Hess's law

Hess's law, or Hess's law of constant heat summation, is the rule laid out by Germain Henri Hess in 1840 that the reaction enthalpy, a thermodynamic quantity equivalent to the total heat content of a system, does not change when converting reactants into products regardless of the path taken to get the reaction. In other words, enthalpy is a state function, meaning that its value depends on the state of the system at the moment. The law is a consequence of the first law of thermodynamics, although it keeps its name because of its importance for calculating heats of reactions.

The first law of thermodynamics states that if heat is a form of energy, the total energy of a system, including its surroundings, is conserved, or remains constant. Hess's law is important to calculations in thermochemistry due to its ability to calculate the transfer and absorption of heat no matter how many steps are taken in a chemical reaction.

Brief History

Germain Henri Hess was born on August 7, 1802, in Geneva, Switzerland, and moved at an early age to Russia. He received his medical degree at the University of Dorpat and practiced medicine in Irkutsk, Russia. In 1828, he became an adjunct professor in chemistry at the Imperial Academy of Sciences in St. Petersburg. His work mainly focused on the analysis of inorganic and organic substances. He began experimenting with an ice calorimeter, a large insulated container of ice and water used to measure the amount of heat involved in a reaction, and soon developed his thermochemical laws. His law of heat summation was published in a paper in 1840 and became widely accepted. In 1842, he proposed another law called the law of thermoneutrality, which stated that no heat is evolved by the exchange reactions of neutral salts in aqueous solutions, a solution in which the solvent is water.

Hess's advances in thermochemistry laid the groundwork for the laws of thermodynamics, which cover the relationships between work, temperature, and energy, with an emphasis on entropy. In the laws of thermodynamics, heat is a form of energy that corresponds to a definite amount of chemical work. Temperature in thermodynamics is often measured in Kelvin (K), an absolute temperature scale wherein zero is the theoretical absolute zero, rather than Celsius or Fahrenheit. Although enthalpy is a measure of heat, Hess's law deals with the heat of formation, which is measured in Joules because it is a measurement of energy.

The basic steps to implementing the equations for Hess's law start with writing out the thermochemical equations for the step reactions, then writing the balanced chemical equation for the overall target reaction. Each step reaction has a corresponding ΔH (change in enthalpy) that can easily be found on a change in enthalpy table. In the equation, the step reactions are reversed as needed so products or reactants match the target reaction, and they are scaled with a multiplicative factor so the products or reactants that are not in the target reaction cancel out mathematically. The ΔH sign may need to be reversed if the step reactions are reversed. The step reactions are added, then the resulting reaction is scaled so it lines up with the target reaction. The ΔH sign is added to get the total change in enthalpy.

Impact

Hess's law of heat summation is an important proponent of modern chemistry because it states that the change in enthalpy for a particular reaction stays constant no matter the steps taken to complete the reaction. To put Hess's law into a simpler example, it is like trying to move from the ground floor to the third floor of a building. By taking the elevator, a person could go straight from the ground floor to the third floor. Alternatively, they could go up to the second floor, stop there for a while, and then continue on to the third floor. Just like in Hess's law, no matter how many stops (chemical reactions) the elevator takes to get to the third floor (target product), where it starts and where it ends will be the same; the difference in height between the ground floor and the third floor (the change in enthalpy from the initial to final product) will remain the same.

Hess's law is applicable in everyday life, from the food people consume to the cars they drive. This law can be used to find the energy content in food (i.e., the calories). The calories commonly referred to on nutritional labels are actually kilocalories, the equivalent of 4184 Joules per one kilocalorie. The measurement of calories in a kind of food refers to the standard enthalpy of combustion in a bomb calorimeter. Although the use of a bomb calorimeter is different from the human body, the initial and final states of the reaction are the same and can be measured as such. By using Hess's law to measure the change in enthalpy for the start and end product of the food, a caloric value can be given to the food for reference.

Another application of Hess's law is measuring the energy content of fuels. Certain fuels have a higher energy density than other fuels, and measuring the amount of energy that it takes to combust the fuel can help companies determine which fuels are better suited for their needs, both for their energy content and their cost.

Car companies can use Hess's law to economize how their cars' engines use gasoline. By calculating the energy output of the cars' engines, the manufacturers can find ways to burn gasoline differently and create engines that use the reaction more efficiently. Companies can also experiment with using other fuels to make the standard combustion engine run more efficiently. For example, race car engineers have experimented with hydrogen peroxide, which releases some heat when it decomposes into oxygen and water, but the reaction is generally slow enough that heat is not noticed. Given the proper catalyst, however, it may produce a large enough quantity of heat to fuel a race car.

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