What is Enthalpy?

Core Concepts

In this tutorial, you will learn about the definition and equation of enthalpy. In addition, you will learn about some of its applications, as it relates to thermodynamics.

Topics Covered in Other Articles

Enthalpy Definition

Enthalpy ( $H$ ) has to do with thermodynamics; it is a state function used in chemical and biological systems. This means that enthalpy depends only on the final energy, pressure, and volume and not the path the system took to get to the final state. In most solution based chemical systems, where the pressure and volume stay constant, the enthalpy is equal to the internal energy of the system, meaning the change in enthalpy is the heat absorbed or released in a thermodynamic process, like a chemical reaction. This is due to the law of energy conservation. If the pressure or volume changes, such as if a gas is produced during a reaction, then the enthalpy change will no longer be equal to the heat change.

Enthalpy Formula

This property is the sum of a system’s internal energy and the product of the pressure and the volume.

$\begin{gather*} H = U + pV \end{gather*}$

$H$ : Enthalpy
$U$ : Internal energy of the system
$p$ : Pressure
$V$ : Volume

You also may see $pV$ expressed as $W$ , known as work.

$\begin{gather*} H = U + W \end{gather*}$

The SI unit of measurement for enthalpy is the Joule (J). However, sometimes you may see the units of calorie or British thermal unit (BTU).

Temperature Dependence

A factor that comes into play when determining enthalpy, even though it is not seen in the equations above, is temperature. The same reactants in a reaction can vary in the amount of heat they can transfer when at different temperatures. Additionally, different reactants may be in different phases of matter at different ambient temperatures, which can also play a role in heat transfer.

Endothermic vs Exothermic

When a process is endothermic, it absorbs heat into the system. This causes the $\Delta H$ value to be positive. On the other hand, when a process is exothermic, it releases heat into the environment. Remember that despite there seeming to be a change in energy, these reactions, like all chemical reactions, obey the law of conservation of energy.

There are chemical reactions going on everywhere in the real world, and there are various examples of enthalpy seen in everyday life. One example is portable hand warmer packets. These tiny packets undergo an iron oxidation reaction under constant pressure in a closed system, which is why when you break in the packet, it gets warm quickly and easily. This is an example of an exothermic reaction because energy is being released from the system in the form of heat. Another example of enthalpy can be seen in refrigerator compressors. Here, there is a vaporization reaction occurring, again under the exact condition of constant pressure. Energy is being used and absorbed in the system in the form of heat as the refrigerant chemicals are vaporized in an endothermic matter.

Enthalpy Practice Problems

Problem 1

When water vapor condenses into liquid, heat is released into the environment. Is condensation, therefore, exothermic or endothermic?

Problem 2

Consider the equation for change in Gibbs free energy:

$\begin{gather*} \Delta G = \Delta H -T \Delta S \end{gather*}$

If a certain chemical reaction involves a negative change in entropy ( $\Delta S < 0$ ) and a negative change in Gibbs free energy ( $\Delta G < 0$ ), is it exothermic or endothermic? (Hint: since temperature is expressed in Kelvin, we know $T > 0$ )