How is it possible for a reaction to be spontaneous yet endothermic?

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Understand that a spontaneous reaction is one that occurs without needing to be driven by an external force. It is determined by the change in Gibbs free energy (ΔG).

Recall the Gibbs free energy equation: ΔG = ΔH - TΔS, where ΔG is the change in free energy, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.

Recognize that for a reaction to be spontaneous, ΔG must be negative.

Consider that an endothermic reaction has a positive ΔH, meaning it absorbs heat from the surroundings.

Realize that if the increase in entropy (ΔS) is large enough and the temperature (T) is high enough, the TΔS term can outweigh the positive ΔH, resulting in a negative ΔG, thus making the reaction spontaneous.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Spontaneity of Reactions

A reaction is considered spontaneous if it occurs without external intervention. This is often determined by the change in Gibbs free energy (ΔG); if ΔG is negative, the reaction is spontaneous. Spontaneity is influenced by both enthalpy (ΔH) and entropy (ΔS) changes during the reaction.

Endothermic Reactions

Endothermic reactions absorb heat from their surroundings, resulting in a positive change in enthalpy (ΔH > 0). While these reactions require energy input to proceed, they can still be spontaneous if the increase in entropy (ΔS) is sufficiently large to offset the energy absorbed, leading to a negative ΔG.

Gibbs Free Energy Equation

The Gibbs free energy equation, ΔG = ΔH - TΔS, relates the change in free energy to enthalpy and entropy changes at a given temperature (T). For a reaction to be spontaneous, the term TΔS must be greater than ΔH, allowing for a negative ΔG even when ΔH is positive, thus enabling endothermic reactions to occur spontaneously.