NEET · Chemistry

Chemical Thermodynamics for NEET: Delta U, Enthalpy, Entropy and Gibbs Energy

Own official Unit 4 with explicit chemistry sign conventions: system/surroundings, state functions, internal energy, heat/work, enthalpy, heat capacity, Hess law and enthalpy changes, entropy, Gibbs energy, spontaneity and the standard Gibbs/equilibrium relation.

Subject: Chemistry
Syllabus unit: Current official NEET UG 2026 scope

In short

Thermodynamics tracks energy and state changes. With the chemistry convention used here, Delta U = q + w, where work done on the system is positive; pressure-volume expansion against an external pressure has w = -P_ext Delta V. At constant temperature and pressure, Gibbs energy provides the spontaneity criterion: negative Delta G favours spontaneous change in the stated direction, and equilibrium has Delta G = 0.

Syllabus mapping

Syllabus mapping
Unit	Topics
Current official NEET UG 2026 scope	State and path quantities, First law, Enthalpy, Entropy and Gibbs energy, Equilibrium connection

Unit
Current official NEET UG 2026 scope
Topics
State and path quantities, First law, Enthalpy, Entropy and Gibbs energy, Equilibrium connection

Before this chapter

Concepts in this chapter

1. State and path quantities

Internal energy U, enthalpy H, entropy S and Gibbs energy G are state functions: their changes depend only on initial and final equilibrium states. Heat q and work w describe energy transfer along a process path and are not state functions.

2. First law

Using the chemistry convention, Delta U = q + w: heat absorbed by the system is positive, and work done on the system is positive. For pressure-volume expansion against a constant external pressure, w = -P_ext Delta V; expansion gives positive Delta V and therefore negative work.

3. Enthalpy

H = U + PV. At constant pressure with only PV work in the standard treatment, the heat exchanged is related to Delta H. Heat capacity must be tied to the specified condition or quantity basis. Hess's law follows from enthalpy being a state function: enthalpy changes for reaction steps can be added algebraically when equations are added, with reaction reversal changing the sign and stoichiometric multiplication scaling Delta H.

4. Entropy and Gibbs energy

Entropy is a thermodynamic state function used with the second law to describe directionality. Gibbs energy is G = H - TS, so at constant T for a state change, Delta G = Delta H - T Delta S when the quantities are applied consistently. At constant T and P, Delta G < 0 indicates spontaneous change in the written direction, Delta G > 0 the reverse direction is favoured, and equilibrium has Delta G = 0.

5. Equilibrium connection

For standard states, Delta G° = -RT ln K. A large K corresponds to a more negative standard Gibbs energy for the reaction as written. Do not confuse Delta G under the current composition with Delta G° under standard-state reference conditions.

6. Decision aids for this chapter

Thermodynamic Sign Ledger: heat direction, volume change, work sign and resulting Delta U shown before arithmetic.
Hess Equation Transformer: reverse/multiply reaction -> transform Delta H -> add steps -> verify final equation.
Spontaneity Matrix: signs of Delta H/Delta S plus temperature dependence, without turning qualitative cases into absolute predictions when values matter.

Formula sheet

Delta U = q + w: energy, usually J or kJ. Chemistry sign convention stated
Delta U = q + w
Use when — Chemistry sign convention stated
w = -P_ext Delta V: P x V energy units. Constant external pressure PV work
w = -P_ext Delta V
Use when — Constant external pressure PV work
H = U + PV: energy. Definition
H = U + PV
Use when — Definition
Delta H via Hess law: kJ mol^-1 reaction as written. Same thermodynamic states; equations added consistently
Delta H via Hess law
Use when — Same thermodynamic states; equations added consistently
Delta G = Delta H - T Delta S: energy per amount; T K. Same T and compatible units
Delta G = Delta H - T Delta S
Use when — Same T and compatible units
Delta G° = -RT ln K: R and Delta G units compatible. Standard reaction, thermodynamic K
Delta G° = -RT ln K
Use when — Standard reaction, thermodynamic K

Worked examples

A system absorbs 500 J of heat and expands by 2.00 L against a constant external pressure of 1.00 bar. Using the chemistry convention and 1 L bar = 100 J, calculate work and Delta U.

Original NEET-style item: A system absorbs 500 J of heat and expands by 2.00 L against a constant external pressure of 1.00 bar. Using the chemistry convention and 1 L bar = 100 J, calculate work and Delta U.

Reasoning: Expansion gives w = -P_ext Delta V = -(1.00)(2.00) L bar = -200 J. Therefore Delta U = q+w = 500-200 = +300 J.

Common mistakes and what they actually indicate

Cannot separate state function from path function
Knowledge gap
Why it happens
Preparation Intelligence classification: Knowledge Gap.
How it is corrected
Classify U/H/S/G vs q/w.
Forgets chemistry work sign
Recall gap
Why it happens
Preparation Intelligence classification: Recall Gap.
How it is corrected
Retrieve w=-Pext Delta V with expansion sign.
Mixes L bar with J directly
Execution error
Why it happens
Preparation Intelligence classification: Execution Error.
How it is corrected
Convert 1 L bar = 100 J.
Uses Delta G sign criterion without constant T,P context
Decision / selection error
Why it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
State the criterion conditions.
Standard state or phase is unclear
Needs review
Why it happens
Preparation Intelligence classification: Needs Review.
How it is corrected
Do not compare enthalpy/Gibbs values without state definitions.

FAQ