For which of the following reactions is ΔH∘rxn equal to ΔH∘f of the product(s)?

2Na(s)+Cl2(g)→2NaCl(s)
H2(g)+12O2(g)→H2O(g)
Na(s)+12Cl2(g)→NaCl(s)
H2O2(g)→12O2(g)+H2O(g)
2H2(g)+O2(g)→2H2O(g)
Na(s)+12Cl2(l)→NaCl(s)

1 Answer
Truong-Son N.
Jan 14, 2018

Well, which of these are proper formation reactions that make one mol of product?

First of all, many of these are imbalanced. They should be:

#1)# #2"Na"(s) + "Cl"_2(g) -> 2"NaCl"(s)#
#2)# #"H"_2(g) + 1/2"O"_2(g) -> "H"_2"O"(g)#
#3)# #"Na"(s) + 1/2"Cl"_2(g) -> "NaCl"(s)#
#4)# #"H"_2"O"_2(g) -> 1/2"O"_2(g) + "H"_2"O"(g)#
#5)# #2"H"_2(g) + "O"_2(g) -> 2"H"_2"O"(g)#
#6)# #"Na"(s) + 1/2"Cl"_2(l) -> "NaCl"(s)#

  • As mentioned, there is by convention #"1 mol"# of product made. So we can eliminate #(1)# and #(5)# immediately.
  • They must also use elements in their elemental state, and chlorine is a gas. Thus, we strike out #(6)#.
  • They obviously must form a compound to be a nontrivial formation reaction, so that eliminates #(4)#, which is the opposite of a formation, AND water is not an element in its elemental state.

Why are #(2)# and #(3)# valid?

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