Chem – Spontaneous (ΔG)

What is a spontaneous process or what is spontaneity?

Spontaneous has to do with completing a process without any outside help. For purposes of chemistry we will be talking about the chemical reaction (chemical equation) when we talk about the process. So, what is meant by outside help? The best and easiest way to think about outside help, in terms of chemistry, is that outside help is any form of energy put into the process. In other words, if I am cooking an egg I am putting energy into the process of transforming the egg from liquid to solid. This process would not happen without the help of the energy (heat) from the stove. Therefore, it is not a spontaneous process. What is an example of a spontaneous process? Well, if you use hydrogen peroxide (H2O2(l)) to clean your wounds you know that it will spontaneously bubble. The bubbles are oxygen (O2(g)) and it also breaks down into water (H2O(l)). This is a spontaneous process because no energy is required (although energy will help the process move faster).


How do we represent spontaneous or spontaneity in chemistry?

The symbol for the spontaneous or spontaneity is G. The G comes from the fact that it is called Gibbs Free Energy. It is a very deceiving name so don’t worry about trying to figure out why they call it Gibbs Free Energy. The change in Gibbs Free Energy is represented by Δ G. When Δ G is a negative number the process is spontaneous. When Δ G is a positive number the process is not spontaneous or non-spontaneous. We can demonstrate this by putting the examples we just discussed of the egg and hydrogen peroxide into chemical equations and demonstrating how they may be shown in a chemistry test or book.


2 H2O2(l) ——> O2(g) + 2 H2O(l)  Δ G = – 300 kJ/mol

The above chemical reaction can be said to have a negative Δ G or is spontaneous. All reactions with a negative Δ G are SPONTANEOUS.


Egg(l) ——> Egg(s)  Δ G = +4500 J

The above chemical reaction can be said to have a positive Δ G or is non-spontaneous. All reactions with a positive Δ G are NON-SPONTANEOUS.


This demonstration also tells us the units of G. They are usually in Joules (J). However, because chemistry likes to account for everything per mole you can also see the units of G as J/mol or kJ/mol.


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