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Gibbs free energy is the usable energy available for a system to perform work at constant temperature and pressure. It accounts for both the system's energy change (enthalpy) and the disorder increase (entropy). This measurement helps scientists compare energy releases between different reactions and predict whether chemical reactions will occur spontaneously in cells.

The change in free energy (ΔG) is calculated using the equation ΔG = ΔH − TΔS, where ΔH is enthalpy change, T is absolute temperature in Kelvin, and ΔS is entropy change. This formula shows that free energy depends on both the system's total energy change and the temperature-weighted entropy change, allowing prediction of reaction spontaneity.

A negative ΔG indicates a spontaneous reaction that releases usable energy and can perform work. The change in free energy predicts spontaneity by incorporating both enthalpy and entropy changes of the system alone, eliminating the need to measure the difficult-to-quantify entropy change of the surroundings directly.

Standard ΔG values are calculated at pH 7.0, 25 degrees Celsius, and 100 kilopascals pressure, but actual cellular conditions vary considerably from these parameters. Because free energy depends on temperature and other environmental factors, the standard calculated values for biological reactions will differ from the actual free energy changes occurring inside living cells.

The entropy of the surroundings equals the negative enthalpy change of the system divided by temperature. When a system releases heat, the surroundings absorb it, increasing their randomness. This relationship allows scientists to determine spontaneity using only the system's properties, without directly measuring the surroundings' entropy change.

The second law of thermodynamics states that all energy transfers result in some energy lost as heat, creating entropy. Gibbs free energy specifically accounts for this entropy loss by measuring only the usable energy remaining after entropy is considered, making it a practical tool for predicting whether reactions can do work in biological systems.

Spontaneous processes increase the entropy of the universe, but measuring total entropy is difficult. The Gibbs free energy equation rearranges to show that the negative ratio of free energy change to temperature equals the entropy change of the universe. This allows scientists to predict spontaneity using only the system's enthalpy and entropy changes.