which of the reactions are spontaneous favorable

3 min read 28-02-2025

which of the reactions are spontaneous favorable

Determining whether a reaction will proceed spontaneously under a given set of conditions is crucial in chemistry and many other fields. Spontaneity isn't simply about speed; it's about whether a reaction will occur naturally without external input. This article explores the key thermodynamic concept that dictates spontaneity: Gibbs Free Energy.

Gibbs Free Energy: The Decisive Factor

The Gibbs Free Energy (ΔG) is a thermodynamic potential that measures the maximum reversible work that may be performed by a thermodynamic system at a constant temperature and pressure. Its value directly indicates the spontaneity of a reaction:

ΔG < 0: The reaction is spontaneous (favorable) under the given conditions. It will proceed without requiring external energy.
ΔG = 0: The reaction is at equilibrium. The rates of the forward and reverse reactions are equal.
ΔG > 0: The reaction is non-spontaneous (unfavorable) under the given conditions. It will not proceed without external energy input.

Factors Influencing Gibbs Free Energy

Gibbs Free Energy is a function of two other thermodynamic properties: enthalpy (ΔH) and entropy (ΔS):

ΔG = ΔH - TΔS

Where:

ΔH: Change in enthalpy (heat content) of the system. Exothermic reactions (ΔH < 0) release heat, while endothermic reactions (ΔH > 0) absorb heat.
T: Absolute temperature in Kelvin.
ΔS: Change in entropy (disorder) of the system. Reactions that increase disorder (ΔS > 0) are favored.

Let's examine how these factors influence spontaneity:

1. Exothermic Reactions (ΔH < 0) and Increased Entropy (ΔS > 0)

This is the most favorable scenario. A negative ΔH and a positive ΔS both contribute to a negative ΔG, guaranteeing spontaneity at all temperatures. Many combustion reactions fall into this category.

2. Exothermic Reactions (ΔH < 0) and Decreased Entropy (ΔS < 0)

In this case, the negative ΔH favors spontaneity, but the negative ΔS opposes it. The spontaneity depends on the temperature. At lower temperatures, the negative ΔH dominates, making the reaction spontaneous. At higher temperatures, the TΔS term becomes larger, potentially making ΔG positive and the reaction non-spontaneous.

3. Endothermic Reactions (ΔH > 0) and Increased Entropy (ΔS > 0)

Here, the positive ΔH opposes spontaneity, while the positive ΔS favors it. Similar to the previous case, the temperature plays a crucial role. At high temperatures, the positive TΔS term can overcome the positive ΔH, resulting in a negative ΔG and a spontaneous reaction. Many phase transitions (like melting ice) fall under this category.

4. Endothermic Reactions (ΔH > 0) and Decreased Entropy (ΔS < 0)

This is the least favorable situation. Both ΔH and ΔS contribute to a positive ΔG, making the reaction non-spontaneous at all temperatures.

Determining Spontaneity: A Practical Approach

To determine whether a specific reaction is spontaneous, you need to:

Determine ΔH and ΔS: This often involves using standard enthalpy and entropy values from tables.
Calculate ΔG: Use the equation ΔG = ΔH - TΔS. Remember to use the temperature in Kelvin.
Interpret the Result: If ΔG < 0, the reaction is spontaneous; if ΔG > 0, it's non-spontaneous; if ΔG = 0, it's at equilibrium.

Examples of Spontaneous Reactions

Combustion of methane: This reaction releases a significant amount of heat (ΔH < 0) and increases the disorder (ΔS > 0), making it highly spontaneous.
Dissolution of table salt in water: While the enthalpy change is relatively small, the increase in entropy due to the mixing of ions and water molecules leads to a spontaneous process.
The rusting of iron: The oxidation of iron is a spontaneous reaction due to a large negative change in Gibbs Free Energy.

Conclusion

Understanding Gibbs Free Energy and its dependence on enthalpy and entropy is essential for predicting the spontaneity of chemical reactions. While exothermic reactions with increased entropy are always spontaneous, the temperature plays a crucial role in determining the spontaneity of other reaction types. By calculating ΔG, we can gain valuable insight into the feasibility of a reaction under specific conditions. Remember that spontaneity doesn't imply speed; a spontaneous reaction may proceed slowly.