Error 1: Confusing $\Delta G$ with $\Delta G^\circ$

Wrong: "At equilibrium, $\Delta G^\circ = 0$" Correct: At equilibrium, $\Delta G = 0$ (not $\Delta G^\circ$). $\Delta G^\circ = -RT\ln K$; it equals zero only when $K = 1$.

Error 2: Including Solids in $\Delta n_g$

Wrong: For $CaCO_{3}$(s) → CaO(s) + $CO_{2}$(g), $\Delta n_g = 2 - 1 = 1$ Correct: $\Delta n_g = 1 - 0 = 1$ (only $CO_{2}$ is gaseous; both solids are excluded)

Error 3: Wrong Sign for Work in Expansion

Wrong: "Work done by gas during expansion is positive" Correct: In IUPAC convention, work done BY the system is negative ($w < 0$ for expansion). Only work done ON the system is positive.

Error 4: Using Wrong Formula for Irreversible vs Reversible

Wrong: Using $w = -nRT\ln(V_2/V_1)$ for an irreversible process Correct: For irreversible against constant $P_{ext}$: use $w = -P_{ext}\Delta V$. The $\ln$ formula applies ONLY to reversible isothermal.

Error 5: Assuming All Endothermic Reactions Are Non-Spontaneous

Wrong: "Since $\Delta H > 0$, the reaction is non-spontaneous" Correct: Spontaneity depends on $\Delta G = \Delta H - T\Delta S$, not just $\Delta H$. Endothermic reactions CAN be spontaneous if $T\Delta S > \Delta H$.

Error 6: Wrong Temperature Units

Wrong: Using $T = 25$ °C in $\Delta G = \Delta H - T\Delta S$ Correct: Always convert to Kelvin: $T(K) = T(°C) + 273$. Using °C gives wrong answers.

Error 7: Wrong Bond Counting in Polyatomic Molecules

Wrong: For $C_{2}H_{6}$, counting 2 C-C bonds and 4 C-H bonds Correct: $C_{2}H_{6}$ (ethane) has 1 C-C bond and 6 C-H bonds. Draw the structure: $CH_{3}$-$CH_{3}$.

Error 8: Forgetting Standard State of Carbon

Wrong: $\Delta H_f^\circ(C, \text{diamond}) = 0$ Correct: Standard state of C is graphite. $\Delta H_f^\circ(C, \text{graphite}) = 0$; $\Delta H_f^\circ(C, \text{diamond}) = +1.9$ kJ/mol.