IB Chemistry - Energetics

IB Chemistry home > Syllabus 2025 > Energetics > Standard enthalpy changes

All of the processes involved in science have corresponding energy changes, however there are some chemical and physical changes that are of particular interest, either historically, or because they can be used to obtain other information about energy changes that are difficult to find directly.

The actual value of a specific change can be affected by the temperature at which it is measured, so a standard set of conditions is defined to ensure that the data produced in an experiment is relevant. Standard state conditions are 1 atmosphere pressure and molar quantities.

Note: This is not to be confused with S.T.P. which, in terms of the gas laws, is defined as 273K and 1 atmosphere pressure. Energy measurements are usually quoted under standard conditions and represented by the subway sign (shown right, more correctly called a superscript plimsoll line). The temperature, while not formally included in standard state conditions is quoted and is usually 298K.

Syllabus reference

Reactivity 1.1.4 - The standard enthalpy change for a chemical reaction, ΔH⦵, refers to the heat transferred at constant pressure under standard conditions and states. It can be determined from the change in temperature of a pure substance.

  • Apply the equations Q = mcΔT and ΔH = − Q/n in the calculation of the enthalpy change of a reaction.


  • The units of ΔH are kJ mol–1.
  • The equation Q = mcΔT and the value of c, the specific heat capacity of water, are given in the databooklet.

Tools and links

  • Tool 1, Inquiry 1, 2, 3 - How can the enthalpy change for combustion reactions, such as for alcohols or food, be investigated experimentally?
  • Tool 1, Inquiry 3 - Why do calorimetry experiments typically measure a smaller change in temperature than is expected from theoretical values?

In Chapter 4.2