Change in Enthalpy

Enthalpy changes are associated with chemical processes, and are important in understanding how the reaction has changed a chemical system.

Learning Objective

Define change in enthalpy

Key Points

A change in enthalpy refers to the amount of energy released or absorbed in a chemical reaction.
Hess's law states the enthalpy change for a reaction is independent of the pathway.
A positive change in enthalpy signifies an endothermic reaction, while a negative change in enthalpy signifies an exothermic reaction.

Terms

enthalpy
In thermodynamics, a measure of the heat content of a chemical or physical system.
enthalpy of reaction
the energy released during a chemical tranformation
enthalpy change
defined as the difference between the final and initial enthalpy of the system
state function
property of a system that depends on the current state of the system, not the way in which the system acquired that state; independent of pathway chosen

Full Text

Enthalpy

Enthalpy (H) is a measure of the total energy of a thermodynamic system. Any time a thermodynamic system undergoes a transformation or a chemical reaction, there is an energy (enthalpy) change associated with the process. Since the enthalpy of a system cannot be directly measured, we often concern ourselves with the change in enthalpy after a reaction has taken place. Any chemical reaction can be characterized by a change in enthalpy, denoted as: $\Delta H = \Delta H_{final} - \Delta H_{initial}$

Changes in Enthalpy

Energy and Chemical Bonds (Enthalpy)

This lesson introduces Enthalpy and the energy of chemical bonding. We discuss where the energy in chemical bonds comes from in terms of internal energy and enthalpy, as well as how to approximate the overall heat of reaction using bond enthalpies.

By absorbing heat, the temperature, and thus the enthalpy of a substance increases. The relationship is shown as: $\Delta H = C\rho \Delta T$, where $C\rho$ is the heat capacity of the material. By determining the value of the change of enthalpy, you can determine whether the reaction is endothermic (positive change) or exothermic (negative change).

Hess's Law

Sometimes reactions of interest take place in several steps. Hess's law addresses how to calculate the enthalpy for the overall reaction. The law states that the enthalpy change for a reaction is the same whether it occurs in one or many steps. Since enthalpy is a state function, or pathway independent, the route that the reaction takes does not change the enthalpy value. Hess's law states that the standard enthalpy change of the overall reaction is the sum of the enthalpy change of all the intermediate reactions that make up the overall reaction. For example, if you convert reactants A into products B, the overall enthalpy change will be the same whether you do it in one step or many steps.

Hess's Cycle

Since enthalpy is a state function, it can be calculated in one step or in many intermediate steps.

Being able to understand changes in enthalpy is essential in understanding the nature of the overall reaction. Many reaction enthalpies have been experimentally determined and documented. Often, you will encounter an overall reaction whose enthalpy is not listed; however, you may see that the overall reaction may be split into intermediate reactions. By remembering and employing Hess's Law, the change in enthalpy for the overall reaction can be determined by adding up the enthalpies of the intermediate reactions.

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