Change in Gibbs free energy at equillibrium

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SUMMARY

The change in Gibbs Free Energy (ΔG) at equilibrium is definitively 0, which leads to the relationship -ΔH = TΔS. This indicates that while a reaction is at equilibrium, enthalpy (ΔH) and entropy (ΔS) can still change during the process, as seen in the melting of ice at 0 degrees Celsius. The system can temporarily deviate from equilibrium when external heat is applied, such as with a Bunsen burner, but ultimately returns to equilibrium, reaffirming that G, H, and S are state functions relevant only at the initial and final states of the system.

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  • Basic principles of chemical equilibrium
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I understand that the change in Gibbs Free Energy at equillibrium is 0 and this leads to the equation -deltaH=TdeltaS. My questions here is that if a reaction is at equillibrium, how can there be any change in enthalpy or entropy at all? Why wouldn't these terms be 0?
 
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As an equilibrium process, consider the melting of ice. At 0 degrees Celsius, the amount of ice and water in equilibrium is arbitrary. You can now melt some ice with a Bunsen burner, but afterwards, the system will be in equilibrium again. The heat you added to the system equals the change in H. As G, H and S are state functions, the only point which is relevant is that the system is in equilibrium at the beginning of a process and at the end. In the meantime, the system may deviate from equilibrium to drive the reaction.
 

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