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Consider these two reactions: A + B → C + D, with ΔG° = +x kJ D + E → F, with ΔG° = −y kJ For the overall reaction A + B + E → C + F to be spontaneous, which statement is correct?

1. 2x = y
2. x > y
3. x < y
4. x = y × TΔS

Correct answer: x < y

Solution

Adding the two steps, the overall dG = (+x) + (-y) = x - y. For spontaneity the overall dG must be negative, so x - y < 0, i.e. x < y.

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