What prevents everyday objects from reacting?

Question

This may be a stupid question but I would like to know what prevents chemical reactions from happening among common objects in everyday life? The opposite would be, what requirements must be met for chemical reactions?

For example, as I am typing this what prevents no reaction between my fingers and the keyboard, or everyday objects such as chairs, tables, etc. In other words, why do we not constantly see chemical processes happening all around us constantly? Why don't my feet chemically react with the ground, my pencil reacts with the air, my fridge with the floor, etc...

There must be electron transfer such as conductivity, and charging but why no chemical reactions? Even as I am in AP Chemistry and understand Thermodynamics, Hess' Law, and so on, I am too embarrassed to ask my teacher with such a silly question.

Answer 1

As I see it, two factors: thermodynamics and kinetics.

Thermodynamics: Many of the reactions that "don't happen" in everyday life are not favored thermodynamically, we call them non-spontaneous. This means that the products would be higher in energy than the reactants. Such reactions can certainly occur if you supply external energy (charging your cell phone battery, for instance, is a non-spontaneous process). But, without applied energy, non-spontaneous processes generally will not occur. Reactions that we observe in nature are generally spontaneous, favored by thermodynamics (with lower-energy products). Some thermodynamically spontaneous processes are not observed to occur, even when we might naively expected them to. Example: Sugars decomposing to $CO_2$ and water is a very energetically favorable reaction, in fact the energy released from this process fuels nearly all known life. However, we do not observe our table sugar decomposing in its container because without outside help like enzymes and applied heat, the process is terribly slow kinetically.

Kinetics: For us to observe a reaction happening, it must proceed at rate that is fast relative to our human scale of time. Perhaps your fingers fusing to your keyboard is thermodynamically favored/spontaneous, but if it takes hundreds of years to occur then for our human purposes we may regard it as "no reaction". Indeed, the decomposition of most types of the biomolecules constituting our bodies is favored thermodynamically, but the kinetics are so slow (440 years for table sugar)°. There are a few ways to increase the rate at which a reaction occurs. To occur, a reaction requires a certain amount of activation energy. Think of this as hill you have to climb before you can sled down to the products (or up, potentially). The reaction does not occur if the starting material molecules do not possess enough energy to climb the activation energy barrier. You may help them overcome the barrier by supplying heat; thus, heating your table sugar will allow it to decompose into $CO_2$ and water before your eyes (the water will be released as steam of course). The other main way to increase the kinetics of the reaction is to lower the hill, instead of adding the energy to climb it. That is, use a catalyst like an enzyme to lower the activation energy barrier of the reaction. Without such external factors like applied heat and use of a catalyst, many reactions that are thermodynamically favored will be disfavored by kinetics, and not proceed at an observables rate.

So the reason the universe isn't a useless, constantly rearranging chemical soup boils down to thermodynamics and kinetics. Even if a process is thermodynamically spontaneous, it may be kinetically slow. I should point out that in a lot of places, such as the centers of stars, there is indeed a copious abundance of uncontrollable chemical and nuclear reactions. And planets like Jupiter pretty much are a useless chemical soup (no offense to Jupiterians). But in our favorite places on Earth, at room temperature and atmospheric pressure, the carbon in our bodies isn't going to undergo nuclear fission and become lead (thankfully).

By the way, this is in not a silly question. In fact, if a chemistry student doesn't wonder this, one may question if they are actually learning the subject. I applaud you for your curiosity.

Answer 2

This question is more than a bit ironic; without the millions and billions of chemical reactions that we each are composed of, it couldn't be asked. Two books come to mind here: The Watchmen (specifically Dr. Manhattan) and Godel, Escher Bach by Hofstadter, which you may (or may not) enjoy reading. Also this:https://en.wikipedia.org/wiki/The_Siphonaptera. There are only two thermodynamically stable minerals: quartz and darn it, I've forgotten the other one! So when you look around you, virtually nothing you see is chemically or physically stable. (Arguably, water is stable too but it is slowly being lost to space, although you and I won't have to worry about that.) So we need to be aware that most of what is going on around us we are unaware of. (Thank goodness, most of the creepy crawlies living on and in us are the stuff of nightmares..)

Anyway, while you are unaware of the sea of microbes that we are surrounded by, and of the countless chemical reactions going on around us (did you know that concrete generally has a lifetime of only 20-50 years?) that shouldn't imply that our environment is chemically inert.E^-pusher mentioned activation energy, chemists generally think of it as the hill between two valleys, where the two valleys are "stable" chemical compounds and the hill is the "activated species".

For instance carbohydrate (like wood is mainly composed of) is one valley, and $\ce{CO2}$ is the other (lower energy) valley. Wood doesn't seem to do much until you light a match to it (providing enough energy to push some of it up the hill). A lot of our natural world is composed of polymers. Unlike small molecules like water or sugar, polymers (like carbohydrate, protein, plastic, paint, DNA, RNA, etc.) are typically very slow to react. It's also very difficult to get them to diffuse or dissolve (many of them, anyway).

At the other extreme, take a piece of aluminum - say an aluminum nail or screw. Cut it (or just scratch it). Within microseconds, the surface of the $\ce{Al}$ is oxidized to form a film of $\ce{Al2O3}$. Most of us don't ever realize that aluminum is a very reactive metal, much more reactive than iron or steel. One reason that we don't see many reactions occurring, is because if they did, then the object they were occurring on/in wouldn't be very useful. (Of course, cooking is just a series of (useful) chemical reactions, which allowed us to reduce the energy we needed to digest food and was probably one reason we developed intelligence).

Anyway, the basic reason why we can surround ourselves with all sorts of materials which don't seem to degrade, is due to the need for the reaction to get up that activation energy hill. Even a reaction as simple as $\ce{2H2 + O2 -> 2H2O}$ doesn't proceed very fast at room temperature (unless a catalyst, such as $\ce{Pt^0}$ (with a clean surface) is present and then whoosh!).

By the way, all of my kids are long grown up and gone, but I've still got some of their polyethylene toys around. The plastic looks almost like new, but squeeze it, or drop it, and it cracks and breaks, even though when it was new, the toy was very tough. Even polyethylene ages, and undergoes oxidation reactions. Makes you wonder about all of the plastic plumbing pipes in homes, huh?