Chapter 18 Notes

Chapter 18 Notes - Chapter 18 Thermodynamics:...

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Unformatted text preview: Chapter 18 Thermodynamics: Directionality of Chemical Reactions Why are equilibria product- or reactant- favored? Why do some reactions occur spontaneously? Why do others require help (heat, spark)? Exothermic reactions are often product favored (but not always). Reactant- & Product-Favored Processes 2 Al(s) + 3 Br 2 () 2 AlBr 3 () Product favored; spontaneous Energy will spread out (disperse) unless it is hindered from doing so. Dispersal of energy is probability driven: E is more likely to spread over many atoms than over just a few. Chemical Reactions & Dispersal of Energy Most exothermic reactions are product favored. E is transferred to the surroundings. Bond (potential) E dispersed into many more atoms and molecules. E packet A**B C D A* B* C D A* B C* D A* B C D* A B**C D A B* C* D A B* C D* A B C**D A B C* D* A B C D** Probability & Dispersal of Energy A*B* + C D Excited atoms (A, B) What are the possible outcomes if 2 excited atoms make contact with 2 more atoms? E stays with original atoms in only 3 (of 10) cases: 70% chance that E will disperse. More atoms: E concentration is much less likely. Unexcited atoms E is dispersed when a system expands. Gases expand to fill a container. Dispersal of Matter Quantum theory: E-levels for gas motion get closer together when volume expands. More E-levels become accessible at a given T. E will disperse over more levels. removable barrier In summary : Processes are product favored if E is dispersed from a few levels into many levels. Measuring Dispersal of E : Entropy In thermodynamics, nanoscale E dispersal is measured by the Entropy ( S) of a system. This typically occurs during: exothermic reactions (bond E dispersal). processes in which matter is dispersed. At constant T : S = S final S initial = q rev T rev: Only applies to reversible changes. Absolute T A reversible process: can be reversed by a slight change in conditions. e.g. ice melting at 0.0 C and 1 atm is reversible. small decrease in T will cause it to refreeze . Measuring Dispersal of E : Entropy Calculate S when 25.0 g of Al(s) melts at its normal melting point (660.3 C; 1 atm). H fus (Al)=10.7 kJ mol-1 T = 660.3 C = 933.5 K Measuring Dispersal of E : Entropy S = q rev T = H fus T q p = H Reversible if normal melting 25.0 g 26.98 g mol-1 n Al = = 0.9266 mol S = 0.9266 mol(10.7 kJ mol-1 ) 933.5 K = 0.01062 kJ K-1 S = 10.6 J K-1 A perfect crystal, at absolute zero (0 K) has: Minimum molecular motion (min. E dispersal). S = 0 (perfect crystal at 0 K) (The 3 rd law of thermodynamics) S = S final S initial = q rev T S room T = S S initial = S Measure the heat to change from 0 K room- T (reversibly). Gives S and the absolute S at room T ....
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This note was uploaded on 01/27/2012 for the course CH 1213 taught by Professor Xia during the Spring '07 term at Mississippi State.

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Chapter 18 Notes - Chapter 18 Thermodynamics:...

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