Enthalpy — ΔH, ΔU, relation between them
Thermodynamics & Thermochemistry
9
JEE Qs
8%
Hard
60
min
Master the conditions and correct application of the ΔH = ΔU + Δn_g RT relation, paying close attention to phases and units.
🧮 Key Formulas
✅ Key Points for JEE
- 1Enthalpy change (ΔH) represents the heat exchanged by a system at constant pressure (q_p), while change in internal energy (ΔU) represents heat exchanged at constant volume (q_v).
- 2The relation ΔH = ΔU + Δn_g RT is crucial for gaseous reactions where Δn_g is the change in moles of gaseous products minus gaseous reactants; remember to consider only gaseous species for Δn_g.
- 3For reactions involving only solids and liquids, or when the number of moles of gaseous reactants equals products (Δn_g = 0), ΔH is approximately equal to ΔU because the PΔV term is negligible or zero.
- 4Exothermic reactions (heat released) have negative ΔH and ΔU values, while endothermic reactions (heat absorbed) have positive ΔH and ΔU values.
⚠️ Common Mistakes
- ✕Incorrectly calculating Δn_g by including moles of liquid or solid reactants/products instead of only gases.
- ✕Using inconsistent units for R (e.g., L.atm/mol.K) with ΔH/ΔU values given in Joules or kJ, or failing to convert temperature to Kelvin.
- ✕Confusing the conditions for ΔH (constant pressure) and ΔU (constant volume) and interchanging q_p and q_v without considering work.
- ✕Incorrectly applying sign conventions for heat (q) and work (w) in ΔU = q + w, leading to errors in ΔH and ΔU calculations.
📝 Practice Questions
See allQ32.Given are statements for certain thermodynamic variables, (A) Internal energy, volume (V) and mass (M) are extensive variables. (B) Pressure (P), temperature (T) and density ( ρ ) are intensive variables. (C) Volume (V), temperature (T) and density ( ρ ) are intensive variables. (D) Mass (M), temperature (T) and internal energy are extensive variables. Choose the correct answer from the options given below : (1) (B) and (C) Only (2) (C) and (D) Only (3) (D) and (A) Only (4) (A) and (B) Only
Q64.Ice at −5∘C is heated to become vapor with temperature of 110∘C at atmospheric pressure. The entropy change associated with this process can be obtained from 2025 (23 Jan Shift 1) JEE Main Previous Year Paper (1) (2) (3) (4)
Q53.A liquid when kept inside a thermally insulated closed vessel at 25∘C was mechanically stirred from outside. What will be the correct option for the following thermodynamic parameters ? (1) ΔU < 0, q = 0, w > 0 (2) ΔU = 0, q = 0, w = 0 (3) ΔU > 0, q = 0, w > 0 (4) ΔU = 0, q < 0, w > 0
Q31.Water of mass m gram is slowly heated to increase the temperature from T1 to Tz The change in entropy of the water, given specific heat of water is 1Jkg−1 K−1 , is : (1) m ln ( T2T1 ) (2) zero (3) m ln ( T1T2 ) (4) m (T2 −T1)
Q74.Niobium ( Nb ) and ruthenium (Ru) have " x " and " y " number of electrons in their respective 4 d orbitals. The value of x + y is ______ -.
Q70.The correct stability order of the following species/molecules is: (1) q > r > p (2) r > q > p (3) q > p > r (4) p > q > r Q71. 1 The standard enthalpy and standard entropy of decomposition of N2O4 to NO2 are 55.0 kJ mol−1 and 175.0 J/K/mol respectively. The standard free energy change for this reaction at 25∘C in J mol−1 is ______ (Nearest integer)
NCERT Chapters
- Class 11 Chemistry Ch 6: Thermodynamics