Term
enthalpy
Symbol
Definition
enthalpy change or heat of
reaction
difference between the enthalpy of the
reactants and products expressed in kJ;
the amount of heat released/absorbed
standard enthalpy change
the energy change at standard ambient
temperature and pressure
(______________________________)
Enthalpy change for exothermic reactions are shown by a _____________ sign.
Enthalpy change for endothermic reactions are shown by a ____________ sign.
molar enthalpy
the energy change associated with a
physical, chemical, or nuclear change
involving one mole of a substance
standard molar enthalpy
the energy change with one mole of a
substance at SATP
To calculate molar enthalpy,
Some Molar Enthalpies of Reaction (Hx)
Type of Molar Enthalpy
solution (Hsoln)
combustion (Hcomb)
vaporization (Hvap)
condensation (Hcond)
freezing (Hfr)
neutralization (Hneut)
formation (Hf)
Example of Change
solid  aqueous
 carbon dioxide + water
liquid  gas
gas  liquid
liquid  solid
acid + base  a salt + water
X + Y  XY
Note that vaporization and condensation are opposite processes.
Similarly, melting and freezing are opposite processes.
=
=
Example 1: Freon-12 has a molar mass of 120.91 g/mol and Hvap of 34.99 kJ/mol.
If 500.0 g of Freon is vaporized, what is the enthalpy change?
Using Calorimetry to Find Molar Enthalpies
A useful assumption is that the energy change of the system equals the quantity of heat
that flows from the system to its surroundings, or from the surroundings to the system.
This is consistent with the law of conservation of energy.
H system = + lq surroundingsl
Three simplifying assumptions used in the experimental technique:



No heat is transferred between the calorimeter and the outside environment
Any heat absorbed/released by the calorimeter is negligible.
A dilute aqueous solution is assumed to have a density and specific heat
capacity equal to that of pure water.
density = 1.00 g/mL
specific heat capacity = 4.18 J/gC
Remember
and since
therefore
Example 2: In a calorimetry experiment, 7.455 g of KCl is dissolved in 100.0 mL of
water with an initial temperature of 24.1C. The final temperature of the solution is
20.0C. What is the molar enthalpy of a solution of KCl?
Example 3: What mass of LiCl must have dissolved if the temperature of 200.0 g of
water increased by 6.0C? The molar enthalpy of a solution of LiCl is -37 kJ/mol.