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# lecture15 - Lecture 15 Thermochemistry part 2 Internal...

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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

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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
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.
5 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

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lecture15 - Lecture 15 Thermochemistry part 2 Internal...

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