Exploring Enthalpy, Entropy, and Gibb’s Free Energy
We have already discussed five ways in which we can determine ΔH:
1. Calorimetry: ΔH = qsys/mols of limiting reactant
2. Bond energies: ∆H = ∑ Bond Energies broken − ∑ Bond Energies formed
3. Stoichiometry: Use ΔH as your conversion factor between kJ and moles of reaction
4. Standard heats of formation: ∆H° = ∑ ∆ H°f (products) − ∑ ∆ H°f (reactants)
(NMSI affectionately calls this the “Big Mamma equation”)
5. Hess’s Law: Flip equations, multiply coefficients, add equations, etc.
We can apply method’s 3-5 to entropy, ΔS, and Gibb’s free energy, ΔG, calculations.
For example: the “Big Mamma equation” can be applied to ΔS and ΔG.
∆S = ∑ S°(products) − ∑ S°(reactants)
∆G° = ∑ ∆ G°f (products) − ∑ ∆ G°f (reactants)
Enthalpy, entropy, and Gibb’s free energy are the fundamentals to understanding
thermodynamics. The “Granddaddy Equation” relates all three thermodynamic terms. It is a
very useful tool, not only for calculations, but also predicting thermodynamic favorability of
chemical and physical processes. Learn how to use this “Granddaddy equation”!
ΔG = ΔH – TΔS
A word about units…
Keeping track of units in thermodynamics is very important! Historically, the AP Exam cares a
lot about units in thermodynamics calculations. You will not earn points unless the correct unit
is on your answers.
Thermodynamics units are as follows:
Enthalpy
ΔH
kJ/molrxn
Entropy
ΔS
J/(molrxn • K)
Gibb’s free energy
ΔG
kJ/molrxn
