How to Calculate Enthalpies
Typically, enthalpies of reactions need to be calculated. Enthalpy (abbreviated as "dH") is the amount of heat absorbed (dH >0) or released (dH<0) in a chemical reaction. Enthalpies are usually computed at standard conditions (the pressure of 1 bar and the temperature 25 degrees Celsius) based on tabulated values of enthalpies of formation of reagents. As an example, we will calculate the enthalpy of the oxidation reaction of benzene (C6H6) given with the chemical equation: C6H6(l) + 4.5O2(g)= 6CO2(g) + 3H2O(l). The substance gas or liquid phases are denoted as "(g)" and "(l)."
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Instructions
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1
Open a web browser and navigate to the enthalpy of formation table to find enthalpies of formation for all chemical components of the reaction. Note that enthalpies of formation of elements at standard conditions are equal to zero.
In our example,
dH(C6H6(g))= 48.95 kJ/mol
dH(O2(g)=0
dH(CO2(g))= -393.509 kJ/mol
dH(H2O(l))= -285.83 kJ/mol. -
2
Add up enthalpies of formation of the initial reagents multiplying each enthalpy value on the corresponding reaction coefficient. In our example, the initial reaction components are C6H6 and O2 and
dH(initial)= 48.95 kJ/mol + 4.5 x 0=48.95 kJ/mol. -
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3
Add up enthalpies of formation of the final reagents multiplying each enthalpy value on the corresponding reaction coefficient. In our example, the final reaction components are CO2 and H2O and
dH(final)=6 x(-393.509 kJ/mol) + 3 x (-285.83 kJ/mol) =- 3218.544 kJ/mol. -
4
Subtract the value obtained in Step 2 from the one obtained in Step 3 to calculate the enthalpy of the reaction. dH(reaction) = dH(final)-dH(initial) .
dH(reaction) = -3218.544 kJ/mol - 48.95 kJ/mol)= -3267.494 kJ/mol.
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