Which of the following reactions as written

Question

corresponds to a standard formation reaction?
1. 2H (g) + O2(g) ! H2O2 (`)
2. Na(s) +
1
2
Cl2(g) ! NaCl(s)
3. CO2(s) ! CO2(g)
4. SO2(g) +
1
2
O2(g) ! SO3(g)
5. 2 H2(g) + O2(g) ! 2 H2O(`)

Answer 1

1. 2H(g)+O2(g)->H2O2
2. Na(s)+1/2Cl2->NaCl(s)
3. Co2(s)->Co2(g)
4. So2(g)+1/2O2->So3(g)
5. 2H2(g)+O2(g)->2H2O

Answer 2

1 is not.
2 is if you have 1/2 Cl2(g)
3. I don't understand the 2 but I think this is not because for Co I would write Co and not Co2.
4 is if you write 1/2 O2(g)
5 is not. You can make it one by
H2(g) + 1/2 O2(g) ==> H2O(g)