Gibbs Energy Change — Definition

Definition

Imagine you want to know if a chemical reaction or a physical change will happen on its own, without any external push. This 'happening on its own' is what we call spontaneity. For a long time, scientists thought that reactions that release heat (exothermic reactions, where enthalpy change $\Delta H$ is negative) were always spontaneous.

Then, they realized that reactions which increase disorder (increase in entropy $\Delta S$) could also be spontaneous, even if they absorbed heat. So, how do we combine these two factors – heat change and disorder change – to get a single, reliable predictor for spontaneity?

That's where Gibbs energy change, or $\Delta G$, comes in.

Gibbs energy, named after Josiah Willard Gibbs, is a thermodynamic potential that measures the 'useful' or process-initiating work obtainable from an isothermal, isobaric thermodynamic system. Think of it as the energy available to do work. The change in Gibbs energy, $\Delta G$, tells us whether a process will occur spontaneously at a given constant temperature and pressure. It's like a universal 'go/no-go' signal for reactions.

The core idea is captured by the equation: $\Delta G = \Delta H - T\Delta S$. Let's break this down:

$\Delta H$ (Delta H): This is the change in enthalpy, which is essentially the heat absorbed or released by the system at constant pressure. If $\Delta H$ is negative, the reaction releases heat (exothermic) and tends to be spontaneous. If $\Delta H$ is positive, it absorbs heat (endothermic) and tends to be non-spontaneous.
$T$ (Temperature): This is the absolute temperature in Kelvin. Temperature plays a crucial role because it scales the importance of the entropy term. As temperature increases, the $T\Delta S$ term becomes more significant.
$\Delta S$ (Delta S): This is the change in entropy, which measures the change in disorder or randomness of the system. If $\Delta S$ is positive, the system becomes more disordered, which favors spontaneity. If $\Delta S$ is negative, the system becomes more ordered, which disfavors spontaneity.

Now, how do we interpret $\Delta G$?

If $\Delta G < 0$ (negative): The process is spontaneous. It will happen on its own without external intervention.
If $\Delta G > 0$ (positive): The process is non-spontaneous. It will not happen on its own; you need to put in energy to make it occur.
If $\Delta G = 0$: The system is at equilibrium. There is no net change, and the forward and reverse processes are occurring at equal rates.

So, Gibbs energy change provides a comprehensive criterion for spontaneity, considering both the energy (enthalpy) and the disorder (entropy) aspects of a system, weighted by temperature. It's a powerful tool for predicting the direction of chemical and physical changes.