Chemistry and Physics
Predicting the Future
So far we've been talking about thermodynamics the way that physicists and engineers do. And rightfully so. They did all the heavy lifting that gave birth to the field of thermodynamics. But there is a whole other world of thermodynamics: the chemist's thermodynamics.
In the late 1800s, an American scientist by the name of Willard Gibbs realized that the laws of thermodynamics not only applied to heat engines and helium balloons, but also to chemical reactions.
His insight went even further than that, though. He hypothesized that the laws of thermodynamics could predict the future—they could be used to determine whether a chemical reaction would happen or not. As you can imagine, chemists thought (and think) this is Kind of a Big Deal.
The First Law of Thermodynamics remains the same in chemical thermodynamics; the total energy of the universe stays the same, regardless of what happens. Y'know, energy is conserved. The energy might leave whatever system we're looking at, but not the universe. Though that would be undeniably cool.
The gist of the Second Law remains the same, but it's worded a little differently. It says that all spontaneous processes increase the total entropy, or disorder, in the universe.
Physicists said that heat will always flow from a warm object to a cold object, not the other way around. That's because giving a cold object some heat increases its disorder, or entropy. Kind of like giving a kid a bunch of Legos also increases the amount of disorder in the area. A warmer object's molecules move about more quickly than a colder object's molecules, so the warm object's molecules are in greater disorder.
Because of the chemical reactions they observed, Gibbs and his students added two new ideas to our understanding of entropy: enthalpy and Gibbs' free energy. Don't worry, they're not as '70s as they sound.
Chemical reactions involve the breaking and forming of bonds between atoms. These reactions either take up heat or give off heat. Enthalpy, H, is a measure of the change in heat when a chemical reaction occurs. It tells whether the reaction is endothermic (and so requires energy or heat input), or whether it is exothermic (and gives off energy in the form of heat).
Whether chemical reactions are spontaneous (like a pile of fireworks sitting in the middle of a forest fire) or not spontaneous (like those same fireworks sitting in your freezer) depends on both the change in entropy as the reaction occurs, and the change in enthalpy. In other words, it depends both on whether disorder is increased or not and on whether the reaction requires energy or releases energy. Gibbs' free energy, ΔG, combines the change in entropy and the enthalpy of a reaction, and tells whether or not this reaction will occur spontaneously. It's a chemical soothsayer.
If ΔG is negative, the reaction will occur spontaneously without any energy added. If ΔG is positive, the reaction is not spontaneous because it requires additional energy, and if ΔG is zero, the reaction is in equilibrium, neither giving off nor taking in energy. It's zen.