Experimental determination of an
enthalpy of reaction (Δ
rH) involves measuring the
heat absorbed or released at constant pressure (
qp) for a specific
amount of reaction (
n).
The heat transferred between a reaction system and its surroundings depends on the amount and the state of substances reacting This depends on the pressure and temperature at which the measurement is made.
Because it is highly
desirable to make
comparisons between enthalpy changes for various reactions measured in different laboratories, it is important that the
temperature and
pressure which apply to any measurements are
standard.
This activity is concerned with
standard thermodynamic states of substances and
standard reactant enthalpies. On completion of the activity you should be able to
- identify the standard state for common elements and compounds at 25 ° C
- recognize the reaction having an enthalpy change equal to the standard enthalpy of combustion for a hydrocarbon
- write the equation having the standard enthalpy change which is equal to the standard enthalpy of formation of a substance
- recognise substances for which the standard enthalpy of formation is zero