 NO 3 (s) NH 4 + (aq) + NO 3 - (aq) H = +25.7 kJ.mol -1 So is: 2N 2 O 5 (s) 4NO 2 (g) + 2O 2 (g) H = +109.5 kJ.mol -1 a process does not have to be exothermic to be spontaneous. Chemical Thermodynamics something else besides sign of H must contribute to determining whether a process is spontaneous or not.
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Chemical Thermodynamics That something else is: ENTROPY (S) ENTROPY (S) extent of disorder! More disordered larger entropy Entropy is a state function S = S final -S initial Units: J K -1 mol -1
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Chemical Thermodynamics Examples of spontaneous processes where entropy increases: Dissolution of ammonium nitrate: NH 4 NO 3 (s) NH 4 + (aq) + NO 3 - (aq) H = +25.7 kJ.mol -1 Decomposition of dinitrogen pentoxide: 2N 2 O 5 (s) 4NO 2 (g) + 2O 2 (g) H = +109.5 kJ.mol -1
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Chemical Thermodynamics However, entropy does not always increase for a spontaneous process At room temperature: Spontaneous Non-spontaneous
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Chemical Thermodynamics SECOND LAW OF THERMODYNAMICS The entropy of the universe increases in any spontaneous process. S universe = S system + S surroundings Spontaneous process: S universe > 0 Process at equilibrium: S universe = 0 Thus S univ is continually increasing! S univ must increase during a spontaneous process, even if S syst decreases.
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Chemical Thermodynamics For example: Rusting nail = spontaneous process 4Fe(s) + 3O 2 (g) 2Fe 2 O 3 (s) S syst <0 S univ >0 BUT reaction is exothermic , entropy of surroundings increases as heat is evolved by the system thereby increasing
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This note was uploaded on 08/22/2010 for the course CHS 1440 taught by Professor Staff during the Spring '06 term at University of Central Florida.

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1440Ch19trans - Chapter 19 Chemical Thermodynamics Chemical...

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