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Imagine a 1kg block of ice that has completely melted at room temperature. If melting is a spontaneous reaction, then wouldn't that mean the products (1kg of water) would have less Gibbs free energy than the reactants (1kg of ice)? I am confused because water is more disordered than ice and less stable, so it should have more free energy than ice.

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    $\begingroup$ Note that the Gibbs energy change of phase transition is temperature dependent, being zero at the melting point. $\endgroup$
    – Poutnik
    Sep 27, 2023 at 8:24

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The liquid phase does not have more free energy than the solid phase (when the liquid is stable).

True, the liquid has more total energy, or more accurately more enthalpy (which is essentially internal energy plus the work put into it from the outside pressure). But part of that energy is tied up in the greater disorder, or entropy, of the liquid versus the solid. When you add everything up according to the relation $G=H-TS$, the bigger enthalpy ($H$) of the liquid is outweighed by the bigger entropy ($S$) if the temperature ($T$) is high enough. Then the liquid has less of its energy as free energy.

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  • $\begingroup$ If the temperature is high enough, as in greater than 0 degrees celsius? Also, this would mean that in a Gibbs free energy diagram, the curve at the liquid phase would be lower than the same curve during the solid phase? $\endgroup$ Sep 27, 2023 at 2:12

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