Baking soda and coke react because coke contains an acid and baking soda can behave as a base in this case. The phosphoric acid in coke gives a hydrogen ion (called a proton) to the baking soda, which produces carbon dioxide, which is a very stable gas. This can be calculated by thermodynamics, which in many cases governs whether a reaction will take place. There are two parts that are generally considered 1) Enthalpy, and 2) Entropy. Without saying too much, and without even talking about the thermodynamics on the side of the phosphoric acid, the formation of strong bonds in CO₂ leads to favourable enthalpy of the reaction, and the production of CO₂ gas molecules leads to favourable entropy of the reaction. Both of these things favour the reaction of the phosphoric acid in the coke with the baking soda.

Baking soda and coke react like baking soda and vinegar. It's the reaction of an alkali, or base [baking soda] with an acid, which is an exciting reaction that releases lots of energy. It's fun to take something like juice from squished blueberries and mix it with baking soda and with coke or vinegar to see the colors you get.

Coke is acid, and the acid joins up with the carbonate in baking soda to form hydrogen carbonate. Hydrogen carbonate then breaks up to become carbon dioxide and water. The carbon dioxide then bubbles off as a gas.