# Free energy vs. Free energy change Khan Q

Discussion in 'MCAT Study Question Q&A' started by DanniD, Aug 5, 2015.

1. ### DanniD

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How does the free energy (G) of a system containing one mole of ADP compare to the free energy of a system containing one mole of AMP under standard physiological conditions?

hi guys I dont get why ADP has more positive free energy compared to AMP. according to Khan "The free energy of a system can never be negative. Remember, free energy (G) is not the same as free energy change (∆G)."

2. ### Neuroplasticity 2+ Year Member

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ADP has 1 more phosphate group, giving it a higher enthalpy because of the energy stored in those bonds. G=H-TS, so higher enthaply gives higher free energy (G).

3. ### DanniD

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hey thanks for the explanation, my problem is why they say that "free energy (G) can never be negative"

I guess because free energy is found from this equation GA = GA° + RT ln PA whereas, free energy CHANGE is sum of the free energies of the products, minus that of the reactants: ΔG = GC + GD – GA – GB so thats why free energy CHANGE could be negative whereas free energy can never be negative?

4. ### Neuroplasticity 2+ Year Member

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Sorry, I just saw you said "I dont get why ADP has more positive free energy compared to AMP" and responded to that. Yes, I think you have understood your confusion. It is similar to kinetic energy. An object can have a negative change in kinetic energy, but it can't have negative kinetic energy (unless some physicist wants to comment on objects with negative mass or something).

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