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Lecture 15 - Thermochemistry Part 2

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

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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 (cal) is the amount of heat necessary to raise the temperature of 1 g of ' 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
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 ) is the heat required to of a ) is the quantity of heat needed to raise the temperature of some 4-1
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 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) < 50 °C B) 50 °C C) > 50 °C

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6 Specific Heat Capacity of Al and Fe Consider the following arguments for each answer and vote: A. The specific heat capacity of Fe(s) is smaller than that of Al(s), so heat from both the Al(s) and the water will be required to warm the Fe(s).
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Lecture 15 - Thermochemistry Part 2 - Oct 5 2011 Lecture 15...

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