Final answer:
If the reaction X → Y is spontaneous, implying a negative ΔG, the reverse reaction Y → X is non-spontaneous as it would increase the system's free energy and require external energy to proceed.
Step-by-step explanation:
If the reaction X → Y is spontaneous, it implies that the change in free energy (ΔG) is negative for this direction, and the process occurs naturally without external work being done on the system. Based on thermodynamics, if the reaction in one direction is spontaneous, the reverse reaction, in this case, Y → X, would be non-spontaneous, meaning it would not occur without the input of energy. This is because the free energy would be at its minimum at equilibrium, and any deviation from equilibrium would require work to be done on the system.
The tendency of a reaction to proceed toward equilibrium can also be understood in terms of the reaction quotient (Q) and the equilibrium constant (K). If Q < K, the reaction proceeds spontaneously towards products, while if Q > K, the reaction is spontaneous in the reverse direction, converting products back to reactants.
In summary, if the reaction X → Y is spontaneous, we can infer that the reverse reaction Y → X is non-spontaneous under the same conditions because reactions proceed spontaneously in the direction that minimizes free energy and establishes equilibrium.