MOLAR ENTHALPY
Recall : one mole (n) of a pure substance represents 6.02 x 10 23 entities (atoms/molecules/ions)
– Avogadro’s Constant (NA)
Formula :
n = m/M, where
m= mass of sample (g or kg)
M = Molar Mass (g/mol)
Molar Enthalpy (definition) – the energy change involved in a physical, chemical or nuclear
chemical system involving ONE MOLE of a pure substance. For pure substances, molar
enthalpy is a characteristic physical property, like density or hardness.
Symbol :
in text)
∆Hx, where x refers to the TYPE of chemical process (see Table 1 & 2 Pg.306/307
Formula :
∆H = (n)(∆Hx)
Recall that
∆H = +/- q = mC∆T , then
∆H = +/- q = mC∆T = n∆Hx
The above can be rearranged to solve for an unknown, depending on the information available.
Ex. 1
What is the enthalpy change if 5.3 g of sodium metal is vapourized?
Ex 2
Propane gas is used to heat a tank of pure water in a plastics manufacturing facility. If the tank
contains 500.0 L of water, what mass of propane will be required to raise its temperature from
20.0 0C to 77.5 0C (the molar enthalpy of combustion for propane is (-)2220 kJ/mol)?