There isn't just one chemical equation for carbon dioxide (CO2). You need to specify what reaction you're interested in.

Here are some examples:

1. Formation of Carbon Dioxide:

* Combustion of a hydrocarbon (e.g., methane):

CH4 + 2O2 → CO2 + 2H2O

* Reaction of a carbonate with an acid:

CaCO3 + 2HCl → CaCl2 + CO2 + H2O

2. Decomposition of Carbon Dioxide:

* Photosynthesis (simplified):

CO2 + H2O → C6H12O6 + O2

3. Other Reactions Involving Carbon Dioxide:

* Reaction with water to form carbonic acid:

CO2 + H2O ⇌ H2CO3

Please provide more context about the reaction you're interested in, and I can give you the specific chemical equation.