1.4.2 – Gibbs energy
📌 Entropy changes
- Exothermic reactions cause an increase in entropy of the surroundings
- Thus, we know that entropy has a proportional relationship to the negative change in enthalpy (more exothermic means greater increase in entropy)
- The impact of temperature on entropy is also considered based on the dispersal of molecules/atoms
- Therefore, we can also make the statement that the entropy of the surroundings is inversely proportional to the negative value of the absolute temperature (T)
- The following expression represents these two statements :
ΔS surroundings = (-ΔH)/T
here, the entropy refers to the surroundings and the enthalpy refers to the system
📌 Entropy calculations
- We know that the expression given above gives us the change in entropy of the system. We also know that the total entropy change will be the change in entropy of the surroundings plus the change in entropy of the system
- This is represented by the following :
ΔS TOTAL = ΔS surroundings + ΔS system
This can then be re-written as the following :
ΔS TOTAL = [(-ΔH)/T ] + ΔS system
📌 Gibbs energy
- To know the true feasibility of a reaction, we must find an expression that combines both the enthalpy and entropy
- This is known as ‘Gibbs energy’ which is the measure of the quality of energy available for a reaction
- It is given by the symbol ‘G’
- The following expression represents how the formula for change in Gibbs energy can be derived :
ΔS TOTAL = [(-ΔH)/T ] + ΔS system
We can multiply both sides with T to get :
TΔS TOTAL = [(-ΔH)] + TΔS system
We then multiply either side by -1 to get :
-TΔS TOTAL = ΔH – TΔS system
We then combine the left hand side into one expression for Gibbs (G)
ΔG = Δ H – TΔS system
- When the change in Gibbs energy is negative, the reaction is spontaneous
- If we know the signs for the change in entropy and enthalpy, we can also guess whether the reaction will be spontaneous or not