Consider the following four distinct thermochemical equations:\ (i) $\mathrm{C}_{( ext{graphite})}

Question

Consider the following four distinct thermochemical equations:\ (i) $\mathrm{C}_{(	ext{graphite})} + \mathrm{O}_{2(g)} ightarrow \mathrm{CO}_{2(g)} \quad \Delta H = -X_1$\ (ii) $\mathrm{C}_{(	ext{diamond})} + \mathrm{O}_{2(g)} ightarrow \mathrm{CO}_{2(g)} \quad \Delta H = -X_2$\ (iii) $\mathrm{CO}_{(g)} + \frac{1}{2}\mathrm{O}_{2(g)} ightarrow \mathrm{CO}_{2(g)} \quad \Delta H = -X_3$\ (iv) $\mathrm{CH}_{4(g)} + 2\mathrm{O}_{2(g)} ightarrow \mathrm{CO}_{2(g)} + 2\mathrm{H}_2\mathrm{O}_{(l)} \quad \Delta H = -X_4$ 
Which specific parameter exactly defines the 'Standard Enthalpy of Formation' of Carbon Dioxide ($\Delta_f H^\circ_{	ext{CO}_2}$)?

Professor PC Thomas Classes · Answer

(a) : The 'Standard Enthalpy of Formation' ($\Delta_f H^\circ$) is rigidly defined as the enthalpy change accompanying the formation of exactly one mole of a pure substance directly from its constituent elements, with all substances being strictly in their standard reference states. For carbon, the most stable standard reference state at 1 atm and 298 K is exclusively Graphite, not Diamond. Thus, equation (i) correctly represents this specific process.