Laws of Thermodynamics — Chemistry Mnemonic
Target: mnemonic for laws of thermodynamics chemistry
Why is this hard to memorize?
Thermodynamics governs energy changes in chemical reactions. NEET and JEE test three laws and related concepts: First Law (energy is conserved, ΔU = q + w), Second Law (entropy of universe always increases), and Third Law (entropy approaches zero at absolute zero). You also need Hess's Law (enthalpy is a state function — path doesn't matter), Gibbs free energy (ΔG = ΔH − TΔS), and the sign conventions for exothermic/endothermic processes. The ability to predict spontaneity using ΔG is perhaps the most frequently tested concept.
Classic mnemonics you should know
"ΔG < 0 = Spontaneous. ΔG > 0 = Non-spontaneous. ΔG = 0 = Equilibrium."
ΔG = ΔH − TΔS. If ΔG is negative → reaction goes forward spontaneously. Positive → non-spontaneous. Zero → at equilibrium. This single equation predicts spontaneity for ALL reactions at constant T and P.
"EXO = Exit heat = ΔH negative(−). ENDO = Enter heat = ΔH positive(+)."
Exothermic: heat exits the system → ΔH < 0 (negative). Endothermic: heat enters → ΔH > 0 (positive). "EXO = EXIT = negative, ENDO = ENTER = positive."
"H(−)S(+)=Always spon. H(+)S(−)=Never spon. H(−)S(−)=Low T spon. H(+)S(+)=High T spon."
ΔG = ΔH − TΔS: (1) ΔH<0, ΔS>0 → ΔG always negative. (2) ΔH>0, ΔS<0 → ΔG always positive. (3) ΔH<0, ΔS<0 → favourable at low T. (4) ΔH>0, ΔS>0 → favourable at high T.
The complete list
- First Law: ΔU = q + w
- Second Law: ΔS(universe) > 0
- Third Law: S → 0 as T → 0 K
- ΔG = ΔH − TΔS
- ΔG < 0 → spontaneous
- Exothermic: ΔH < 0
- Endothermic: ΔH > 0
- Hess's Law: ΔH is path-independent
Frequently asked questions
How do I predict if a reaction is spontaneous?
Calculate ΔG = ΔH − TΔS. If ΔG < 0, the reaction is spontaneous at that temperature. If both ΔH is negative AND ΔS is positive, the reaction is ALWAYS spontaneous at every temperature. If ΔH and ΔS have the same sign, spontaneity depends on temperature.
What is the difference between ΔH and ΔU?
ΔH (enthalpy change) = ΔU (internal energy change) + PΔV (work done by expansion). For reactions with no gas change or in solution, ΔH ≈ ΔU. For gas-phase reactions, ΔH = ΔU + ΔngRT, where Δng = moles of gaseous products − moles of gaseous reactants.
How do I apply Hess's Law?
Hess's Law says enthalpy change is the same regardless of the pathway. To find ΔH for a target reaction: write it as a sum of known reactions (flip reactions to change sign, multiply to scale). ΔH(target) = sum of ΔH values of the component reactions. This is essentially thermodynamic algebra.
Explore more Chemistry mnemonics
We have 100+ memory tricks for Chemistry — periodic table groups, reactions, exceptions, and more.
Browse all Chemistry mnemonics