What does it mean when a reaction is spontaneous?

Question

Natalie Anton · Accepted Answer

It just means the reaction is energetically downhill, i.e. energy is released and the system overall becomes more thermodynamically stable as a result.

Spontaneity is a measure of how energetically feasible the reaction is, assuming that it would occur quickly if it can occur. That is, if the kinetics are fast enough for us to observe in our lifetime, AND the reaction is spontaneous, then we can see it happen.

Spontaneity is measured by the change in the Gibbs' free energy, #DeltaG#, the maximum amount of non-pressure/volume work that can be done by the system.

A common equation that is true at constant temperature is:

#DeltaG = DeltaH - TDeltaS#

where #DeltaH# is the change in enthalpy (heat flow at constant pressure) and #DeltaS# is the change in entropy (tendency of energy dispersal) at a certain temperature #T# in #"K"#.

#DeltaH# and #DeltaS# are commonly tabulated at #T = "298.15 K"# and #"1 atm"# as standard enthalpies of formation #DeltaH_f^@# and standard molar entropies #S^@#, and thus, the spontaneity of known reactions at #"298.15 K"# and #"1 atm"# can be (re-)predicted.

If the reaction is at equilibrium but is NOT at #"298.15 K"# and #"1 atm"#, then in general, #DeltaG ne DeltaG^@#, and

#cancel(DeltaG)^(0) = DeltaG^@ + RTlncancel(Q)^(K)#

#DeltaG^@ = -RTlnK#

And thus, at that temperature that is not #"298.15 K"#, the equilibrium constant can be estimated.

Luna Chen · Answer

undefined A reaction that is spontaneous can happen without the need for outside energy, and it will proceed in the direction of reducing free energy to form products from reactants.

Caleb Adams · Answer

undefined A spontaneous reaction is a chemical reaction that occurs naturally without the need for external intervention. It proceeds on its own without requiring energy input from the surroundings. Spontaneous reactions tend to occur in the direction that leads to a decrease in the overall free energy of the system. This means that the products of the reaction have lower free energy than the reactants. In thermodynamic terms, spontaneous reactions have a negative change in Gibbs free energy ($ \Delta G < 0 $).