How do I write a balanced equation if the elements hydrogen, carbon, and oxygen combine to form carbonic acid?

Answer 1

Hypotetically (because it doesn't actually happen that way), the reaction would be:

2H + C + 3O -> H2CO3

The actual series of reactions would be something like:

2H2 + O2 -> 2H2O
C+O2 -> CO2
H2O + CO2 <-> H2CO3

Answer 2

CO2 + H2O ⇌ H2CO3

Answer 3

If they do, then:

2H2 + 2C + 3O2 ===> 2H2CO3