Reaction A has K = 1 × 10⁻⁵ and reaction B has K = 1 × 10⁸. If B is coupled to A so that their equations add, what is K for the net reaction?

Core Concept

Adding chemical equations multiplies their equilibrium constants, since each step's quotient contributes multiplicatively to the overall quotient. Here Knet = (1 × 10⁻⁵)(1 × 10⁸) = 1 × 10³ = 1000. This mirrors free energy, where ΔG° values add to a negative total through ΔG°net = −RT ln(KA × KB). The strongly favorable B step pays for the unfavorable A step.