The spontaneous reaction is an irreversible process in which the total Gibbs energy is less than zero while the total entropy tends to increase.

Outline

## Spontaneous Chemical Reactions

Some processes happen naturally in a certain direction e.g. expansion of gases and heat transfer from a hotter to a colder body. Likewise, some chemical reactions that run in one direction and happen without any external energy are spontaneous. A spontaneous reaction proceeds in a certain direction with no external energy needed.

Spontaneous reactions are unidirectional irreversible reactions in either a closed or open system. In these reactions, the enthalpy of the reaction changes along with entropy. The spontaneity of reaction depends upon these factors. Therefore to explain the spontaneous reaction an important factor has to be encountered known as Gibbs free energy.

## Gibbs Free Energy

Gibbs’s free energy is used to calculate the maximum amount of work done at constant temperature and pressure. It is expressed as the difference between enthalpy and product of absolute temperature and change in entropy.

ΔGº = ΔHº – TΔSº

where,

• ΔGº = Standard change in Gibbs free energy
• ΔHº = Standard change in enthalpy
• T = Absolute temperature
• ΔSº = Standard change in enthalpy

ΔGº is the standard change in free energy named after an American scientist Josiah W. Gibbs (1839-1903). For a reaction to be spontaneous, the Gibbs free energy should be negative which means the release of free energy. A reaction in which free energy is released to its surrounding is said to be exergonic.

The spontaneity of any process depends upon both enthalpy and entropy. In some reactions, an activation free energy (ΔG) is required to start a reaction. Once the reaction starts, then the reaction will continue as spontaneous, e.g. combustion reactions.

## CRITERIA For Spontaneous Reactions

For a reaction to be spontaneous the change in Gibbs free should be negative. Since both changes in enthalpy and entropy of reaction can be either negative or positive. So there is some combination that provides the negative value of ΔG. The sign of change in enthalpy corresponds to an exothermic (ΔH < 0) or endothermic (ΔH > 0) reaction. Furthermore entropy of a reaction increases (ΔS > 0) or decreases (ΔS < 0) depending upon the degree of randomness. Following are the combinations which depict the spontaneity of reaction when ΔG < 0.

Table.

## Concepts Berg

What does it mean when a reaction is spontaneous?

A spontaneous reaction does not need any external force or energy to initiate the process. For example, the melting of ice.

How do you know if a reaction is spontaneous?

When the value of change in Gibbs free energy comes out negative the reaction is spontaneous. Gibbs free energy equation is given as

ΔGº = ΔHº – TΔSº

So if the value of ΔGº is negative for any reaction, the reaction becomes spontaneous.

What causes a spontaneous reaction?

a spontaneous reaction depends upon two factors

1. Enthalpy (ΔH)
2. Entropy (ΔS)

So based on these two factors following possibilities should be formed for spontaneous reaction

1. ΔH < 0 and ΔS > 0 then ΔG < 0 (always spontaneous)
2. ΔH < 0 and ΔS < 0 then ΔG < 0 at lower temperature
3. ΔH > 0 and ΔS > 0 then ΔG < 0 at higher temperature

What are entropy and spontaneity?

Entropy is the measure of randomness of the system. According to the second law of thermodynamics, the entropy of the system must increase for the reaction.

What are some examples of spontaneous reactions?

Some examples of spontaneous reactions are rusting of iron, melting of water, and roaring fire.

Why do all spontaneous reactions not take place immediately?

The spontaneous reaction has relation to Gibbs free energy which is a thermodynamic factor. The rate of the reaction depends upon a different factor studied in chemical kinetics. So it is not linked to the spontaneity of a reaction.

For example rate of reaction decrease at lower temperature but a reaction may be spontaneous if the enthalpy of the reaction is negative.

Can spontaneous reactions be reversible?

NO, spontaneous reactions are always irreversible. According to the second law of thermodynamics entropy of spontaneous processes always increased. So there is no chance that the randomness of reactions can be reversed.

What are some non-spontaneous reactions?

Boiling water into steam, photosynthesis, and burning of fuel are some examples of non-spontaneous reactions.

References

Spontaneity in Thermodynamics by byjus.com

Spontaneous reactions by chem. libretexts