Hess Law

Jaleidy Z Hernandez Gonzale

Hess's law was invented by Germain Hess (1802- 1850), a Russian chemist and doctor. He was mostly distinguished by his thermo chemical studies in physical chemistry. Hess’s law states the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps. This is also called the law of constant heat summation. Hess’s law is particularly based on the principle of conservation of energy, and it can be used to predict energy changes that are not easily measured. It states that the change in energy is not dependent from its path, it is a state function. The amount of energy only depends on the states of the reactants and the state of the products, but not on the intermediate steps. Energy (enthalpy) changes in chemical reactions are the same, regardless whether the reactions occur in one or several steps. The total energy change in a chemical reaction is the sum of the energy changes in its many steps leading to the overall reaction.

Hess law allows the enthalpy change for a reaction to be calculated even when it cannot be measured directly. This can be accomplished using arithmetic operations such as multiplying, dividing, adding, or subtracting. When an equation is multiplied by a constant its ΔH must also be multiplied by the same number. If the equation is reversed, the ΔH must also be reserved. A net equation is led by an addition of chemical equations. If the net enthalpy is negative, then the reaction will be exothermic and will be spontaneous. An exothermic reaction is a process or reaction that releases energy usually in the form of heat. If the ΔH of the reaction is positive, than it is an endothermic reaction. An endothermic reaction is a process or reaction that absorbs energy in the form of heat. Hess law says that enthalpy changes are additive. For example, we want the enthalpy of formation of ethane gas, 2C (gr) + 3H2 → C2H6 (g). We can use the following enthalpies to determine the desired one.

C2H6 (g) + 7/2 O2 (g) → 2CO2(g) + 3H2O (l) ΔH°298 = -1560 KJ/mol (1)

C(gr) +O2 (g) → CO2 (g) ΔH°298 = -393 ½ KJ/mol (2)

H2 (g) + ½ O2 (g) → H2O (l) ΔH°298 = -286 KJ/mol (3)

The desired reaction has 2 moles of C on the left, and multiplication of reaction (2) by 2 will give 2 moles of C on the left. Similarly, we multiply reaction (1) by -1 to give 1 mole of C2H6 on the right and multiply reaction (3) by 3 to give 3 moles of H2 on the left. Multiplication of reactions (1), (2), and (3) by -1, 2, and 3, followed by addition, gives the desired equation. Hence ΔH°298 is [-(-1560) + 2(-393 ½) + 3(-286)] kJ/mol. The procedure of combining heats of several reactions to obtain the heat a desired reaction is Hess’s law.