Topic 5: Energetics/Thermochemistry

5.1 Measuring Energy Changes


  • 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

5.2 Hess' Law

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

5.3 Bond Enthalpies

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.

  • CFC's are chemicals that enhance the ozone depletion as shown in the reaction pathway bellow: is a student initiative to provide free material to help international students prepare for the IB exams.

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