In environmental science and economics, the phrase "free energy" is sometimes used to refer to renewable resources or any energy that does not require monetary payment. Free energy may also refer to the energy that powers a hypothetical perpetual motion machine. Such a device violates the laws of thermodynamics, so this definition currently
Exergonic reactions are also called spontaneous reactions, because they can occur without the addition of energy. Reactions with a positive ∆ G (∆ G > 0), on the other hand, require an input of energy and are called endergonic reactions. In this case, the products, or final state, have more free energy than the reactants, or initial state.
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