Enthalpy, Entropy & Gibbs Free Energy

Gibb's free energy
 Enthalpy  Entropy Gibbs Free Energy
 icon-enthalpy  icon-entropy
Enthalpy is the amount of heat energy transferred (heat absorbed or emitted) in a chemical process under constant pressure. Entropy measures the amount of heat dispersed or transferred during a chemical process. Gibbs Energy is also known as energy available to initiate a chemical process and is determined under constant pressure and temperature.
It is expressed as a change in enthalpy (ΔH) because the total enthalpy (H) of a system cannot be measured directly.  Entropy can be thought of as the degree to which energy is dispersed throughout a system. For example, water has a greater entropy than ice because energy is more spread out in water than in ice. Some reactions are spontaneous (eg. rusting). A spontaneous process happens by itself without any energy added to the system (apart from the activation energy to start the process).


A non-spontaneous process will not take place unless it is continuously driven by an external source of energy.



Gibbs Free Energy (ΔG) is a way to figure out if a process will happen on its own, without any help. If delta G is negative, then the process will happen spontaneously, all by itself. If delta G is positive, then the process won’t happen on its own – it needs some help to get it started and also to keep it going!


Let’s break down the equation:


Where ΔH is the change in the system’s energy, T is the temperature, and ΔS is the change in the system’s entropy.

So, to determine if a process is spontaneous or not using Gibbs Free Energy, you just need to use the values of  ΔH, ΔS, and the temperature, plug them into the equation, and see if delta G is negative or positive. If it’s negative, then the process will happen spontaneously, and if it’s positive, then it will not happen spontaneously.


From the equation, you can see the “fight” or struggle between ΔH, and T*ΔS
  • If the reaction has to absorb a lot of energy (a large positive ΔH for example), and has a very small (T*ΔS) value, then  ΔG will end up being positive.
  • In this case, the process will not be spontaneous.
  • This is very logical if you think of melting ice in a freezing environment.
But if the temperature of the air is increased, at room temperature for instance (298 K or 25ºC)
  • And the value of (T*ΔS) is larger than delta H
  • Then ΔG is negative.
  • And the ice will melt spotaneously without having to put it on a stove to heat it up!



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