Which thermochemical equation gives the value of the standardenthalpy of formati

Question

Which thermochemical equation gives the value of the standardenthalpy of formation for NH4ClO4 (s)?
(Please note: the molecular formula contain subscripts, e.g. 2 H2(g) refers to two moles of molecular hydrogen gas).

a. N (g) + 4H (g) + Cl (g) + 4 O (g) --> NH4ClO4 (s)
b. N (g) + 2 H2(g) + Cl2 (l) + 2 O2 (g) --> NH4ClO4 (s)
c. N2 (g) + 4 H2(g) + Cl2 (l) + 4 O2 (g) --> 2 NH4ClO4 (s)
d. N2 (g) + H2 (g)+ Cl2 (g) + O2 (g) --> NH4ClO4 (s)
e. ½ N2 (g)+ 2 H2 (g) + ½ Cl2 (g) + 2 O2 (g) --> NH4ClO4 (s)

is it (e)? If so, WHY? How would I answer a similartype problem on an exam please?

Explanation / Answer

enthalphy of formation is defined as: enthalpy change from thereference forms of the reactants to makeone mole of the product. - this means only 1 mol of NH4ClO4 so it is not C - Cl in liquid state is not a reference form so it is not B - Monotomic Nitrogen in A is not a referene form so it isnt A - in D, the equation is not balanced so its not D - It is E, the equation is balanced, all reactants are inreference form, and only 1 mole of product is made! * when doing these problems refer to your table ofHf . Referenceform of an element means that its Hf is 0.

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