Difference between Enthalpy and Entropy

Updated on June 28, 2017

Enthalpy and entropy are related concepts that refer to functions of measurements in chemical thermodynamics. Both terms are also related to the heat changes involved in a reaction. However, these terms are characterized by significant differences which this article aims to explain.

Descriptions

Enthalpy
A diagram showing the relationship between enthalpy and heat energy

Enthalpy is a scientific and engineering concept that measures the total amount of energy in a thermodynamic system.. It describes the amount of heat and work that have been added or removed from a substance when it changes because of constant pressure. For example, when the size of a substance changes, energy is spent up or released. Enthalpy explains this displaced energy. As enthalpy is not measured directly, scientists measure the change in enthalpy instead. Enthalpy is typically expressed in either joules or calories.

Dutch scientist Heike Kamerlingh Onnes coined the term from enthalpos, which is a Greek term for “to put heat into.”

A substance can either become warmer or colder during a chemical reaction. This causes heat to travel to or from objects surrounding it until its temperature returns to its original value. With pressure at a constant, the change in enthalpy is determined by this amount of heat.

If gasoline is burned in an open area, heat is released. If 100 kilojoules of heat is released, then the same amount of kilojoules was reduced from the enthalpy of the gasoline. This kind of reaction is called exothermic. On the other hand, if the reaction leads to heat absorption, then enthalpy of this reaction increased. This is referred to as an endothermic reaction.

Entropy
A diagram showing the shift of solid to liquid and to gas

Entropy is the measure of randomness or uncertainty in a chemical reaction. The second law of thermodynamics states that the entropy of an object will always increase. This means that chemical reactions are more likely to lead to more disorder or uncertainty because of the various states of the atoms and molecules involved in the reaction. This reaches a point where the process spreads and becomes stable. For instance, a substance phase shifts from being solid to liquid and then to gas. The molecules and atoms in the substance move more freely at each new phase. Entropy is that increase in freedom matched by the increase in disorder.

Enthalpy vs Entropy

So what’s the difference between enthalpy and entropy? Enthalpy is a scientific and engineering concept that measures the total amount of energy in a thermodynamic system. It describes the amount of heat and work that has been added or removed from a substance when it is subjected to constant pressure. Entropy, on the other hand, is the measure of randomness and disorder in a chemical reaction or in a system of atom and molecules. Measured in joules per unit kelvin, entropy is a crucial component of the third law of thermodynamics. Enthalpy is measured in kilojoules while entropy is expressed in j/K (joules/Kelvin).

Comparison Chart

Enthalpy Entropy
Measure of the total amount of energy in a thermodynamic systemMeasure of the amount of possible arrangements of atoms in a chemical reaction
Expressed in kilojoulesExpressed in joules per Kelvin

Video

Here’s a thorough explanation of enthalpy and entropy.