My question is why does something that rusts chemical reaction soooooo much slower than another chemical reaction like mentos in coke which happens so fast?
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You’re asking about something called reaction rates, which can be a complicated area of chemistry. This is usually the unit of freshman chemistry students struggle with the most, and I know that held true for me as well. There are a great deal of factors that determine how fast certain reactions happen, for example the presence of a catalyst. In fact, this is the very definition of what a catalyst is: something that increases the rate of a reactions without participating in the reaction. However, what makes explosions (and combustion) different from rust is that they are a chain reaction, where as rust is not a chain reaction.
So, what makes a chain reaction happen? I’m going to talk about combustion because it’s easier. Combustion is an exothermic reaction, meaning it releases energy when it occurs. but gasoline doesn’t just explode on its own, you need to provide a little encouragement in the form of a spark. The spark is what we call activation energy, the little bit of energy that needed to be added to get the exothermic reaction to happen. You can think of it as the initial push down a hill. Now, in a chain reaction, the reactions releases more than enough energy to supply the activation energy to the molecule next to it, which also quickly combusts, which provides the activation energy for the molecule next to it, which….you get the idea.
Now, not all reactions will release enough energy to supply their own activation energy. Other reactions might not have another molecule close by to utilize the energy being released as activation energy for its own reaction. There are many different aspects of chemical reactions that govern their rates, many of which fall under the umbrella of thermodynamics. Things can get complicated if we go too far down that rabbit hole, but also some cool things crop up like the [Iodine Clock Reaction](https://youtu.be/es1voPWXrlU) which are very difficult to explain but basically involve a fast reaction and a slow reaction competing for the same compound.
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