What is Entropy, and it's Importance
Spontaneous change is a change that will occur naturally, without external intervention. In relation to chemical reactions, reactions that occur naturally will often relase energy, as so are exothermic. This makes sense since energy is released as most stable products are formed. However, some spontaneous reactions are endothermic, so there must be another factor we need to consider.
This factor is entropy. Entropy is the degree of disorder there is in a system. If there is more disorder in a system, there will be a higher value of entropy (S). The disorder of a system is very closely linked to the phase it is in.
if you take the above example of a jar of bromine and next to an empty jar; when the divider between the two of them is removed, the bromine will fill both jars evenly. But why doesn't it fill the other entirely? It is because there are many more probabilities of there being an even distribution than not, ie. it will spontaneously diffuse evenly.
Calculating Entropy Change
Entropy is affected by temperature. This is shown in the diagram below. Entropy increases gradually with temperature, but then there is a sudden jump when the phase changes, and the biggest jump occurs from liquid to gas. This is because there are more ways of arranging a gas than a liquid. Ions and molecules in solution also have higher entropy.
Calculating entropy change simply requires you to use given entropy values for particular substances and use the following equation. They have the units J K-1 mol-1. It is possible to have absolute entropy since at 0 Kelvin, there is no entropy.
Feasibility of Reactions
As discussed above, reactions that are exothermic happen quite easily, and reactions that are endothermic but result in an increase in disorder are also likely to happen. Therefore it is possible to calculate the feasability (how likely something is) of a reaction by calculating the Gibbs free-energy change (ΔG).
A reaction is said to be feasible if ΔG is negative.
For example: Determine whether the following reaction is feasible at 298K given that it has an enthalpy change of -825 kJ mol-1 and an entropy change of -272 J K-1 mol-1.
2Fe (s) + 1.5O2 (g) ® Fe2O3 (s)
ΔG = ΔH - TΔS
ΔG = -825 - (298 x -272)/1000
ΔG = -744 kJ mol-1
Since ΔG is negative, the reaction is feasible, and the reaction will occur. Notice also how it was necessary to divide the ΔS by 1000 to convert it from J to kJ, alternatively you could multiply ΔH by 1000 to convert this from kJ to J.