Study Guides/Chemistry/When is a Chemical Reaction Spontaneous?
Study Guide · Chemistry

The Reaction is Spontaneous When... (Thermodynamics)

In Class 11 Chemistry (Thermodynamics), one of the most critical questions scientists ask is: Will this chemical reaction happen on its own without any outside help? A reaction that occurs naturally under given conditions is called a Spontaneous Reaction.

Question (Click to Flip)

If a reaction is spontaneous, does it mean it will happen very fast?

Answer

No! Spontaneity has nothing to do with speed (kinetics). It only means the reaction can happen on its own. For example, the transformation of diamond into graphite is thermodynamically spontaneous (ΔG < 0), but it takes millions of years to occur.

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Key Facts

Rusting of iron is a classic example of a spontaneous reaction. Once it starts, it continues naturally without any continuous input of energy, even though it happens very slowly.

The Golden Rule (Gibbs Free Energy)

A chemical reaction is spontaneous when the change in Gibbs Free Energy (ΔG) of the system is Negative (less than zero).

Condition for Spontaneity: ΔG < 0

If ΔG is positive, the reaction is non-spontaneous. If ΔG is exactly zero, the reaction is in a state of equilibrium.

How do we calculate ΔG?

The spontaneity of a reaction is determined by the famous Gibbs equation: ΔG = ΔH - TΔS

Where:

  • ΔH (Enthalpy change): The heat absorbed or released. (Nature prefers exothermic reactions, where ΔH is negative).
  • T (Temperature): The absolute temperature in Kelvin.
  • ΔS (Entropy change): The measure of randomness or disorder. (Nature prefers chaos, so it wants ΔS to be positive).

Winning Combinations for Spontaneity

  1. Always Spontaneous: If a reaction releases heat (ΔH is -ve) AND creates more disorder (ΔS is +ve), it will ALWAYS be spontaneous, no matter what the temperature is.
  2. Temperature Dependent: If a reaction absorbs heat (ΔH is +ve) but creates a lot of disorder (ΔS is +ve), it will only be spontaneous at very high temperatures (because the TΔS part becomes large enough to overcome the positive ΔH).

Questions and Answers

If a reaction is spontaneous, does it mean it will happen very fast?+

No! **Spontaneity has nothing to do with speed (kinetics)**. It only means the reaction *can* happen on its own. For example, the transformation of diamond into graphite is thermodynamically spontaneous (ΔG < 0), but it takes millions of years to occur.

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