Bond enthalpy

  • The energy required to break/form 1 mol of bonds in gaseous covalent molecules under standard conditions. Bonds formed release energy and bonds broken require energy.

  • Values in Data Booklet are approximations derived from experimental data

  • Bond enthalpy is proportional to bond strength.



  • Important for absorbing long and short wave length UV radiation in the atmosphere

  • Ozone is more decomposed by UV light than Oxygen gas, due to the lower dissociation enthalpy.


  • Form of energy transferred from a warmer body to a cooler body


  • Study of the heat changes that occur during chemical reactions

  • Change in enthalpy       is the heat transferred by a closed system during a chemical reactions

  • Exothermic reactions:

    • Energy is released to the surroundings​

    •      <0

  • Endothermic reactions:

    • Energy is absorbed by the system

    •     > 0

Molar enthalpy change for reaction 

  • Definition: The change in energy (heat) in a reaction

  • q: heat

  • m: mass

  • c: specific heat

  •      change in temperature

​​​Standard enthalpy of formation

  • Energy change upon the formation of 1 mol of substance 

Standard enthalpy of combustion

  • Energy change upon the complete combustion of 1 mol of substance

Standard conditions

  • 25 degrees c/ 298 K

  • 100 kPa

Concept: Regardless of the route by which the reaction proceeds, the enthalpy change will always be the same providing the initial and final states of the system are the same

  • CFC's are chemicals that enhance the ozone depletion as shown in the reaction pathway bellow:

Topic 5: Energetics/Thermochemistry

5.1 Measuring Energy Changes

5.2 Hess' Law

5.3 Bond Enthalpies and are student initiatives to provide free material to help international students prepare themselves for the IB exams.

Contact: Fernando Alonso Bilfinger |