Chapter 100 of 2363 min read

First law, enthalpy and Hess's law

Why this is asked: NCERT's sign convention is ΔU = q + w with w the work done ON the system — mix that up with the older "w = work done BY the system" convention and every work term flips sign. For gas-phase reactions, ΔH and ΔU differ by Δn(gas)×RT, a correction it's easy to forget even when Δn(gas) isn't zero.

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Test yourself

•The standard heat of formation, in kcal/mol, of $B a^{2 +}$ is: [Given: standard heat of formation of the $S O_{4}^{2 -}$ ion (aq) = –216 kcal/mol, standard heat of crystallisation of $BaS O_{4} (s)$ = –4.5 kcal/mol, standard heat of formation of $BaS O_{4} (s)$ = –349 kcal/mol]
• $C (s) + 2 H_{2} (g) \to C H_{4} (g)$ ; $Δ H = - 74.8 kJ mo l^{- 1}$ . Which of the following diagrams (shown) gives an accurate representation of the above reaction? [R → reactants; P → products]

•Match List I with List II.

List I (Process)	List II (Condition)
A. Isothermal process	I. No heat exchange
B. Isochoric process	II. Carried out at constant temperature
C. Isobaric process	III. Carried out at constant volume
D. Adiabatic process	IV. Carried out at constant pressure

Choose the correct answer from the options given below:

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