Molar Enthalpies Introduction
Is the enthalpy change associated with a physical, chemical or nuclear
change involving one mole of a substance.
ΔHx , where x represents the type of change that is occurring
Example: ΔHcomb = -241.8 kJ/mol (energy released when hydrogen
combusts)
A “-“ enthalpy (ΔH) means that energy was released (exothermic)
A “+” enthalpy means that energy was absorbed (endothermic)
Types of molar enthalpies
Type
Solution ΔHsol
Combustion ΔHcomb
Vapourization ΔHvap
Freezing ΔHfr
Neutralization ΔHneut
Neutralization (not a typo) ΔHneut
Formation ΔHf
Example
Molar enthalpies can be found in reference books or table 2 p307…
usually in kJ
Because this is molar enthalpy, if you had two moles, multiply by 2… 3.5
mol, multiply by 3.5… etc…
Example:
Freon (M= 120.91 g/mol) absorbs heat in refrigerators, then condenses
outside the refrigerator resulting in a transfer of energy from inside the
fridge to outside the fridge. The molar enthalpy of vaporization for
Freon is 34.99kJ/mol. If 500g of refrigerant is vapourized, what is the
expected enthalpy change?
Find the # mol of refrigerant
Calculate the enthalpy change
Example:
What amount (in moles) of ethylene glycol (see table 2 p307) for molar
enthalpy would vapourize while absorbing 200.00kJ of heat?