17-Thermodynamics and Spontaneous Change

17-Thermodynamics and Spontaneous Change - 5.111 Lecture...

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5.111 Lecture Summary #17 October 19, 2009 Reading for today: Sections 7.1, 7.2, 7.8, 7.12, 7.13, 7.15, 7.16 Exam Wednesday, October 28 THERMODYNAMICS AND SPONTANEOUS CHANGE Spontaneous change — given enough time, occurs by itself without outside intervention. These reactions occur spontaneously at constant pressure… 2 Fe(s) + 3/2 O 2 (g) Fe 2 O 3 (s) Δ H° = –824.2 kJ/mol H 3 O + (aq) + OH (aq) 2H 2 O( A ) Δ H° = –55.9 kJ/mol ATP –4 (aq) + 2H 2 O( A ) ADP –3 (aq) + HPO 4 –2 (aq) + H 3 O + (aq) Δ H° = –24 kJ/mol But so do these … H 2 O(s) H 2 O( A ) Δ H° = +6.95 kJ/mol + +→ + 32 3 44 NH NO (s) H O( ) NH (aq) NO (aq) A Δ H° = +25.7 kJ/mole Is Δ H the key? NO! Condition for spontaneity under constant p & T involves Δ G… Δ G = Δ H – T Δ S GIBBS FREE ENERGY – a state function Figuring this out was a towering achievement of thermodynamics! Δ S is the change in entropy, a measure of disorder, a state function Δ G < 0 spontaneous process Δ G > 0 non-spontaneous process Δ G = 0 equilibrium Under constant p,T, a process is spontaneous when Δ G < 0, not necessarily when Δ H < 0. For example… CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O( A ) Δ H° = –890.3 kJ/mol, Δ S° = –0.242 kJ/K mol N CH N C C C N HC N NH 2 HC CH CH O HC CH 2 O OH OH P O O O P O O O P O O O N CH N C C C N HC N NH 2 HC CH CH O HC CH 2 O OH OH P O O O P O O O
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5.111 Lecture Summary #17 October 19, 2009 Δ G° = –890.3 kJ/mol – T(–0.242 kJ/K mol) at T = 298K Δ G° = –890.3 kJ/mol + 72.1 kJ/mol = –818.2 kJ/mol Δ G° is still negative, but not as negative as Δ H°. Still spontaneous. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O Δ H° = –2,816 kJ/mol Δ S° = +0.233 kJ/mol Δ G° = –2,816 – 298(0.233) = –2,885 kJ/mol Δ G° more negative than Δ H°. Δ S° positive — increase in disorder Δ S° negative — decrease in disorder Disorder of gas > liquid > solid Molecules in gas can move around freely, not locked in ordered locations Internal degrees of freedom – e.g. multiple molecular configurations, many quantum states that can be occupied - also contribute to entropy ENTROPY Δ S° FOR REACTIONS
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This note was uploaded on 02/08/2010 for the course CHEM 143 taught by Professor Mike during the Spring '10 term at École Normale Supérieure.

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17-Thermodynamics and Spontaneous Change - 5.111 Lecture...

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