The accuracy of heats of combustion determined in the closed calorimeter is in favourable cases about one-half per cent.
With knowledge then of the heats of formation of the substances involved in any chemical action, we can at once calculate the thermal effect of the action, by placing for each compound in the energy-equation its heat of formation with the sign reversed, i.e.
Thus if we wish to ascertain the thermal effect of the action Mg+CaO =MgO+Ca, we may write, knowing the heats of formation of CaO and Mg0 to be 131000 and 146000 respectively, 0-131000 = 0-146000+x x =15000 cal.
Since heats of formation afford such convenient data for calculation on the above method, they have been ascertained for as many compounds as possible.
Now we know the heats of formation of carbon dioxide (from diamond) and of liquid water to be 94300 cal.
