Enthalpy of a system measures the total energy of a thermodynamic system. Knowing how to calculate enthalpy of a system, you can calculate the total amount of energy contained in the system. Enthalpy of a system is denoted by the symbol “H”. We cannot measure the total enthalpy of a system directly, so we measure the enthalpy change of the system, which is denoted as ∆ H. ∆ H conveys more information than H of a system.
How to calculate enthalpy
The equation for calculating enthalpy is as follow:
H = U + pV
In the above formula,
- H = enthalpy of the system
- U = internal energy
- P = pressure
- V = volume
Enthalpy of a system can be calculated experimentally, if you know the values for U, V and p.
How to calculate enthalpy change
Enthalpy change is the measure of amount of heat absorbed or released in a reaction. If in a chemical reaction, heat is absorbed by the system, then enthalpy change is positive. If the energy is released by the system, then enthalpy change is negative.
There are two methods of calculating enthalpy change, depending upon the data available with you. You can calculate enthalpy change from the bond energies or the enthalpy change from the enthalpies of formation.
Enthalpy change is calculated at standard conditions with pressure of 1 bar and temperature 25 degrees to get accurate results, though there is a small fraction of error in all calculations even after maintaining these standard conditions.
Formula for calculating enthalpy change using energies of formation
Hreaction = ∑ (Hproducts) – ∑ (Hreactants)
In the above formula,
- Hreaction = enthalpy change of the reaction
- ∑ (Hproducts) = sum of enthalpies of products
- ∑ (Hreactants) = sum of enthalpies of reactants
Example:
Suppose the standard enthalpies of formation of H(C6H6(g))= 48.95 kJ/mol, H(O2(g))=0, H(CO2(g))= -393.509 kJ/mol and H(H2O(l))= -285.83 kJ/mol.
The equation can be stated as,
C6H6 (g) + O2 (g) → 6CO2 (g) + 3 H2O (l)
- Step 1: Add the enthalpies of formation of the reactants and multiply each enthalpy value on the corresponding coefficient of the reaction.
Therefore,
∑ (Hreactants) = 48.95 kJ/mol + 4.5 × 0
∑ (Hreactants) = 58.95 kJ/mol
- Step 2: Add the enthalpies of formation of the products and multiply each enthalpy value on the corresponding coefficient of reaction.
Therefore,
∑ (Hproducts) = 6 x (-393.509 kJ/mol) + 3 x (-285.83 kJ/mol)
∑ (Hproducts) = -3218.544 kJ/mol
- Step 3: Subtract the enthalpy of products from the enthalpy of reactants inserting the calculated values in the above formula.
Hreaction = ∑ (Hproducts) – ∑ (Hreactants)
Hreaction = -3218.544 kJ/mol – 58.95 kJ/mol
Hreaction = -3267.494 kJ/mol
As the value of enthalpy change in the above reaction is negative, this shows that energy of 3267.494kJ/mol is released during the reaction.
Calculating enthalpy change from the bond energies
Formula for calculating enthalpy change using bond energies is:
Hreaction = ∑BEreactants - ∑BEproducts
In the above formula,
- Hreaction = enthalpy change
- ∑BEreactants = sum of bond energies of reactants
- ∑BEproducts = sum of bond energies of products
We can calculate enthalpy change using the bond energies in the similar manner as we calculated enthalpy change using the values of enthalpy of formation, so follow similar steps and use bond energies instead of enthalpy of formation.
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