Question

for which one of the following is the enthalpy of the reaction the same as the enthalpy of formation?
answer:
a $2fe(s)+\frac{3}{2}o_2(g)\to fe_2o_3(s)$
b $hcl(aq)+nh_3(g)\to nh_4cl(s)$
c $asf_3(s)\to as(s)+\frac{3}{2}f_2(g)$
d $n_2(g)+3h_2(g)\to 2nh_3(g)$

Explanation:

The enthalpy of formation is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states.

• For option A: \(2Fe(s)+\frac{3}{2}O_{2}(g)\to Fe_{2}O_{3}(s)\), \(Fe_{2}O_{3}\) is formed from its elements (\(Fe\) and \(O_{2}\)) in their standard states (\(Fe\) is solid, \(O_{2}\) is gas at standard conditions).
• For option B: \(HCl(aq)\) is not an element (it is a compound in aqueous solution), so this reaction is not an enthalpy of formation reaction.
• For option C: This is a decomposition reaction (\(AsF_{3}(s)\to As(s)+\frac{3}{2}F_{2}(g)\)), not a formation reaction (enthalpy of formation is for formation of a compound from elements, not decomposition of a compound).
• For option D: The reaction \(N_{2}(g)+3H_{2}(g)\to 2NH_{3}(g)\) forms 2 moles of \(NH_{3}\). Enthalpy of formation is defined for the formation of 1 mole of a compound from its elements in standard states.

Answer:

A. \(2Fe(s)+\frac{3}{2}O_{2}(g)\to Fe_{2}O_{3}(s)\)