Topic 5: Energetics/thermochemistry - 5.1 Measuring energy changes

Nature of science: Fundamental principle-conservation of energy is a fundamental principle of science. (2.6) Making careful observations-measurable energy transfers between systems and surroundings. (3.1)


Essential idea: The enthalpy changes from chemical reactions can be calculated from their effect on the temperature of their surroundings.

Heat is a form of energy. Temperature is a measure of the average kinetic energy of the particles.

Total energy is conserved in chemical reactions.

Chemical reactions that involve transfer of heat between the system and the surroundings are described as endothermic or exothermic.

The enthalpy change (ΔH) for chemical reactions is indicated in kJ mol-1.

ΔH values are usually expressed under standard conditions, given by ΔH°, including standard states.

Applications and skills

Calculation of the heat change when the temperature of a pure substance ischanged using q=mcΔT

A calorimetry experiment for an enthalpy of reaction should be covered and the results evaluated


Enthalpy changes of combustion (ΔHc° ) and formation (ΔHf°) should be covered.

Consider reactions in aqueous solution and combustion reactions. Standard state refers to the normal, most pure stable state of a substance measured at 100 kPa. Temperature is not a part of the definition of standard state, but 298 K is commonly given as the temperature of interest.

The specific heat capacity of water is provided in the data booklet in section 2.

Students can assume the density and specific heat capacities of aqueous solutions are equal to those of water, but should be aware of this limitation.

Heat losses to the environment and the heat capacity of the calorimeter in experiments should be considered, but the use of a bomb calorimeter is not required.