Basic Chemical Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 24

on the system, the most common choices are the following (Kondepudi, 2008):

10 5 Pa ) , m 0 =

=

298.15 K, P 0 =

= ×

T 1 atm ( 1.013 1 mol/kg,

1 mol/dm 3 .

C 0 =

For any pure substance at a given temperature, the standard state is the most stable
state (gas, liquid, or solid) at a pressure of 1 atm (

10 5 Pa). In the gas phase,
the standard state of any compound, either pure or as a component in a mixture, is the
hypothetical state of ideal gas behavior at P

= 1.013 ×

1 atm. For a condensed phase (liquid
or solid), the standard state, either as pure or as a component in a mixture, is that
of the pure substance in the liquid or solid form at the standard pressure of P 0 .For
a solute in solution, the standard state is a hypothetical state of an ideal solution at
standard concentration C 0 at the standard pressure of P 0 .
There are three important enthalpy terms that merit discussion. These are standard
enthalpy of reaction (

=

H r ) , standard enthalpy of formation ( H f ) , and standard

Δ Δ

H c ) . The standard enthalpy of reaction is the enthalpy
change for a system during a chemical reaction and is the sum of the standard enthalpy
of formation of the products minus the sum of the standard enthalpy of formation of
the reactants. The standard enthalpy of formation is the enthalpy change to produce
1 mol of the compound from its elements, all at the standard conditions. The standard
enthalpies of reaction may also be combined in various ways. This is succinctly
expressed in Hess's law of heat summation: the standard enthalpy of a reaction is
the sum of the standard enthalpies of reactions into which an overall reaction may
be divided, and holds true if the referenced temperatures of each individual reaction
are the same . The standard enthalpy of combustion is that required to burn or oxidize
1 mol of the material to a final state that contains only H 2 O (l) and CO 2 .

enthalpy of combustion ( Δ

E XAMPLE 2.3 E NTHALPY OF AN A CID -B ASE R EACTION
Determine the enthalpy of reaction at 298 K for the neutralization of hydrochloric acid
with sodium hydroxide:

HCl ( aq ) + NaOH(aq) NaCl(aq) + H 2 O ( aq ) . (2.28)

First calculate the heat of formation of each species ( Δ H f ) from the heat of formation
of the constituent ions ( Δ H f ) . These values are obtained from Appendix 2:
Δ H f ( HCl, aq ) = Δ H f (H + ,aq ) + Δ H f ( Cl − ,aq ) = 0 − 167.2
=− 167.2 kJ/mol
Δ H f ( NaOH, aq ) = Δ H f ( Na + ,aq ) + Δ H f ( OH − ,aq ) =− 239.7 − 229.7
=− 469.4 kJ/mol