Entropy in chemistry
Gibb's free energy
Gibb's energy is interesting because it tell us whether a reaction can occur spontaneously or not.
(spontaneous reactions may need an energy input to get started; for instance, cooking gas burns spontaneously but a spark is needed to get it started - The spark supplies the activation energy for the raction , which is studied in CHEMICAL KINETICS)
At constant temperature and pressure, chemical reactions are spontaneous in the direction of decreasing Gibb's energy.
In other words, when ΔG < 0.
The formula for Gibb's energy is: ΔG = ΔH - TΔS
The Greek letter Δ represents variation and it is much used in physics and chemistry.
ΔH means Hf - Hi , which is final enthalpy (of products) minus initial enthalpy (of reagents).
Likewise for ΔG and ΔS (variation of entropy).
For ΔG to be negative, so that we have a spontaneous reaction, it is convenient that ΔH is also negative and that ΔS is positive.
A negative ΔH indicates an exothermic reaction. It is common sense to suppose that exothermic reactions are spontaneous (like burning butane, for instance).
A positive ΔS indicates an increase in entropy as the reaction proceeds. That is what we would expect, because we know that spontaneous change goes on the direction of increased disorder. Important to note that there is a temperature factor on the entropy term.
For instance, reactions that produce gases out of solids (like fire) are favoured because gases have much higher entropy than solids.
If a chemical reaction is entropy driven, e.g., it occurs because it provides a large increase in entropy and it is not exothermic, it will be more favourable at higher temperatures.
A typical example of such process is dissolving sodium chloride in water. It is not exothermic (we don't see heat being produced) but it is definitely increasing entropy, because as the salt dissolves, the sodium and the chloride ions get dispersed in the water. They were initially close together and they end up all over the place. We would expect this process to be favoured by higher temperature, according to the ΔG equation. And that is exactly what is observed: as the temperature of the solvent (water)increases, the slat is dissolved more easily and quickly!
© Ricardo Esplugas de Oliveira - ricardochemistry@gmail.com