Same factor b if an eqn is reversed sign of h changes

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Unformatted text preview: me factor. b) If an eqn. is reversed, sign of )H changes c) All substances NOT appearing in desired eqn. MUST cancel 12 F) Enthalpy of Formation Enthalpy change for the formation of a compound from its elements )Hf 1) Standard enthalpy change Enthalpy change when all reactants and and products are in their standard states )H° 13 2) Standard State Most stable state of a substance in its pure form under standard pressure (1 atm) & some specified temp. of interest (usually 25 °C) 3) Thermochemical Standard States A) solid or liquid Pure substance at 1 atm b) gas pressure of 1 atm c) species in solution Conc. of 1 M 14 4) Standard Enthalpy of Formation )H for the rxn in which 1 mole of a cmpd. is formed from its elements with ALL substances in their standard states (in kJ/mol) )Hf° Note: )Hf° = 0 for an element in its standard state )Hf° 1/2 N2(g) + 3/2 H2(g) 6 NH3(g) ! 46.2 Na(s) + 1/2 Cl2(g) 6 NaCl(s) ! 411.0 C(graphite) 6 C(diamond) + 1.897 15 16 C) Determine )Hrxn from )Hf° ° )Hrxn = 3 n )Hf° ! 3 m )Hf° ° prod. react. n = coef. in bal. eqn. for each product m = coef. in bal. eqn. for each reactant 17 I) Spontaneous Processes Spontaneous Process Process which proceeds on own without outside assistance - occurs in one direction only NONspontaneous Process Can NOT proceed w/o outside assistance - reverse of spont. process Processes which are spont. in one direction are nonspont. in the reverse direction. 18 Mix 2 ideal gases Open stopcock Spontaneously mix Reverse process is NOT spont. 19 Will all spont. rxns. be exothermic or vice versa? Will all nonspont. rxns. be endothermic or vice versa? NO! Spontaneity can depend on temp. & pressure T > 0 °C : melting spont. T < 0 °C : freezing spont. T = 0 °C : equilibrium and neither conversion occurs spont. 20 A) Reversible & IRreversible Processes 1) Reversible Process Change made in a way that system & surroundings can be restored to their original states by exactly reversing chg NO net chg. to system or its surr. 2) IRreversible Process Can NOT simply be reversed to restore system & surroundings to their original states A reversible change produces the max. amt. of work which can be done by a system on its surroundings. 21 3) Isothermal Expansion of a Gas Expansion of ideal gas at constant temperature. Remove partition - gas spont. expands to fill the whole cylinder. No work is done by the system. Compressing the gas back to the original state requires surroundings to do work on the system. Process is IRreversible 22 Can be done reversibly by doing the expansion and compression infinitely slowly so the external and internal pressures are always in equilibrium. - can not actually be done in real processes - ALL real processes are IRreversible 4) Conclusions a) A spont. process is an IRreversible process b) Driving force is tendency to go to a less ordered state or state of higher probability. 23 nd II) Entropy, S & 2 Law of Thermodynamics Measure of randomness or diso...
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This note was uploaded on 10/03/2013 for the course CHEM 1220 taught by Professor Zellmer during the Fall '13 term at Ohio State.

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