Question

Standard enthalpy of formation of P₄O₁₀(s) is equal to [$\Delta$H⁰c = Standard enthalpy of combustion] :-

• A. $\Delta$H⁰c (P₄, solid, red)
• B. $\Delta$H⁰c (P₄, solid, black)
• C. $\Delta$H⁰c (P₄, solid, white)
• D. -$$\Delta$H⁰c (P₄, solid, white)

Answer 1

Answer: (C)

$\Delta$H⁰c (P₄, solid, white)

Answer 2

The formation reaction for phosphorus pentoxide is

$$\text{P}_4(\text{white}) + 5\,\text{O}_2(g) \to \text{P}_4\text{O}_{10}(s)$$

Since white phosphorus is the standard state of phosphorus, its standard enthalpy of combustion (ΔH⁰c) when combined with oxygen gives the standard enthalpy of formation of $\text{P}_4\text{O}_{10}$. Thus,

$$\Delta H^\circ_f(\text{P}_4\text{O}_{10}(s)) = \Delta H^\circ_c(\text{P}_4, \text{white})$$