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Lecture26

# Lecture26 - Calorimetry Calorimetry an experimental method...

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Calorimetry Calorimetry : an experimental method for observing and measuring the flow of heat into and out of a system. The heat measured with constant volume ( q v ) is different from the heat measured with constant pressure ( q p ). The First Law tells us: Δ E = q + w If only PV -work is present, w = P Δ V . Then, Δ E = q P Δ V At constant V , Δ V = 0, thus Δ E = q v .

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Enthalpy ( H ) What is the heat flow at constant P ( q p )? Define a new state function enthalpy : H = E + P V Define a new state function Then, the change in enthalpy: Δ H = Δ E + Δ ( P V ) We already know: Δ E q P Δ V if only PV work is involved We already know: = , if only -work is involved. Also, Δ ( P V ) = P Δ V , at constant P . Therefore, Δ H = q P Δ V + P Δ V = q p Heat under constant V Heat under constant P Δ E = q v Δ H = q p h i d h i Exo thermic Endo thermic Δ H < 0, or Δ E < 0 Δ H > 0, or Δ E > 0 losing energy Ä gaining energy losing energy gaining energy
Heating/Cooling Curve of H 2 O at 1 atm R P

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Δ H
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