Page 34 - Bruno Linder Elementary Physical Chemistry

August 25, 2010 9:36 9in x 6in b985-ch02 Elementary Physical Chemistry

The First Law of Thermodynamics 19

of formation is the standard enthalpy of reaction of the formation of a
substance formed from its elements in their standard states. The standard
enthalpy of formation of an element is taken to be zero.

2.13. Variation of Enthalpy with Temperature

Returning to the concept of heat capacity, C = q/∆T , or more precisely
C = lim (q/∆T )= dq/dT (2.23)
∆T →0

we can write for C P
(∂H/∂I) P = C P and dH = C P dT (2.24)
If the temperature, originally at T , changes to T and C P is constant

H(T )= H(T )+ C P (T − T ) (2.25)

For a chemical reaction,
∆ r C P =Σ i n iC P,ni(products) − Σ i n iC P,ni (reactants) (2.26)
o o
∆ r H =Σn i H ¯ i (prod) − Σn i H ¯ i (react) (2.27)
o o
∆ rH (T )= ∆H (T )+ ∆C P ∆T (2.28)

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