There are three variables in determining whether or not a reaction will happen: enthalpy, entropy, and temperature. Enthalpy can be roughly described as how much heat the reaction uses or gives off. A positive enthalpy means that the reaction draws in heat from its surroundings, and a negative enthalpy means that the reaction gives off heat to its surroundings. Entropy is the number of possible arrangements of the system or the potential of the system to do work, and temperature is just temperature (in Kelvin, though).
These three variables are related in the Gibbs free energy equation: Gibbs free energy = Enthalpy - (Entropy * Temperature). If Gibbs energy is negative, the reaction is spontaneous, and will happen by itself. If Gibbs energy is positive, the reaction is non-spontaneous, and work will have to be done in order to make the reaction happen.
The thing that interested me most about this equation is how it explains so much about what goes on in the universe. Consider ice melting-- it has to take in heat in order to turn to liquid. In other words, the enthalpy of ice melting is positive. If entropy were out of the picture, Gibbs energy would always be positive, and ice would never melt by itself. The entropy of this reaction, however, is negative (which is favorable for a spontaneous reaction). Liquid water has more possible arrangements than solid water. Taking another look at the Gibbs free energy equation, you can see that the effect entropy has on a reaction depends on temperature. If the temperature is 0 K, entropy * temperature would also be zero, and the ice wouldn't melt.
Now take a random guess at the temperature at which entropy * temperature cancels out enthalpy, and Gibbs energy becomes negative. It is, of course, at 293 K, or 0 degrees Celcius, that the reaction of water melting becomes a spontaneous reaction. There's still a lot I don't know, but entropy is becoming one of my favorite scientific concepts.
No comments:
Post a Comment