Question

Classify the possible combinations of signs for a reaction's Δ and Δ values by the resulting spontaneity.

Answer 1

The spontaneity of a reaction depends on the signs of ΔH and ΔS, along with the absolute temperature (T). There are four possible sign combinations, leading to different spontaneity outcomes based on the Gibbs free energy equation ΔG = ΔH - TΔS.

Step-by-step explanation:

To understand the spontaneity of a reaction based on ΔG (Gibbs free energy change) and ΔH (enthalpy change) and ΔS (entropy change), we must consider the four possible sign combinations for ΔH and ΔS:

• ΔH is negative and ΔS is positive: Reaction is spontaneous at all temperatures because the free energy change, ΔG, is always negative.
• ΔH is negative and ΔS is negative: Reaction is spontaneous at low temperatures since the negative ΔH dominates initially, but at high temperatures, the TΔS term becomes larger and can make ΔG positive, thus non-spontaneous.
• ΔH is positive and ΔS is positive: Reaction is non-spontaneous at low temperatures but can become spontaneous at high temperatures once the TΔS term overpowers the positive ΔH.
• ΔH is positive and ΔS is negative: Reaction is non-spontaneous at all temperatures because ΔG remains positive.

These outcomes are determined by the Gibbs free energy equation ΔG = ΔH - TΔS, where T is the absolute temperature in Kelvin, and ΔG must be negative for a reaction to be spontaneous.