The difference in overall energy between the reactants and products can determine the favorability of a reaction. And we're going to say overall energy is represented by the symbol ΔE, and it equals products minus reactants. We're going to say connected to this idea of overall energy, we can talk about enthalpy and Gibbs free energy specifically. Enthalpy, which is ΔH, is when the overall energy is classified as thermal energy. Because when we say overall energy, it could be nuclear, it could be solar, it could be cosmic, it could be all types of energy. When we say enthalpy, we're talking specifically about thermal energy, energy dealing with heat.
Now Gibbs free energy is ΔG. This is when the overall energy is connected to the favorability of product formation. If you're able to make products in your chemical reaction, that's a good thing. That means the chemical reaction is favorable. That's the whole point of chemical reactions, to change reactants into products.
Now if we take a look here at our energy diagrams, we're going to say that ΔE, remember we said, is products minus reactants. Here's your product line at about 15 and your reactant line at about 40, so that's ΔE = -25 kilojoules. Now remember, ΔE is the umbrella term, but this side here could easily represent Gibbs free energy, or it could say enthalpy, it doesn't matter. They are all types of energy. This would also be -25 for ΔH and -25 for ΔG when discussing thermal or Gibbs free energy. When ΔH is negative, it is exothermic, which means energy releasing. When ΔG is negative, it's exergonic. This is favorable in product forming because your products have lower energy at 15 kilojoules. In chemistry, lower energy means greater stability. That's a good thing.
Going to the other side, products minus reactants, it looks here that my product line is around 50, my reactant line is around 30, so that's a positive 20. This would also be +20 for ΔH if the energy here was specifically thermal energy and +20 for ΔG if this was Gibbs free energy. When it's positive, it's endothermic for ΔH, which means energy absorbing. The reactants absorb energy, and that's why my products at the end have more energy. Here, if ΔG is positive, it's called endergonic. This is not favorable. You don't want to end with products that have more energy than your reactants did. More energy is less stable. So, keep this in mind when we talk about overall energy. That's an umbrella term, and then when we want to be more specific, we can go into enthalpy or Gibbs free energy.