Hess's law (law of conservation of total heat) expresses conservation of energy: “Reaction heat is determined only by the initial and final states, regardless of the pathway.”
Basic Knowledge
Using Hess's law, multiple thermochemical equations can be added or subtracted to calculate the ΔH of a desired reaction. For example, the enthalpy of formation of CO from incomplete combustion of C is difficult to measure directly, but it can be found as follows:
(a) C(s)+O2(g)→CO2(g), ΔHa=-394 kJ
(b) CO(g)+½O2(g)→CO2(g), ΔHb=-283 kJ
(a)-(b): C(s)+½O2(g)→CO(g), ΔH = ΔHa-ΔHb = -111 kJ
This law holds because enthalpy is a state function and is independent of the pathway.
Hess's law（conservation of total heat; ΔH is independent of the pathway）
State function（a quantity determined only by the initial and final states. H, U, S, G）
Addition and subtraction of thermochemical equations（coefficients may be multiplied and equations added）
Reverse reaction（ΔH has the opposite sign）
Energy diagram (enthalpy diagram)（visualization using height differences）
Table of standard enthalpies of formation（basis for calculating ΔH of any reaction）
Further Study (Principles and Applications)
Hess's law is an application of the first law of thermodynamics (conservation of energy) to chemical reactions. Because enthalpy is a state function, the energy change of a complex multistep reaction can be calculated by combining simple fundamental reactions.
Applications include estimating ionization energy and lattice energy of crystals using the Born–Haber cycle, thermodynamic analysis of ATP metabolic pathways in biochemistry, combustion design for fuels, and heat-balance calculations in industrial processes.
- Hess's law = independent of the pathway
- Thermochemical equations can be handled algebraically
- Multiplying reaction coefficients also multiplies ΔH by the same factor
- For the reverse reaction, reverse the sign of ΔH
- ΔHreaction=ΣΔHf°(products)-ΣΔHf°(reactants)
- Visualization with an enthalpy diagram is effective
- Born–Haber cycle = an application
Points of Caution (Common Confusions and Frequent Mistakes)
① When multiplying equation coefficients, multiply ΔH by the same factor. ② Always change the sign for a reverse reaction. ③ Match the states of matter (g/l/s), taking the enthalpy of vaporization into account. ④ In ΣΔHf° calculations, do not reverse the order of products - reactants.
Practice
- From (a) C+O2→CO2 (ΔH=-394 kJ), (b) H2+½O2→H2O(l) (ΔH=-286 kJ), and (c) CH4+2O2→CO2+2H2O(l) (ΔH=-891 kJ), find the enthalpy of formation of CH4.
- What is the ΔH of the reverse reaction CO2→C+O2?
- What property of enthalpy is the basis of Hess's law?