Why dont reactions continue on forever?

Because eventually the reactants or the fuel is eventually used up.

Explanation: Take the reaction between baking soda and vinegar as an example. The reaction starts when you combine the two, and stops when either the baking soda, the vinegar, or both are completely gone.

c-14 is carbon-14, or radiocarbon.   it is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons.