Calorimetry:
Experimental calculation of molar enthalpy
Theory
Chemists determine the enthalpy change in a system by examining the energy changes in the surroundings. Using the 1st law of thermodynamics chemists assume that the energy change in the system will be equal and opposite to the energy change in the surroundings.
E system = - E surroundings
E surroundings
To determine the energy change in the surrounding chemists use substances that will change temperature. They can therefore calculate the energy change in the surroundings using the formula:
Q = m * c * t
Q = energy or heat change (J or kJ )
m = mass of substance in surroundings (g or kg)
c = specific heat capacity in (J/g *oC )
t = temperature change in surroundings
E system
To determine the energy change in the system , chemists use the following formula:
Q = n * H substance
Q = energy or heat change (J or kJ )
n = moles of substance reacted in system (mol.)
H substance = Molar enthalpy of substance in reaction.(kJ/mol.)
The formula above can be used to develop a new formula for calculating molar enthalpy.
1) Formula from 1st Law
E system
=
- E surroundings
2) becomes
Q
=
- Q
3) becomes
n * H substance
=
m * c * t
4) solving for molar enthalpy
H substance
=
m * c * t
n