“Negative means it goes”
Reading ΔG, ΔH and ΔS
Negative ΔG means spontaneous. Negative ΔH means heat released. Positive ΔS means more disorder. Only ΔG decides whether a reaction happens.
Chemistry, Class 11 and 12 · Gujarat Board, CBSE, GUJCET, JEE and NEET
Chemical Thermodynamics formulas
Thermodynamics in chemistry answers one question: will this reaction go by itself? Enthalpy, entropy and Gibbs energy together give the answer, and the sign conventions are the whole difficulty.
17 formulas, 3 memory hooks, and the 5 mistakes that cost the most marks.
First law and enthalpy
| What | Formula | Symbols and notes |
|---|---|---|
| First law | ΔU = q + w | q heat added to the system, w work done on the system |
| Work of expansion | w = −pextΔV | negative when the gas expands |
| Enthalpy | H = U + pV, ΔH = ΔU + ΔngRT | Δng is the change in moles of gas |
| At constant volume | qv = ΔU | no expansion work is done |
| At constant pressure | qp = ΔH | the usual laboratory condition |
| Heat capacity relation | Cp − Cv = R | per mole, for an ideal gas |
Thermochemistry
| What | Formula | Symbols and notes |
|---|---|---|
| Hess's law | ΔH is the same by any route | so thermochemical equations can be added |
| From formation enthalpies | ΔH = ΣΔHf(products) − ΣΔHf(reactants) | elements in their standard state have ΔHf = 0 |
| From bond enthalpies | ΔH = Σ(bonds broken) − Σ(bonds formed) | breaking costs energy, forming releases it |
| Exothermic | ΔH negative | heat is released, the beaker warms |
| Endothermic | ΔH positive | heat is absorbed, the beaker cools |
Entropy and Gibbs energy
| What | Formula | Symbols and notes |
|---|---|---|
| Entropy change | ΔS = qrev/T | unit J K−1 mol−1 |
| Gibbs energy | ΔG = ΔH − TΔS | the test for spontaneity |
| Spontaneous | ΔG < 0 | at the given temperature |
| At equilibrium | ΔG = 0 | the forward and reverse tendencies balance |
| Gibbs energy and K | ΔG° = −2.303 RT log K | links thermodynamics to equilibrium |
| Second law | the entropy of the universe always increases | for any spontaneous change |
Taught by Darshit Bhatt, founder and lead faculty at Darshit Sir Institute. Instrumentation and Control engineer, with a master's degree and a chartered degree, teaching in Paldi, Ahmedabad since 2006.
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Memory hooks
How to remember this chapter.
“Both negative, cold helps; both positive, heat helps”
When temperature decides
If ΔH and ΔS are both negative, low temperature makes ΔG negative. If both are positive, high temperature does. If signs differ, temperature cannot change the answer.
“Break costs, make pays”
Bond enthalpy calculations
Breaking bonds always absorbs energy and forming them always releases it. Bonds broken minus bonds formed gives ΔH, in that order.
Where marks go
The mistakes that cost the most.
- Mixing sign conventions for work. In chemistry w is the work done on the system, which is the opposite of the physics convention.
- Using ΔH = ΔU without the ΔngRT term when the number of moles of gas changes.
- Deciding spontaneity from ΔH alone. Endothermic reactions can be spontaneous if ΔS is large enough.
- Using °C in ΔG = ΔH − TΔS. It needs kelvin.
- Forgetting that ΔH is usually in kilojoules and ΔS in joules per kelvin, so one must be converted.
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