Hess’s Law in Chemistry: Principles and Applications
by Yuvi K - December 16, 2023
What is Hess’s Law?
Hess’s Law, developed by a German chemist named Germain Hess in 1840, states that the total enthalpy change for a chemical reaction (also known as the heat of reaction) is the same whether the reaction takes place in one step or multiple steps. It is based on the fact that the energy of a system depends only on its initial and final states, and not on the particular path taken to get from one to the other.
Enthalpy is a thermodynamic quantity equal to the internal energy of the system plus the product of its pressure and volume. It can be used to measure the heat change of a reaction (ΔH) using the equation ΔH=E+PV. Using this information, Hess’s Law allows chemists to determine the heat of reaction for a complex chemical reaction by breaking it down into simpler parts and adding their individual heat of reaction values together.
Hess’s Law in Chemistry: Principles and Applications
Hess’s Law is an important tool in chemistry, particularly laboratory chemistry. It provides a way to predict the energy change of a multistep chemical reaction from the energy change of its simpler component steps. This can be used to compare different methods of reaction or to estimate the energy required for a reaction to occur. In addition, Hess’s Law can be applied to a variety of chemical equilibria to understand how changes in temperature and pressure affect a system.
Calculating Enthalpy Changes: The Heat of Reaction
Hess’s Law states that the overall heat of reaction (ΔH) for any chemical reaction is the same regardless of the number of steps it takes to complete that reaction. This means that a multistep reaction can be broken down into simpler component steps, and the energy change of each step can be added up to get the total energy change for the reaction.
For example, consider the combustion of propene to form carbon dioxide and water (C3H6 + O2 → CO2 + H2O). This reaction can be broken down into two simpler steps: (C3H6 + O2 → CO2 + H2) and (H2 + O2 → H2O). The energy changes for these two steps can be added to give the energy change for the entire reaction (ΔH = ΔH1 + ΔH2).
Using Hess’s Law to Understand Equilibrium
Hess’s Law can also be used to understand how changes in temperature, pressure, and other variables affect the equilibrium of a reaction. This is known as the principle of “enthalpy of reaction.”
Consider the following equation:
N2 + O2 → NO + O
The energy change of this reaction (ΔH) is equal to the energy change of the reverse reaction minus the energy change of the forward reaction. If the temperature or pressure of the system is altered, the energy change of the reaction (ΔH) also changes. This change can be used to determine how the energy levels in the system have been affected by the change in temperature or pressure.
Calculating Enthalpy of Formation Using Hess’s Law
Hess’s Law can also be used to calculate the enthalpy of formation (ΔHf) of a compound. The enthalpy of formation (ΔHf) is the energy change that occurs when a compound is formed from its constituent elements.
For example, consider the formation of water (H2 + O2 → H2O). This reaction can be broken down into two simpler steps: (H2 → 2H) and (O2 → 2O). The enthalpy of formation (ΔHf) of water is equal to the energy change of these two steps added together (ΔHf = ΔH1 + ΔH2).
Conclusion
Hess’s Law is a useful tool in chemistry that can be used to calculate the heat of reaction and enthalpy of formation of compounds, and to understand how changes in temperature and pressure affect a system. By breaking down complicated chemical reactions into simpler steps, chemists can use Hess’s Law to calculate the expected energy change for a reaction and to gain insights into the effects of changes in system conditions on an equilibrium.
