Unsure whether a chemical reaction will be spontaneous (take place without added energy)? Gibbs free energy to the rescue! Learn about the Gibbs free energy change (ΔG) and how it can be used to predict reaction spontaneity.
The Gibbs free energy change (ΔG) and how it’s related to reaction spontaneity and equilibrium.
When you hear the term “free energy,” what do you think of?
Well, if you’re goofy like me, maybe a gas station giving away gas. Or, better yet, solar panels being used to power a household for free.
There’s even a rock band from Philadelphia called Free Energy (confirming my longtime suspicion that many biology terms would make excellent names for rock bands).
These are not, however, the meanings of “free energy” that we’ll be discussing in this article.
Instead, we’re going to look at the type of free energy that is associated with a particular chemical reaction, and which can provide a measure of how much usable energy is released (or consumed) when that reaction takes place.
A process will only happen spontaneously, without added energy, if it increases the entropy of the universe as a whole (or, in the limit of a reversible process, leaves it unchanged) – this is the Second Law of Thermodynamics.
But to me at least, that’s kind of an abstract idea. How can we make this idea more concrete and use it to figure out if a chemical reaction will take place?
Basically, we need some kind of metric that captures the effect of a reaction on the entropy of the universe, including both the reaction system and its surroundings. Conveniently, both of these factors are rolled into one convenient value called the Gibbs free energy.
The Gibbs free energy (G) of a system is a measure of the amount of usable energy (energy that can do work) in that system.
The change in Gibbs free energy during a reaction provides useful information about the reaction’s energetics and spontaneity (whether it can happen without added energy).
We can write out a simple definition of the change in Gibbs free energy as:
ΔG=Gfinal–Ginitial
In other words, ΔG is the change in free energy of a system as it goes from some initial state, such as all reactants, to some other, final state, such as all products. This value tells us the maximum usable energy released (or absorbed) in going from the initial to the final state. In addition, its sign (positive or negative) tells us whether a reaction will occur spontaneously, that is, without added energy.
When we work with Gibbs free energy, we have to make some assumptions, such as constant temperature and pressure; however, these conditions hold roughly true for cells and other living systems.