Courses

Courses for Kids

Free study material

Store

Talk to our experts

1800-120-456-456

Chemistry

Spontaneity in Chemistry and Gibbs Free Energy Concept

Spontaneity in Chemistry and Gibbs Free Energy Concept

Q: 1. What is spontaneity in chemistry?

In chemistry, spontaneity refers to whether a process or reaction can occur on its own under given conditions without continuous external energy input. A spontaneous reaction does not require ongoing work to proceed, although it may need initial activation energy. Spontaneity is determined by Gibbs free energy (ΔG) and not by how fast the reaction occurs. For example, rusting of iron is spontaneous but slow: 4Fe(s) + 3O2(g) → 2Fe2O3(s).

Q: 2. What is the Gibbs free energy equation for spontaneity?

The Gibbs free energy equation is ΔG = ΔH − TΔS, and it predicts whether a reaction is spontaneous at constant temperature and pressure. In this equation: ΔG = change in Gibbs free energy (J or kJ)ΔH = change in enthalpy (heat)T = temperature in kelvin (K)ΔS = change in entropyIf ΔG < 0, the reaction is spontaneous; if ΔG > 0, it is non-spontaneous; if ΔG = 0, the system is at equilibrium.

Q: 3. How do you know if a reaction is spontaneous?

A reaction is spontaneous if the Gibbs free energy change (ΔG) is negative under the given conditions. You can determine spontaneity by: Calculating ΔG = ΔH − TΔSChecking the sign of ΔGComparing ΔG to zeroFor example, if ΔH = −100 kJ and TΔS = −20 kJ, then ΔG = −100 − (−20) = −80 kJ, so the reaction is spontaneous.

Q: 4. What is the relationship between entropy and spontaneity?

An increase in entropy (ΔS > 0) generally favors spontaneity because systems naturally move toward greater disorder. According to ΔG = ΔH − TΔS, a positive ΔS makes the term −TΔS more negative, lowering ΔG. For example, vaporization of water increases entropy: H2O(l) → H2O(g), and at high temperature this process becomes spontaneous because the entropy gain outweighs the enthalpy input.

Q: 5. Is an exothermic reaction always spontaneous?

No, an exothermic reaction (ΔH < 0) is not always spontaneous because spontaneity also depends on entropy and temperature. Even if heat is released, a large decrease in entropy (ΔS < 0) at high temperature can make ΔG positive. For example, if ΔH = −50 kJ and TΔS = −80 kJ, then ΔG = +30 kJ, meaning the reaction is non-spontaneous despite being exothermic.

Q: 6. What is the difference between spontaneous and non-spontaneous reactions?

A spontaneous reaction has ΔG < 0 and can occur without continuous external energy, while a non-spontaneous reaction has ΔG > 0 and requires constant energy input. Key differences include: Spontaneous: thermodynamically favorableNon-spontaneous: thermodynamically unfavorableSpontaneous reactions may still be slow (kinetics is separate)For example, electrolysis of water 2H2O(l) → 2H2(g) + O2(g) is non-spontaneous and requires electrical energy.

Q: 7. How does temperature affect spontaneity?

Temperature affects spontaneity through the TΔS term in the Gibbs free energy equation. As temperature increases, the entropy contribution becomes more significant. The four common cases are: ΔH < 0, ΔS > 0: spontaneous at all temperaturesΔH > 0, ΔS < 0: non-spontaneous at all temperaturesΔH < 0, ΔS < 0: spontaneous at low TΔH > 0, ΔS > 0: spontaneous at high TThis explains why some endothermic reactions become spontaneous only at higher temperatures.

Q: 9. What is the condition for spontaneity at equilibrium?

At equilibrium, the condition for spontaneity is ΔG = 0. When ΔG equals zero: The forward and reverse reaction rates are equalNo net change in concentrations occursThe system is at minimum Gibbs free energyThe relationship between free energy and the equilibrium constant is ΔG° = −RT ln K, where R is the gas constant and T is temperature in kelvin.

Reviewed by:

Ritika Singla

Latest Updates

Book Free Demo :

1 Teacher, 1 Student, 100% Attention (Classes start from ₹800 per hour)

What Is Spontaneity in Chemistry and How Gibbs Free Energy Determines It

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.

[Image will be uploaded soon]

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

Here,

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

Here,

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’.

Conclusion

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 in Chemistry and Gibbs Free Energy Concept

1. What is spontaneity in chemistry?

In chemistry, spontaneity refers to whether a process or reaction can occur on its own under given conditions without continuous external energy input. A spontaneous reaction does not require ongoing work to proceed, although it may need initial activation energy. Spontaneity is determined by Gibbs free energy (ΔG) and not by how fast the reaction occurs. For example, rusting of iron is spontaneous but slow: 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s).

2. What is the Gibbs free energy equation for spontaneity?

The Gibbs free energy equation is ΔG = ΔH − TΔS, and it predicts whether a reaction is spontaneous at constant temperature and pressure. In this equation:

ΔG = change in Gibbs free energy (J or kJ)
ΔH = change in enthalpy (heat)
T = temperature in kelvin (K)
ΔS = change in entropy

If ΔG < 0, the reaction is spontaneous; if ΔG > 0, it is non-spontaneous; if ΔG = 0, the system is at equilibrium.

3. How do you know if a reaction is spontaneous?

A reaction is spontaneous if the Gibbs free energy change (ΔG) is negative under the given conditions. You can determine spontaneity by:

Calculating ΔG = ΔH − TΔS
Checking the sign of ΔG
Comparing ΔG to zero

For example, if ΔH = −100 kJ and TΔS = −20 kJ, then ΔG = −100 − (−20) = −80 kJ, so the reaction is spontaneous.

4. What is the relationship between entropy and spontaneity?

An increase in entropy (ΔS > 0) generally favors spontaneity because systems naturally move toward greater disorder. According to ΔG = ΔH − TΔS, a positive ΔS makes the term −TΔS more negative, lowering ΔG. For example, vaporization of water increases entropy: H₂O(l) → H₂O(g), and at high temperature this process becomes spontaneous because the entropy gain outweighs the enthalpy input.

5. Is an exothermic reaction always spontaneous?

No, an exothermic reaction (ΔH < 0) is not always spontaneous because spontaneity also depends on entropy and temperature. Even if heat is released, a large decrease in entropy (ΔS < 0) at high temperature can make ΔG positive. For example, if ΔH = −50 kJ and TΔS = −80 kJ, then ΔG = +30 kJ, meaning the reaction is non-spontaneous despite being exothermic.

6. What is the difference between spontaneous and non-spontaneous reactions?

A spontaneous reaction has ΔG < 0 and can occur without continuous external energy, while a non-spontaneous reaction has ΔG > 0 and requires constant energy input. Key differences include:

Spontaneous: thermodynamically favorable
Non-spontaneous: thermodynamically unfavorable
Spontaneous reactions may still be slow (kinetics is separate)

For example, electrolysis of water 2H₂O(l) → 2H₂(g) + O₂(g) is non-spontaneous and requires electrical energy.

7. How does temperature affect spontaneity?

Temperature affects spontaneity through the TΔS term in the Gibbs free energy equation. As temperature increases, the entropy contribution becomes more significant. The four common cases are:

ΔH < 0, ΔS > 0: spontaneous at all temperatures
ΔH > 0, ΔS < 0: non-spontaneous at all temperatures
ΔH < 0, ΔS < 0: spontaneous at low T
ΔH > 0, ΔS > 0: spontaneous at high T

This explains why some endothermic reactions become spontaneous only at higher temperatures.

8. Can a spontaneous reaction be slow?

Yes, a spontaneous reaction can be slow because spontaneity depends on thermodynamics, not reaction rate. Thermodynamics determines if a reaction can occur (ΔG), while kinetics determines how fast it occurs (activation energy). For example, diamond converting to graphite is spontaneous under standard conditions but occurs extremely slowly due to high activation energy.

9. What is the condition for spontaneity at equilibrium?

At equilibrium, the condition for spontaneity is ΔG = 0. When ΔG equals zero:

The forward and reverse reaction rates are equal
No net change in concentrations occurs
The system is at minimum Gibbs free energy

The relationship between free energy and the equilibrium constant is ΔG° = −RT ln K, where R is the gas constant and T is temperature in kelvin.

10. How is Gibbs free energy related to the equilibrium constant?

Gibbs free energy is related to the equilibrium constant by the equation ΔG° = −RT ln K, which connects thermodynamics and chemical equilibrium. In this equation:

ΔG° = standard Gibbs free energy change
R = 8.314 J mol⁻¹ K⁻¹
T = temperature (K)
K = equilibrium constant

If K > 1, then ΔG° is negative and the reaction is spontaneous under standard conditions; if K < 1, ΔG° is positive and the reaction is non-spontaneous.