Faithful viewer, One Day at a Time, asked how I knew that the the combustion of wax resulted in carbon dioxide and water vapor. Well, there are two things to this problem: stoichiometry and thermodynamics. Let’s tackle the thermodynamics first.
Every molecule has a certain amount of energy that it took to make it from its atomic parts. This energy is called the “Gibbs Free Energy” or just G for short. There are published tables that show the Gibbs Free Energy (G) for different molecules. For example, here is a short list of G values for various molecules:
| Molecule | G (kJ/mole) |
| CH4 (methane) | -50.8 |
| O2 (oxygen) | 0 |
| CO2 (carbon dioxide) | -394.4 |
| H2O (water) | -237.2 |
Delta G is the term used to describe how much energy is released durnig a chemical reaction. Delta G is calculated from the sum of G for the reactants minus the sum of G for products.
So for this reaction:
We can look up the G values for all four molecules involved. The left hand side has G of -50.8 kJ/mole. The right hand side has G of -868.8 kJ/mole. The difference (delta G) is -818.0 kJ/mole. A negative number like that indicates that the reaction produces a lot of energy. If you were to try running the reaction backwards (ie. to make wax out of water vapor and carbon dioxide), you would find that you had very little success. This is because you would have to add 818 kJ of energy per mole of reactants. Never mind what a kJ is or what a mole is. Suffice to say that that requires a lot of energy for not much stuff.
So in summary, by using thermodynamics and Gibbs Free Energy, we were able to discover that the chemical reaction of turning wax plus oxygen into water and carbon dioxide produces a lot of energy. This is useful, because if it doesn’t produce energy (e.g. has a postive delta G) then the reaction won’t happen.
It’s also import to make sure we don’t create or destroy matter. In other words, there must be the same number and kind of atoms on both sides of a chemical reaction. That is what they mean by stoichiometry. Thus “methane plus oxygen makes water and carbon dioxide” must be balanced. You’ll notice that in the above equation, I added a “2” in front of the water molecule product. That’s because for every molecule of methane that burns, two molecules of water are produced. Also, you’ll notice that I had to put a 2 in front of the oxygen as well. In that form, every molecule is accounted for. Of course, many more molecules are involved when burning an actual candle (on the order of 602,300,000,000,000,000,000,000 molecules), but the ratio ( one methane : two oxygen : one carbon dioxide : two water ) is conserved.
Now quiz yourself. Here are some chemical reactions that aren’t stochiometrically balanced. Try to figure out what terms to add to make the reaction balanced. (click and drag to select and see the hidden answer.)
1. H2 + O2 –> H2O clickndrag—> Answer: 2 H2 + O2 –> 2 H2O —>tohere
2. CH3CH3 + O2 –> CO2 + H2O clickndrag—> Answer: 2 CH3CH3 + 7 O2 –> 4 CO2 + 6 H2O —>tohere
One reply on “Delta G”
the scary part of all this is that somewhere in my brain all of these big words have been stored for long peroids of time and it actually all makes sense once you wake them up form their hibernation……………