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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Mentos in coke isn’t really a chemical reaction like rusting is. The coke has carbon dioxide dissolved in it, which is part of what gives it the “bite” or sharp taste. The carbon dioxide would rather not be dissolved in the coke, but it’s not easy to get out; it needs to find a place to form a bubble, which is hard to do in a smooth plastic bottle or metal can.
However, a mentos candy has a lot of rough surfaces and nooks and crannies that carbon dioxide can use to form a tiny bubble and then float to freedom. It has so many that all the tiny bubbles form and start floating upwards faster than the liquid coke can get out of the way; this forms a foam which gets very rapidly pushed out of the bottle by the other bubbles that are forming on the mentos.
Rust is the direct reaction of oxygen in the air with water and steel or iron, specifically oxidation of the metal. Since this usually happens outside, where it’s relatively cool, it happens very slowly; chemical reactions tend to happen faster at higher temperatures. If you want to see “rusting” at high speeds, you can simply heat up a ball of steel wool in a fireplace and watch it burn to bits; that’s the same reaction, but happening much, much faster.
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.
Same reason guns don’t use C4. The speed of an explosion determines its severity. Iron explodes so slowly you don’t notice. Gunpowder explodes pretty slowly too.
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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