History

• Henri Hess was a Swiss scientist who lived in Russia. His contribution to science is that he discovered an efficient way to calculate the amount of heat used in a chemical reaction. He died in 1850, which was 23 years before the laws of thermodynamics were clearly described by Josiah Josiah Willard Gibbs. However, Hess's Law is evident from the first law of thermodynamics.
  
  

Identification

• The first of the two laws of thermodynamics states that there is a fixed amount of energy within a system. No new energy can be created. No energy can be destroyed. It can, however, be converted or transferred to another kind of energy. The potential thermodynamic energy, or heat, in a system is known as enthalpy. Its scientific symbol is H or h.
  
  

Features

• Within a system or during a chemical reaction, changes in energy can be monitored by measuring changes in volume, temperature and pressure. These variables are very difficult to measure in some chemical reactions such as in the process of producing carbon monoxide gas from graphite.
  
  

Function

• Hess's Law is premised on the fact that the total amount of heat involved in this chemical reaction will be the same whether the carbon monoxide gas is produced via a direct conversion from graphite or whether the process is broken into two discreet steps. Since it is relatively easier to measure the heat used when you first convert graphite to carbon dioxide and then convert the carbon dioxide to carbon monoxide, Hess's Law states that the sum of the amount heat used in both reactions is equal to the amount that would be used in a single step process.
  
  

Effects

• Some expand the name of Hess's Law to "Hess's Law of Heat Summation." No matter how many steps are used in a chemical reaction, the amount of enthalpy will be the same. The principle is that energy is conserved within a system. Hess's Law is a mathematical approach that measures the change of energy during a chemical reaction because the total amount of energy will not change. It is easy to envision how Hess's Law works by remembering how to balance an algebra equation. Both sides of the equation must always remain equal so whatever operation you do on one side of the equal sign, you have to account for with a reciprocal operation on the other side.