Lecture 15 - Thermochemistry Part 2

Lecture 15 - Thermochemistry Part 2 - Oct 5, 2011 Lecture...

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1 Lecture 15 Thermochemistry, part 2 Internal energy changes The internal energy of a system is the sum of all the KE and PE of all of the components of the system. Changes are defined as final minus initial ± ' E = E final -E initial Oct 5, 2011
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2 First Law of Thermodynamics The first law of thermodynamics states that the energy gained or lost by a system must equal the energy lost or gained by surroundings. ± Energy is not created or destroyed ± Energy is transferred via heat and work The calorie (cal) is the amount of heat necessary to raise the temperature of 1 g of water 1 o C. 1 food “Calorie” = 1000 cal The joule (J) is the SI unit: 4.184 J = 1 cal. ' E = q + w ± ' E = change in system’s internal energy (energy gain is positive) ± q = heat (heat absorbed is positive) ± w = work (work done by system is negative) Work done by the system ± Work is force times distance ± many types of work, including “PV” work Change in Internal Energy 2-1 2-2 2-3
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3 Energy flow: heat and work Enthalpy (H) = E + PV Change in Enthalpy ( ' H) = ' E + P ' V At constant pressure q P = ' E + P ' V, therefore q P = ' H ' H = change in enthalpy: an energy flow as heat (at constant pressure) ' H > 0, Endothermic; ' H < 0, Exothermic Enthalpy and Change in Enthalpy
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4 Enthalpy for heating Heat Capacities Molar heat capacity (c p ) is the heat required to raise the temperature of 1 mole of a substance by 1 o C at constant pressure. q = nc p ' T Specific heat (c s ) is the heat required to raise the temperature of 1 gram of a substance by 1 o C at constant pressure. Heat capacity (C p ) is the quantity of heat needed to raise the temperature of some specific object by 1 o C at constant pressure. 4-1
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Heat capacities can vary Substance c p (J/mol·°C) Water (s) 37.1 Water (l) 75.3 Ethanol (l) 113.1 Graphite C(s) 8.54 Al, Cu, Fe (s) 24.4, 24.5, 25.1 Specific Heat Capacity: Example A 1.0 gram block of Al (c s = 0.9 J/°Cg) at 100°C and a 1.0 gram block of Fe (c s = 0.4 J/°Cg) at 0 °C are added to 10 mL of water (c s = 4.2 J/°Cg) at 50°C. What will be the final temperature of the water? A)
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This note was uploaded on 03/12/2012 for the course CHEMISTRY 131 taught by Professor Lacey during the Fall '11 term at SUNY Stony Brook.

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Lecture 15 - Thermochemistry Part 2 - Oct 5, 2011 Lecture...

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