What is the scientific equation for baking soda and lemon juice?

It's really two reactions in one. The first one's a double replacement:

NaHCO3 + HC2H3O2 --> NaC2H3O2 + H2CO3

Then, the carbonic acid immediately decomposes:

H2CO3 --> H2O + CO2

So the two together are:

NaHCO3 + HC2H3O2 --> NaC2H3O2 + H2O + CO2