In fact, there's no chemical reaction happening here. For a chemical reaction to take place, there must be a rearrangement of atoms, i.e., we must form a new substance. In this case, what is happening it only a physical process: the mentos candy is breaking the surface tension of the water in the soda, allowing for the CO2 to be released. The result is that CO2 is releasead much faster than it would normally be, had the surface tension not be disturbed by the mentos candy.
Now, we could use a chemical equation to represent this process of CO2 releasing, but let's remember that it' a phyisical process. The reaction could be written as follows:
[tex]CO_{2(aq)}\rightleftarrows CO_{2(g)}[/tex]
Being a physical process, we don't need to worry about balancing the reaction.
[tex]Ca_2CO_{3(s)}\rightarrow CaO_{(s)}+CO_{2(g)}[/tex]
