Hess' law of constant heat summation, is a relationship in physical chemistry named after Germain Hess, a Swiss-born Russian chemist and physician who published it in 1840. The law states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several steps.
Hess' law is now understood as an expression of the principle of conservation of energy, also expressed in the first law of thermodynamics, and the fact that the enthalpy of a chemical process is independent of the path taken from the initial to the final state (i.e. enthalpy is a state function). It applies to the special case of paths consisting of chemical reactions (or changes of state) at constant temperature and pressure. Hess's law can be used to determine the overall energy required for a chemical reaction, when it can be divided into synthetic steps that are individually easier to characterize. This affords the compilation of standard enthalpies of formation, that may be used as a basis to design complex syntheses.
Hess's law states that the change of enthalpy in a chemical reaction (i.e. the heat of reaction at constant pressure) is independent of the pathway between the initial and final states.
In other words, if a chemical change takes place by several different routes, the overall enthalpy change is the same, regardless of the route by which the chemical change occurs (provided the initial and final condition are the same).
Hess's law allows the enthalpy change (ΔH) for a reaction to be calculated even when it cannot be measured directly. This is accomplished by performing basic algebraic operations based on the chemical equations of reactions using previously determined values for the enthalpies of formation.
Addition of chemical equations leads to a net or overall equation. If enthalpy change is known for each equation, the result will be the enthalpy change for the net equation. If the net enthalpy change is negative (ΔHnet < 0), the reaction is exothermic and is more likely to be spontaneous; positive ΔH values correspond to endothermic reactions. Entropy also plays an important role in determining spontaneity, as some reactions with a positive enthalpy change are nevertheless spontaneous.
Hess's law states that enthalpy changes are additive. Thus the ΔH for a single reaction
- Reactants → Elements
and Elements → Products
- B2O3 (s) + 3H2O (g) → 3O2 (g) + B2H6 (g) (ΔH = 2035 kJ/mol)
- H2O (l) → H2O (g) (ΔH = 44 kJ/mol)
- H2 (g) + (1/2)O2 (g) → H2O (l) (ΔH = -286 kJ/mol)
- 2B (s) + 3H2 (g) → B2H6 (g) (ΔH = 36 kJ/mol)
Find the ΔHf of:
- 2B (s) + (3/2) O2 (g) → B2O3 (s)
After the multiplication and reversing of the equations (and their enthalpy changes), the result is:
- B2H6 (g) + 3O2 (g) → B2O3 (s) + 3H2O (g) (ΔH = -2035 kJ/mol)
- 3H2O (g) → 3H2O (l) (ΔH = -132 kJ/mol)
- 3H2O (l) → 3H2 (g) + (3/2) O2 (g) (ΔH = 858 kJ/mol)
- 2B (s) + 3H2 (g) → B2H6 (g) (ΔH = 36 kJ/mol)
Adding these equations and canceling out the common terms on both sides, we get
- 2B (s) + (3/2) O2 (g) → B2O3 (s) (ΔH = -1273 kJ/mol)
2) a) Cgraphite+O2 → CO2 (g) ;(ΔH = -393.5 kJ) (direct step)
- b) Cgraphite+1/2 O2 → CO (g) ; (ΔH = -110.5 kJ)
- c) CO (g)+1/2 O2 → CO2 (g); (ΔH = - 283.02 kJ)
→In the reactions b) and c), the total ΔH = -393.5 kJ which is equal to ΔH in a)
The difference in the value of ΔH is 0.02 kJ which is due to measurement errors .
Extension to free energy and entropy
The concepts of Hess's law can be expanded to include changes in entropy and in Gibbs free energy, which are also state functions. The Bordwell thermodynamic cycle is an example of such an extension which takes advantage of easily measured equilibria and redox potentials to determine experimentally inaccessible Gibbs free energy values. Combining ΔG
o values from Bordwell thermodynamic cycles and ΔH o values found with Hess's law can be helpful in determining entropy values which are not measured directly, and therefore must be calculated through alternative paths.
For the free energy:
For entropy, the situation is a little different. Because entropy can be measured as an absolute value, not relative to those of the elements in their reference states (as with ΔH
o and ΔG o), there is no need to use the entropy of formation; one simply uses the absolute entropies for products and reactants:
Hess's law of constant heat summation is useful in the determination of enthalpies of the following:
- Heats of very slow reactions
- Heats of formation of unstable intermediates like CO(g) and NO(g).
- Heat changes in phase transitions and allotropic transitions.
- Lattice energies of ionic substances by constructing Born-Haber cycles if the electron affinity to form the anion is known, or
- Electron affinities using a Born-Haber cycle with a theoretical lattice energy
- Leicester, Henry M. (1951). "Germain Henri Hess and the Foundations of Thermochemistry". The Journal of Chemical Education 28 (11): 581–583. Bibcode:1951JChEd..28..581L. doi:10.1021/ed028p581.