Hess’s Law is a method in chemistry for determining the change in enthalpy (\(\Delta H\)) of a chemical reaction that is difficult or impossible to measure directly. This method relies on algebraically combining a series of known reactions with established \(\Delta H\) values. The law works because the total energy change of a reaction is independent of the pathway taken, allowing chemists to calculate the overall heat transfer based on a hypothetical sequence of steps.
Understanding the Law’s Foundation
The theoretical basis of Hess’s Law is rooted in the fact that enthalpy is a state function. A state function is a property whose value depends only on the current state of the system, such as its temperature and pressure, and not on how it reached that state. This means that whether a reaction occurs in a single step or a series of multiple steps, the overall change in enthalpy will remain exactly the same. This concept is a direct consequence of the first law of thermodynamics, which emphasizes the conservation of energy. To apply this law, you must first define the “target equation” (the reaction whose unknown enthalpy change you seek) and gather a set of “known reactions” with established \(\Delta H\) values.
Strategies for Manipulating Chemical Equations
The procedural heart of using Hess’s Law involves carefully manipulating the known reactions to match the reactants and products of the target equation. This process is governed by two main algebraic rules applied consistently to both the chemical equation and its corresponding \(\Delta H\) value.
Reversing the Reaction
The first manipulation is reversing the direction of a known reaction. When you reverse a chemical equation, you must simultaneously reverse the sign of its \(\Delta H\) value; an exothermic reaction (negative \(\Delta H\)) becomes endothermic (positive \(\Delta H\)) upon reversal.
Adjusting Stoichiometry
The second manipulation involves adjusting the stoichiometric coefficients of a known reaction to match the required amounts in the target equation. If the coefficients of a known reaction are multiplied by a factor, the entire equation must be multiplied by that factor. Crucially, the \(\Delta H\) value for that step must also be multiplied by the exact same numerical factor. The overall strategy involves systematically working through the target equation, ensuring each compound is on the correct side and has the correct stoichiometric coefficient.
Calculating the Overall Enthalpy Change
After all the known equations have been manipulated, the final step is to combine them algebraically to find the overall enthalpy change. You begin by adding the adjusted chemical equations together, treating the reaction arrows like equal signs in a mathematical equation. Any chemical species that appear on both the reactant side of one equation and the product side of another equation will cancel out, much like terms in an algebraic sum. Simultaneously, you must algebraically sum all of the newly adjusted \(\Delta H\) values, including any sign changes or multiplication factors applied in the previous step. The final result of this summation represents the \(\Delta H\) for the target reaction, typically expressed in units of kilojoules (kJ) or kilojoules per mole (kJ/mol). This method is useful for processes that are physically difficult to measure directly, such as reactions that proceed too slowly or involve highly unstable, short-lived intermediate compounds. By breaking down the complex process into a series of simpler, measurable steps, the law allows chemists to determine thermodynamic values that would otherwise be inaccessible.