Which of the following reaction defines ΔHf^o ? (a) C (diamond) +O2( g) ⟶CO2( g) (b) (1)/(2) H2(

step 1 Here in this question we are asked that out of the following reactions given, which reactions de

Which of the following reaction defines ΔHf^o ? (a) C...

Which of the following reaction defines $\Delta \mathrm{H}_{\mathrm{f}}^{\mathrm{o}}$ ? (a) $\mathrm{C}$ (diamond) $+\mathrm{O}_{2}(\mathrm{~g}) \longrightarrow \mathrm{CO}_{2}(\mathrm{~g})$ (b) $\frac{1}{2} \mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{~F}_{2}(\mathrm{~g}) \longrightarrow \mathrm{HF}(\mathrm{g})$ (c) $\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{NH}_{3}(\mathrm{~g})$ (d) $\mathrm{CO}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{~g}) \longrightarrow \mathrm{CO}_{2}(\mathrm{~g})$

Which of the following reaction defines $\Delta \mathrm{H}_{\mathrm{f}}^{\mathrm{o}}$ ?
(a) $\mathrm{C}$ (diamond) $+\mathrm{O}_{2}(\mathrm{~g}) \longrightarrow \mathrm{CO}_{2}(\mathrm{~g})$
(b) $\frac{1}{2} \mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{~F}_{2}(\mathrm{~g}) \longrightarrow \mathrm{HF}(\mathrm{g})$
(c) $\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{NH}_{3}(\mathrm{~g})$
(d) $\mathrm{CO}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{~g}) \longrightarrow \mathrm{CO}_{2}(\mathrm{~g})$

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Key Concepts

Standard States

Standard states refer to the most stable physical form of an element or compound under standard conditions. For elements, this means the form in which they naturally exist at 1 atm and 25°C (for example, O?(g) for oxygen and graphite for carbon). Recognizing standard states is key to correctly defining formation reactions.

Balanced Chemical Equations

Balancing a chemical equation ensures that the number of atoms for each element is conserved. In the context of formation reactions, the balanced equation must reflect that one mole of the product is formed from its elements in their standard states.

Standard Enthalpy of Formation

This is the enthalpy change when one mole of a compound is formed from its constituent elements, with all substances in their most stable physical forms (standard states) under standard conditions (usually 1 atm pressure and 25°C). It is a central concept in thermochemistry and is used to calculate the energy changes for reactions.

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