This means that when 12.01 grams of solid carbon (graphite) and 32 grams of gaseous oxygen react completely to yield 44.01 grams of gaseous carbon dioxide, at constant pressure, there is a decrease in heat content, since ΔH is negative, of 94 kilocalories (kcal.), i. e. , 94,000 calories. It is the general practice in modern thermochemical work to express results in kilocalories because the statement of heat changes in calories implies an accuracy greater than is usually attainable experimentally. It should be noted, incidentally, that the ΔH (or ΔE) values always refer to completed reactions, appropriate allowance having been made, if necessary, if the process does not normally go to completion.
The symbols g, /, and s, placed in parentheses after the formula indicate whether the substance taking part in the reaction is gas, liquid or solid. Reactions taking place in aqueous solution are indicated by the symbol aq; thus,
HCl (aq) + NaOH (aq) = NaCl (aq) + H2O)
ΔH= 13.70 kcal.
Strictly speaking the use of aq implies that the reaction is occurring in such dilute solution that the addition of further water causes no detectable heat change.
A negative value of ΔH, as in the two instances quoted above, means that the reaction is accompanied by a decrease in heat content; that is to say, the heat content of the products is less than that of the reactants at a specified temperature, in other words, the reaction at the given temperature is associated with an evolution of heat. It follows, therefore, that when ΔH is negative the reaction is exothermic; similarly, if ΔH is positive the process is endothermic. The same conclusions can be reached directly from the fact that qp, which is equal to ΔH, the heat absorbed in the reaction; hence, when ΔH is negative heat is actually evolved.