What is implied by a positive Δ G value?

A positive ΔG value indicates that the reaction requires an input of energy to proceed. This indicates that the products of the reaction have more free energy than the reactants. As a result, the reaction is not spontaneous under standard conditions and will not occur without the addition of energy from an external source.

In biochemical terms, reactions with positive ΔG are typically endergonic, meaning they absorb energy rather than releasing it. Consequently, such reactions often couple with exergonic reactions (which release energy) in biological systems to facilitate the overall process.

In contrast, a negative ΔG would suggest a spontaneous reaction that releases energy, while ΔG of zero indicates a reaction at equilibrium. The idea of irreversibility is not directly tied to ΔG; many reactions can be reversible even if they have positive ΔG values under certain conditions.