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Unformatted text preview: 1.3 J/(L atm) = -120 J
∆E = q + w = (2.0 kJ) + (-120 J) = 1.9 kJ
6.2 Enthalpy and Calorimetry
H = E + PV where E = internal energy of the system
P = pressure of the system
V = volume of the system Since internal energy, pressure, and volume are all state functions, enthalpy is also a state function.
H = E + PV. Therefore
∆H = ∆E + ∆(PV)
for a process at constant pressure, ∆(PV) = P∆V so
∆H = ∆E + P∆V
6-2 Chapter 6 Thermochemistry at constant P, ∆E = qp + w = qp – P∆V (subscript p means P is constant), so
∆H = qp - P∆V + P∆V = qp
∆H = qp H can be thought of as the heat content of the system, ∆H = heat flow For processes which occur at constant volume, ∆V = 0 so w = 0, therefore
∆E = qP
Enthalpy Changes in Chemical Reactions
In an exothermic reaction, chemical potential energy is converted into thermal energy which is released to the
surroundings as a positive heat flow.
In an endothermic reaction, thermal energy absorbed from the surroundings is converted into chemical
potential energy which results in a negative heat flow.
For a reaction reactantsÆ products ∆H = Hproducts - Hreactants ∆H>...
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