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ch 9 study sheet

# ch 9 study sheet - L M Petrovich Chapter 9 Study Sheet...

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Unformatted text preview: L. M. Petrovich 10/14/04 Chapter 9 Study Sheet First Law: ∆ E universe = ∆ E system + ∆ E surroundings = 0 ∆ E system = - ∆ E surroundings ∆ E system = q + w system does work, w is negative work done on system, w is positive system loses heat, q is negative system gains heat, q is positive Enthalpy At constant pressure, q = ∆ H ∆ H < ∆ H > 0 exothermic endothermic feels hot feels cold Ways to figure out enthalpy 1) How the reaction feels hot = exothermic = ∆ H <0 cold = endothermic = ∆ H >0 2) Bond Enthalpies Sum of bonds broken – sum of bonds formed Be sure to draw Lewis Structures N 2 + 3H 2 ! 2NH 3 3) Phase changes going down = exothermic = ∆ H <0 going up = endothermic = ∆ H >0 Products Reactants H Products Reactants H ∆ H ~+(N N) + 3(H-H)- 6(N-H) ∆ H ~+(N N) + 3(H-H)- 6(N-H) ∆ H ~ + (941) + 3(436)- 6(391) ~ -97 kJ E out E out E out E out Solid Liquid Gas Potential Energy E in E in E in E in Solid Liquid Gas Potential Energy chapter 9 study sheet page 2 of 7 L. M. Petrovich 10/14/04 4) Bond strengths ________Weak bonds (low melting point) ________Strong bonds (high melting point) going down = exothermic = ∆ H <0 going up = endothermic = ∆ H >0 Entropy S = q / T or heat content / temperature in K. A measure of the concentration of energy. ∆ S universe = ∆ S system + ∆ S surroundings > 0 Whenever something happens, the universe is getting messier. Ways to figure out entropy 1) Gas in vs. Gas out 2 mol gas ! 0 mol gas ∆ S < 0 0 mol gas ! 1 mol gas ∆ S > 0 3 mol gas ! 3 mol gas ∆ S ~ 0 2) Phase Changes _______ Gas (dispersed energy, low order) _______ liquid (more concentrated energy, medium order) _______ solid (very concentrated energy, high order) going down = concentrating energy = ∆ S <0 going up = dispersing energy = ∆ S >0 3) Observation. If the reaction is bubbling, it’s making gas and ∆ S >0. If the reaction requires some gas to work, ∆ S <0. chapter 9 study sheet page 3 of 7 L. M. Petrovich 10/14/04 Free Energy From the second law of thermodynamics, the following equation is derived: ∆ G = ∆ H - T ∆ S Whenever ∆ G < 0 for a reaction in the forward direction, the forward reaction is spontaneously happening. Whenever ∆ G > 0 for a reaction in the forward direction, the forward reaction is not happening. However, the reaction is spontaneous in reverse. The sign of ∆ G is determined by the interaction of ∆ H and ∆ S. While you can memorize the chart, it might be easier to think of like this: At low temp, ∆ G is the same sign as ∆ H At high temp, ∆ G is the opposite sign of ∆ S....
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ch 9 study sheet - L M Petrovich Chapter 9 Study Sheet...

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