The Hess's Law calculator computes the sum of enthalpy changes for a reaction based on the changes in series of steps.
INSTRUCTIONS: Choose units and enter the following:
Overall Enthalpy Change(ΔH0rxn): The calculator returns the enthalpy change in kilojoules per mole (kJ/mol). However this can be automatically converted to compatible units via the pull-down menu.
The Hess’s Law formula is a summation of enthalpy changes for a reaction. G. H. Hess published this equation in 1840 and discovered that the enthalpy change for a reaction is the same whether it occurs via one step or several steps. Hess's Law Formula is:
H0rxn= H0a + H0b + H0c + H0d
All inputs have default units of kilojoules per mole (kJ/mol).
If H0rxn is positive, then the reaction is endothermic, which means the reaction requires the absorption of heat to proceed to completion. However, if H0rxn is negative, then the reaction is exothermic, and the reaction proceeds to completion by generating heat.
Whitten, et al. "Chemistry" 10th Edition. Pp. 564