Coke + Mentos isn’t chemical. Mentos have surface area. Coke had 3 atmosphere of CO2 dissolved into it making it unstable. You add surface area to CO2 saturated solution, the extra surface of the mentos allow a location for the CO2 to come out of solution and hence it does. It comes out so fast because Coke is unstable. The gas wants to get out and balance with the normal air pressure.
Rust is an oxidation reaction where Oxygen and Iron are swapping electrons and binding to form Ferrous Oxide. This happens slowly because nothing is highly out of balance and the reactivity of these is not high. On the other hand, you can take something like sodium and add it to water and get a violent reaction because sodium is very reactive.
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