How can the spontaneity of a reaction be determined without performing it?

The spontaneity of a reaction can be determined by calculating the Gibbs free energy change (ΔG).

The Gibbs free energy change (ΔG) is a thermodynamic potential that measures the maximum reversible work that a system can perform at constant temperature and pressure. It is a key factor in determining whether a chemical reaction will occur spontaneously. If ΔG is negative, the reaction will occur spontaneously under the given conditions. If ΔG is positive, the reaction will not occur spontaneously. If ΔG is zero, the system is at equilibrium.

The Gibbs free energy change can be calculated using the equation ΔG = ΔH - TΔS, where ΔH is the change in enthalpy (heat content), T is the absolute temperature in Kelvin, and ΔS is the change in entropy (disorder). The enthalpy change and entropy change can be determined from standard enthalpy and entropy values for the reactants and products, which are available in thermodynamic tables.

In addition to the Gibbs free energy change, the spontaneity of a reaction can also be influenced by the reaction conditions, such as temperature and pressure. For example, an endothermic reaction (ΔH > 0) may become spontaneous at high temperatures, because the TΔS term in the Gibbs free energy equation becomes larger and can outweigh the positive ΔH. Similarly, a reaction that results in an increase in the number of gas molecules may become spontaneous at low pressures, because the increase in entropy (disorder) is favoured.

It's also important to note that the spontaneity of a reaction does not indicate the speed of the reaction. A reaction may be spontaneous but occur very slowly, if there is a high energy barrier (activation energy) that must be overcome. The rate of a reaction is determined by the reaction mechanism and can be influenced by factors such as the presence of a catalyst.