Hess's Law

Understanding Hess’s Law

• Hess’s Law is a fundamental concept in thermochemistry that asserts the total enthalpy change of a chemical reaction, regardless of the number or complexity of steps, is dependent only on the initial and final states, not the path of the reaction.
• This law enables scientists to measure the enthalpy changes that are difficult to determine directly, by using other reactions that include the same chemicals and are easier to measure.

Hess’s Law and Reaction Pathways

• Hess’s Law is particularly useful when dealing with reaction pathways, or the individual steps that make up a larger reaction.
• For instance, if a reaction can be written as the sum of two or more other reactions, the enthalpy change of the reaction is the sum of the enthalpy changes of those reactions.
• This concept can be depicted visually by a series of energy level diagrams that show the different potential reactions and their associated energy changes.

Using Hess’s Law to Calculate Enthalpy Changes

• The standard enthalpy change of formation, ΔHf, is the enthalpy change when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states.
• The standard enthalpy change of combustion, ΔHc, is the enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions.
• Using the ΔHf and ΔHc values for relevant substances in the reaction, we can apply Hess’s Law to calculate the enthalpy change for the overall chemical reaction.

Ideal vs. Real World Conditions

• While Hess’s Law is absolute, it is important to understand that in the real world, conditions may not always be ideal.
• For example, various factors such as pressure, temperature, or the presence of a catalyst may impact the observed enthalpy changes in practice.
• Nonetheless, the foundational principles of Hess’s Law provide a robust methodology for predicting and understanding energy changes in chemical reactions.