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The world of chemistry is filled with reactions, some that happen effortlessly and others that require a push. Understanding which reactions occur on their own, without outside intervention, is crucial. So, what type of reaction is always spontaneous? The answer lies in the realm of thermodynamics, specifically the interplay of enthalpy and entropy.
Exothermic Reactions with Increased Entropy The Key to Spontaneity
The spontaneity of a reaction is governed by Gibbs Free Energy (ΔG). A reaction is considered spontaneous (or thermodynamically favorable) when ΔG is negative. The Gibbs Free Energy equation is: ΔG = ΔH - TΔS, where ΔH is the change in enthalpy (heat), T is the temperature in Kelvin, and ΔS is the change in entropy (disorder). An exothermic reaction releases heat into the surroundings, resulting in a negative ΔH. Increased entropy means the system becomes more disordered, leading to a positive ΔS. Therefore, reactions that are exothermic (negative ΔH) and have an increase in entropy (positive ΔS) are always spontaneous at all temperatures.
Here’s why: When ΔH is negative and ΔS is positive, the term -TΔS becomes negative as well. Since both terms on the right side of the Gibbs Free Energy equation are negative, their sum (ΔG) will always be negative, regardless of the temperature. To further illustrate this, consider these factors influencing spontaneity:
- Enthalpy (ΔH): Favors spontaneity when negative (exothermic).
- Entropy (ΔS): Favors spontaneity when positive (increased disorder).
- Temperature (T): Affects the significance of the entropy term.
Let’s consider a simple example. Imagine a solid breaking apart into gaseous components. This process is likely endothermic (requires energy to break the bonds in the solid), but it also results in a significant increase in entropy as the gas molecules are much more disordered than the solid. However, if the same reaction was exothermic:
- Heat is released.
- Disorder increases
In this case, the reaction will always be spontaneous.
Want to dive deeper into the specifics of Gibbs Free Energy and its calculation? Review your thermodynamics textbook for detailed explanations and examples!