Which of the following reactions is a standard formation reaction (i.e.
Δ
𝐻
𝑟
𝑥
𝑛
∘
=
Δ
𝐻
𝑓
∘
) at T = 298 K?

Question

Which of the following reactions is a standard formation reaction (i.e. 
Δ
𝐻
𝑟
𝑥
𝑛
∘
=
Δ
𝐻
𝑓
∘
) at T = 298 K?

BlackTom AI · Accepted Answer

A standard enthalpy of formation reaction gives exactly one mole of a compound formed from its constituent elements in their standard states (pure forms at 1 bar, 298 K). For Fe2O3(s) the elements are Fe (solid) and O2 (gas, diatomic) in their standard states.

1) Relevant concepts
- Standard formation reaction: produces 1 mole of product from elements in their standard states at 298 K (1 bar).
- Stoichiometry must supply the correct numbers of atoms in the product; oxygen exists as O2 molecules in its standard state.

2) Step-by-step reasoning
- Molecular formula: Fe2O3 contains 2 Fe atoms and 3 O atoms per formula unit.
- We must start from elemental Fe and elemental O in standard states: Fe(s) and O2(g).
- To supply 3 O atoms using O2 molecules: number of O2 molecules = 3 atoms ÷ 2 atoms per O2 = 3/2 = 1.5 O2.
- To supply 2 Fe atoms use 2 Fe atoms (as Fe(s)).
- Combine these to form exactly one formula unit (one mole) of Fe2O3(s).

3) Intermediate calculation steps
- Fe atoms required = 2 → use 2 Fe(s)
- O atoms required = 3 → O2 molecules required = 3/2 = 1.5 O2(g)
- Balanced formation equation: 2 Fe(s) + 3/2 O2(g) → 1 Fe2O3(s)

4) Why this is correct
- The reactants are pure elements in their standard states (Fe solid, O2 gas).
- The reaction produces exactly one mole of the compound Fe2O3(s).
- Therefore it meets the definition of a standard formation reaction (ΔHrxn° = ΔHf° for Fe2O3).

5) Why other options would be incorrect
- Any equation that does not start from elements (e.g., from oxides or ions) is not a formation reaction.
- If elements are not in their standard states (e.g., O atoms instead of O2, or Fe vapor instead of Fe(s)), it is incorrect.
- Equations producing a different amount of product (e.g., forming 2 moles of Fe2O3 without dividing) are not the standard one‑mole formation equation unless scaled to one mole.

Conclusion: The balanced standard formation reaction is 2 Fe(s) + 1.5 O2(g) → Fe2O3(s), which matches the provided correct answer.

Which of the following reactions is a standard formation reaction (i.e. Δ 𝐻 𝑟 𝑥 𝑛 ∘ = Δ 𝐻 𝑓 ∘ ) at T = 298 K?单项选择题

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