Uploaded August 2025 | Updated September 2026, 2 weeks ago
Bond Energy and Heat of Reaction
Dr. DeBacco
Bond Energy and Enthalpy
Bond energy (also called Bond enthalpy) is the amount of energy required to break one mole of a specific type of chemical bond in the gas phase, resulting in neutral atoms or fragments.
It is the difference between the total bond energies of the reactants and the products and shows how much energy the system gains or loses.
Typically expressed in kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol).
Heat of Reaction
Dr. DeBacco
The heat of reaction, also known as enthalpy change (ΔH), is the amount of heat absorbed or released during a chemical reaction at constant pressure.
Remember Enthalpy (ΔH): the sum of a system's internal energy and the product of its pressure and volume.
Higher bond enthalpy = stronger bond
Enthalpy (ΔH) Change
If Enthalpy (ΔH) is Negative (−)
The reaction releases heat
It’s an exothermic reaction
Example: Combustion of propane
If Enthalpy (ΔH) is Positive (+)
The reaction absorbs heat
It’s an endothermic reaction
Example: Dissolving certain salts in water (feels cold to the touch)
Exothermic Reactions- Negative Enthalpy (ΔH)
Bond formation releases energy.
The energy released when new bonds form in the products is greater than the energy required to break the bonds in the reactants.
This excess energy is given off as heat or light.
Example:
Combustion reactions- Burning methane (CH₄).
Bonds in CH₄ and O₂ break, and new bonds in CO₂ and H₂O form, releasing a lot of energy.
Endothermic Reactions- Positive Enthalpy (ΔH)
Bond breaking requires energy.
The energy needed to break the bonds in the reactants is greater than the energy released when new bonds form.
This energy is absorbed from the surroundings.
Example: Photosynthesis
Energy from sunlight is absorbed to break bonds in CO₂ and H₂O and form glucose and oxygen.
Link to Lecture Slides: drive.google.com/file/d/1Oih5vPSrGbP5lqXKLuiu4OIKJgQytKbW/view?usp=drive_link
*Due to the description character limit the full work cited for "Bond Energy and Heat of Reaction" can be viewed at... docs.google.com/document/d/1wXvL4obu2zSFeWvyuFfQCCyAvW3k8vNp/edit?usp=drive_link&ouid=104237452697237972847&rtpof=true&sd=true
Bond Energy and Heat of Reaction
Dr. DeBacco
Bond Energy and Enthalpy
Bond energy (also called Bond enthalpy) is the amount of energy required to break one mole of a specific type of chemical bond in the gas phase, resulting in neutral atoms or fragments.
It is the difference between the total bond energies of the reactants and the products and shows how much energy the system gains or loses.
Typically expressed in kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol).
Heat of Reaction
Dr. DeBacco
The heat of reaction, also known as enthalpy change (ΔH), is the amount of heat absorbed or released during a chemical reaction at constant pressure.
Remember Enthalpy (ΔH): the sum of a system's internal energy and the product of its pressure and volume.
Higher bond enthalpy = stronger bond
Enthalpy (ΔH) Change
If Enthalpy (ΔH) is Negative (−)
The reaction releases heat
It’s an exothermic reaction
Example: Combustion of propane
If Enthalpy (ΔH) is Positive (+)
The reaction absorbs heat
It’s an endothermic reaction
Example: Dissolving certain salts in water (feels cold to the touch)
Exothermic Reactions- Negative Enthalpy (ΔH)
Bond formation releases energy.
The energy released when new bonds form in the products is greater than the energy required to break the bonds in the reactants.
This excess energy is given off as heat or light.
Example:
Combustion reactions- Burning methane (CH₄).
Bonds in CH₄ and O₂ break, and new bonds in CO₂ and H₂O form, releasing a lot of energy.
Endothermic Reactions- Positive Enthalpy (ΔH)
Bond breaking requires energy.
The energy needed to break the bonds in the reactants is greater than the energy released when new bonds form.
This energy is absorbed from the surroundings.
Example: Photosynthesis
Energy from sunlight is absorbed to break bonds in CO₂ and H₂O and form glucose and oxygen.
Link to Lecture Slides: drive.google.com/file/d/1Oih5vPSrGbP5lqXKLuiu4OIKJgQytKbW/view?usp=drive_link
*Due to the description character limit the full work cited for "Bond Energy and Heat of Reaction" can be viewed at... docs.google.com/document/d/1wXvL4obu2zSFeWvyuFfQCCyAvW3k8vNp/edit?usp=drive_link&ouid=104237452697237972847&rtpof=true&sd=true
