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Last updated date: 16th Apr 2024
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Introduction to Spontaneity Chemistry

Spontaneity is part of the first law of thermodynamics. In this section, students can understand the fact associated with an isolated system's fixed energy level.  

There is a direction of heat flow that can be elaborated by establishing a relation between the work done by the system or on the system. This is spontaneity in thermodynamics.

Spontaneous meaning in Chemistry is not that hard to understand. Many natural phenomena are having one straight path of heat flow. They do not have any limitations on their heat flow paths. 

What Is a Spontaneous Reaction?

A spontaneous chemical reaction is an irreversible process where you can’t get the ingredients back without the external agents.  

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Equation of Spontaneous Reaction

We can predict the spontaneity Chemistry of a reaction with the change in its total entropy. This is defined as the spontaneity of any process. Almost all types of chemical reactions come under this category.

Scientists also predicted that the change in enthalpy helps the increase or increase in the randomness of the chemical reactions. They also affect the molecular motions. This is not possible as entropy changes only possible due to spontaneity. Many other processes are also in the queue of participation.

Students can gain proper knowledge about Spontaneity Chemistry and spontaneous equations by studying Gibb’s energy. 

Spontaneous Equation

Gibb’s equation can be the best option to understand spontaneous reactions in chemical composition. It is a state function. Also, Gibb’s equation is an extensive property. At constant temperature, Gibb’s equation shows the energy change. 

It can be expressed as, 

ΔGsys = ΔHsys – TΔSsys


  • Change in Gibbs energy of the system = ΔGsys 

  • Change in enthalpy of the system = ΔHsys 

  • Change in Entropy of the system = ΔSsys 

  • Constant Temperature of the system = T 

Also, if we conduct a spontaneous process, the total change in entropy is always greater than zero. 

Mathematical expression for the above spontaneous reaction meaning expression is

ΔSsys + ΔSsurr = ΔStotal


  • ΔStotal= total change in entropy for the process

  • ΔSsurr = change in entropy of the surrounding

  • ΔSsys = change in entropy of the system

What Is Spontaneous Process?

When the system is in a thermal equilibrium state, the change in temperature between the surroundings and the system is always zero. 

i.e. dT = 0

Do you know how this is happening? It is due to the change of enthalpy. As the amount of enthalpy is lost by the system, the same amount is gained by the surrounding. 

So, scientists have put forward the equations that stand for the change in entropy for both the system and the surrounding.

\[\Delta S_{surr}=\frac{\Delta H_{surr}}{T}=- \frac{\Delta H_{sys}}{T}\]

\[\Delta S_{Total}=\Delta S_{sys}+\left ( -\frac{\Delta H_{sys}}{T} \right )\]

Here, ΔHsurr = enthalpy change of the surrounding

ΔHsys = enthalpy change of the system

As expressed earlier, ΔStotal> 0.

The change in entropy is always more than zero when it is a spontaneous process. So, we conclude that

TΔSsys – ΔHsys > 0

ΔHsys– TΔSsys < 0

When we use Gibb’s equation, it can be said that ‘ΔGsys< 0’.


In a spontaneous chemical reaction, if we notice any energy change in Gibb’s energy of the system as less than zero otherwise, it is not a spontaneous process. 

It can be concluded that relation is also predicted for a spontaneous reaction.

When it is an exothermic reaction, the enthalpy of the system is negative. This is why it makes all exothermic reactions spontaneous. 

When it is an endothermic reaction, Gibbs’s free energy turns into negative. It happens in certain conditions only, such as when the temperature rises or the change in entropy is very high.

FAQs on Spontaneity

1. How Do You Explain Entropy?

Ans: If you want to measure any type of energy dispersal of a certain system, then entropy works as a true measurement. As evidence, we can take our universe. We know our universe tends towards the highest entropy. Also, for example, we can take the campfire. 

This is a perfect example of entropy. Here, wood (solid object) turns into ash, smoke, and gas. This is the end where at first, the energy was there when the wood was burning. So, the woods work as a solid fuel that spreads energy. 

Many things happening around us are related to entropy. We can’t check entropy as a bad or a good chemical process. It is happening to sustain life.

2. Define Enthalpy in Brief.

Ans: Enthalpy is the alteration of the amount of heat in a system when the temperature is constant. Enthalpy is not the same as heat. Heat is an energy that transfers from one system to another or surrounding due to temperature differences. Students can consider enthalpy and heat as a single term under one condition. When there is no work done to the system, then we take both of them as the same. Scientists measure the enthalpy with the (international system of units) SI unit called Joule. Also, they use the unit ‘calorie’ for measuring enthalpy in different regions of Europe (British thermal Unit).