# Entropy vs. Enthalpy: What's the Difference?

Edited by Janet White || By Harlon Moss || Updated on October 4, 2023
Entropy measures disorder or randomness; enthalpy measures heat content or energy.

## Key Differences

Entropy is a measure of the randomness or disorder of a system. The higher the entropy, the more disordered a system is. Enthalpy, on the other hand, is a measure of the total energy of a system, which includes both internal energy and the energy due to pressure-volume work. Both entropy and enthalpy are state functions, which means their values depend only on the current state of the system, not on its history.
In chemical reactions, the change in entropy (∆S) indicates how the disorder of a system changes. If the products are more disordered than the reactants, ∆S is positive. Enthalpy, in a chemical context, refers to the heat content. The change in enthalpy (∆H) for a reaction indicates whether the reaction is endothermic (absorbs heat) or exothermic (releases heat). Both entropy and enthalpy play critical roles in determining the spontaneity of a reaction.
From a thermodynamic standpoint, entropy can be thought of as a measure of the number of possible microstates a system can have. The more ways a system can distribute its energy, the higher its entropy. Enthalpy, meanwhile, relates to the heat flow in or out of a system under constant pressure. Again, both entropy and enthalpy contribute to the overall Gibbs free energy, which predicts the direction and spontaneity of chemical processes.
In everyday terms, if a room becomes messier over time, its entropy has increased. If a hot cup of coffee cools down, it releases heat, reflecting a change in its enthalpy. Entropy and enthalpy, while both related to energy and order, represent distinct thermodynamic concepts, with entropy focusing on disorder and enthalpy on heat content.

## Comparison Chart

### Definition

Measure of disorder or randomness.
Measure of total energy, including heat content.

### Units

Joules per Kelvin (J/K)
Joules (J)

### Chemical Reaction

Indicates change in disorder.
Indicates if reaction is endo/exothermic.

### Thermodynamics

Relates to number of possible microstates.
Relates to heat flow at constant pressure.

### Everyday Analogy

Messiness of a room increasing.
Hot coffee cooling down.

## Entropy and Enthalpy Definitions

#### Entropy

Entropy refers to the number of possible microstates of a system.
As ice melts, its entropy increases due to the greater number of microstates in water.

#### Enthalpy

Enthalpy reflects the heat content of a system at constant pressure.
When ice melts, the system absorbs heat, leading to a positive change in enthalpy.

#### Entropy

Entropy is associated with the unavailability of a system's energy to do work.
The entropy of the universe is always increasing, limiting the energy available for work.

#### Enthalpy

Enthalpy is the measure of the total energy of a system.
The enthalpy of water is different from that of ice or steam.

#### Entropy

Entropy indicates the direction of thermodynamic processes.
A process that increases the entropy of the universe is spontaneous.

#### Enthalpy

Enthalpy determines whether a reaction is endothermic or exothermic.
Combustion reactions release heat, signifying a negative change in enthalpy.

#### Entropy

Entropy describes the level of uncertainty or surprise in a system.
A shuffled deck of cards has higher entropy than an ordered one.

#### Enthalpy

Enthalpy changes correlate with bond breaking and formation.
In chemical reactions, breaking bonds requires energy, while forming bonds releases energy, influencing enthalpy changes.

#### Entropy

Entropy is the measure of randomness or disorder in a system.
The entropy of a gas is higher than that of a solid.

#### Enthalpy

Enthalpy is the sum of internal energy and the product of pressure and volume.
For a given substance, changes in pressure or volume can influence its enthalpy.

#### Entropy

Symbol S For a closed thermodynamic system, a quantitative measure of the amount of thermal energy not available to do work.

#### Enthalpy

Symbol H A thermodynamic function of a system, equivalent to the sum of the internal energy of the system plus the product of its volume multiplied by the pressure exerted on it by its surroundings.

#### Entropy

A measure of the disorder or randomness in a closed system.

#### Enthalpy

A measure of the heat content of a chemical or physical system.
$H = U + p V$, where H is enthalpy, U is internal energy, p is pressure, and V is volume.

#### Entropy

A measure of the loss of information in a transmitted message.

#### Enthalpy

(thermodynamics) a thermodynamic quantity equal to the internal energy of a system plus the product of its volume and pressure;
Enthalpy is the amount of energy in a system capable of doing mechanical work

#### Entropy

The tendency for all matter and energy in the universe to evolve toward a state of inert uniformity.

## FAQs

#### Do all reactions increase entropy?

No, some reactions can decrease entropy, leading to more order.

#### What is entropy?

Entropy is a measure of the randomness or disorder of a system.

#### What does a positive enthalpy change indicate?

It indicates an endothermic reaction, where heat is absorbed.

#### How is entropy used in thermodynamics?

It predicts the direction and spontaneity of processes.

#### Why is enthalpy important in reactions?

It helps determine if a reaction is exothermic or endothermic.

#### How do entropy and enthalpy relate to spontaneity?

Both contribute to Gibbs free energy, which predicts spontaneity.

#### What is enthalpy?

Enthalpy is the total energy of a system, including heat content.

#### Can enthalpy be negative?

Yes, negative enthalpy indicates an exothermic reaction that releases heat.

#### How is enthalpy calculated?

Typically, through calorimetry or by using enthalpy of formation values.

#### Can entropy ever decrease?

In isolated systems, it can, but in the universe as a whole, it tends to increase.

#### What's the unit for enthalpy?

It's typically measured in Joules (J).

#### Are entropy and enthalpy independent of each other?

No, they often interact, especially in determining reaction spontaneity.

#### Why is the entropy of a gas higher than a solid?

Gas particles have more freedom to move, leading to greater disorder.

#### How are entropy and probability related?

Higher entropy corresponds to a higher probability of a system's state.

#### Can we feel changes in enthalpy?

Often, yes. A reaction that gets hot or cold indicates a change in enthalpy.

#### What's the unit for entropy?

It's usually Joules per Kelvin (J/K).

#### How is enthalpy related to heat?

Enthalpy reflects the heat content of a system under constant pressure.

#### Is enthalpy conserved?

Yes, based on the conservation of energy, but its value can change in reactions.

#### How is entropy related to disorder?

Higher entropy indicates greater disorder or randomness.

#### Why is entropy called "time's arrow"?

Because the overall entropy of the universe tends to increase, suggesting a direction in time.