08 Applications

08 Applications - What makes a cold pack work? Cold packs...

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1 1 What makes a cold pack work? Cold packs use an endothermic chemical reaction to spontaneously absorb energy, e.g. NH 4 NO 3 (s) + H 2 O(l) NH 4 + (aq) + NO 3 - (aq) H o = +21 kJ/mol This reaction is spontaneous because A) H > 0 B) H < 0 C) G > 0 D) S > 0 E) S < 0 G = H – T S 2 Exam 01 Wednesday, February 20 8:30 – 10:00 PM Rooms to be announced Bring SB ID card, #2 pencils w/ erasers Bring calculator w/ spare batteries Covers Assignments Jan 28 – Feb 15 See Assignment Schedule Chapters 6 and 18 20 – 22 multiple choice questions Plus 1 bonus question.
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2 3 4 QRT Homework Chapter 18 Wed: 79, 85, 91, 92, 93, 137 Fri: 95, 97, 101, 111, 114, 133 Note error to solution of Ch6 #97 Should have 6 x H o f (H 2 O)
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3 5 G and Spontaneity Changes are spontaneous (favorable) if the entropy of the universe increases. Since G system = -T S universe , this means changes are spontaneous if the free energy of the system decreases, i.e. G system <0. We can influence G of chemical reactions and affect spontaneity by changing the Temperature Concentrations in solution Pressure of gases 6 Temperature Effect G = H – T S (Negative G = spontaneous) A positive S promotes spontaneity: The system has more accessible states as a result of the reaction. A negative H promotes spontaneity: The surroundings have more accessible states when a reaction releases energy. If S is negative and H is negative, then a reaction is spontaneous at low temperatures and can be reversed at high temperatures.
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4 7 An Example: Calcium Sulfate: Desiccant H° = -104.9 kJ S° = -290.2 J K -1 G° = H° - T At 298 K, 298 = -104.9x10 3 – 298x(-290.2) = -18.4 kJ So at 298 K, anhydrous CaSO
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This note was uploaded on 06/29/2008 for the course CHE 132 taught by Professor Hanson during the Spring '08 term at SUNY Stony Brook.

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08 Applications - What makes a cold pack work? Cold packs...

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