Which of the following reaction defines Δ Hf° ?

Question

Which of the following reaction defines Δ Hf° ? (A)  C text(diamond)+O2(g)arrow CO2(g)  (B)  (1/2)H2(g)+(1/2)F2(g)arrow HF(g)  (C)  N2(g)+3H2(g)arrow 2NH3(g)  (D) CO (g)+(1/2) O 2(g) longrightarrow CO 2(g)

Tardigrade Admin · Accepted Answer

Δ Hf° is the enthalpy change when 1 mol of the substance is formed from its elements in the standard state. Reaction does not represent Δ Hf° because standard state of carbon is graphite and not diamond.

Q. Which of the following reaction defines $Δ H_{f}^{\circ}$ ?

$C_{(diamond)} + O_{2} (g) \to C O_{2} (g)$

$\frac{1}{2} H_{2} (g) + \frac{1}{2} F_{2} (g) \to H F (g)$

$N_{2} (g) + 3 H_{2} (g) \to 2 N H_{3} (g)$

$CO (g) + \frac{1}{2} O_{2} (g) ⟶ C O_{2} (g)$

$Δ H_{f}^{\circ}$ is the enthalpy change when $1 m o l$ of the substance is formed from its elements in the standard state. Reaction does not represent $Δ H_{f}^{\circ}$ because standard state of carbon is graphite and not diamond.

Q. Which of the following reaction defines ΔHf∘​ ?

C(diamond)​+O2​(g)→CO2​(g)

21​H2​(g)+21​F2​(g)→HF(g)

N2​(g)+3H2​(g)→2NH3​(g)

CO(g)+21​O2​(g)⟶CO2​(g)