Lecture26 - Calorimetry Calorimetry an experimental method...

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 .

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

Δ H