Uploaded August 2025 | Updated September 2026, 2 weeks ago
Calculating Net Energy Change of a Reaction
Dr. DeBacco
How It's Calculated Using Bond Enthalpies
If you know the bond enthalpies of all bonds broken and formed:
ΔH = Σ (Bond energies of bonds broken) − Σ (Bond energies of bonds formed)
In other words:
Breaking bonds = energy absorbed/input (endothermic)
Forming bonds = energy released/output (exothermic)
Higher bond energy = stronger bond
So the heat of reaction tells you the net energy flow for the whole process.
Net Energy Change
To calculate the net energy change of a chemical reaction, you are essentially figuring out how much energy is absorbed or released when bonds are broken and formed.
Formula for Net Energy Change (ΔH)
ΔH = Σ(Bond energies of bonds broken) − Σ(Bond energies of bonds formed)
Breaking bonds → requires energy (positive values)
Forming bonds → releases energy (negative values)
Step-by-Step Guide
Write the balanced chemical equation.
Make sure all reactants and products are accounted for.
List all bonds broken in the reactants.
Count how many of each type of bond is broken.
List all bonds formed in the products.
Count how many of each type of bond is formed.
Use average bond energies
These are typically found in tables (e.g., C–H ≈ 413 kJ/mol, O=O ≈ 495 kJ/mol).
Plug into the formula.
Add up the energy for all bonds broken and subtract the energy for all bonds formed.
Example: Combustion of Methane
CH₄ + 2O₂ → CO₂ + 2H₂O
Bonds broken:
4 × C–H (413 kJ/mol)
2 × O=O (495 kJ/mol)
Bonds formed:
2 × C=O (799 kJ/mol)
4 × O–H (463 kJ/mol)
ΔH = [4×413 + 2×495] − [2×799 + 4×463] ΔH = [1652 + 990] − [1598 + 1852] ΔH = 2642 − 3450 = −808 kJ/mol
This reaction is exothermic, releasing 808 kJ/mol of energy.
Link to Lecture Slides: drive.google.com/file/d/1RVZ77V5Vt6CWEoUCQJ9D9PV3Ds_sefFR/view?usp=drive_link
*Due to the description character limit the full work cited for "Calculating Net Energy Change of a Reaction" can be viewed at... docs.google.com/document/d/1o4ZSu6akSk--xD_8q9gyRspC_EypGy-b/edit?usp=drive_link&ouid=104237452697237972847&rtpof=true&sd=true
Calculating Net Energy Change of a Reaction
Dr. DeBacco
How It's Calculated Using Bond Enthalpies
If you know the bond enthalpies of all bonds broken and formed:
ΔH = Σ (Bond energies of bonds broken) − Σ (Bond energies of bonds formed)
In other words:
Breaking bonds = energy absorbed/input (endothermic)
Forming bonds = energy released/output (exothermic)
Higher bond energy = stronger bond
So the heat of reaction tells you the net energy flow for the whole process.
Net Energy Change
To calculate the net energy change of a chemical reaction, you are essentially figuring out how much energy is absorbed or released when bonds are broken and formed.
Formula for Net Energy Change (ΔH)
ΔH = Σ(Bond energies of bonds broken) − Σ(Bond energies of bonds formed)
Breaking bonds → requires energy (positive values)
Forming bonds → releases energy (negative values)
Step-by-Step Guide
Write the balanced chemical equation.
Make sure all reactants and products are accounted for.
List all bonds broken in the reactants.
Count how many of each type of bond is broken.
List all bonds formed in the products.
Count how many of each type of bond is formed.
Use average bond energies
These are typically found in tables (e.g., C–H ≈ 413 kJ/mol, O=O ≈ 495 kJ/mol).
Plug into the formula.
Add up the energy for all bonds broken and subtract the energy for all bonds formed.
Example: Combustion of Methane
CH₄ + 2O₂ → CO₂ + 2H₂O
Bonds broken:
4 × C–H (413 kJ/mol)
2 × O=O (495 kJ/mol)
Bonds formed:
2 × C=O (799 kJ/mol)
4 × O–H (463 kJ/mol)
ΔH = [4×413 + 2×495] − [2×799 + 4×463] ΔH = [1652 + 990] − [1598 + 1852] ΔH = 2642 − 3450 = −808 kJ/mol
This reaction is exothermic, releasing 808 kJ/mol of energy.
Link to Lecture Slides: drive.google.com/file/d/1RVZ77V5Vt6CWEoUCQJ9D9PV3Ds_sefFR/view?usp=drive_link
*Due to the description character limit the full work cited for "Calculating Net Energy Change of a Reaction" can be viewed at... docs.google.com/document/d/1o4ZSu6akSk--xD_8q9gyRspC_EypGy-b/edit?usp=drive_link&ouid=104237452697237972847&rtpof=true&sd=true
