Does gravity affect the rate of a chemical reaction?

Question

For example, on planets with lower gravity, would the rate of reactions be lower because the reactants are slower to mix with one another?

Answer 1

It sort of depends what you mean. If by "affect the rate of a chemical reaction", you mean affect the individual steps of some overall reaction as to slow them down, then no, gravity definitely has no effect at all.

This is simply because the gravitational force is so much weaker than the electrostatic force, which will dominate all interactions relevant in chemical reactions, that it does not have sufficient time to make any appreciable difference in the behavior of molecules or atoms.

A kind of standard way of demonstrating this is by imagining a situation in which it is guaranteed that electromagnetism will be stronger than gravity, which is all normal situations in chemistry, and then plugging the numbers into Coulomb's law or Newton's law of gravity.

This, however, is contrived because there are situations where gravity is stronger than electromagnetic interactions. Just never in chemistry. This article does a pretty good job of explaining this, and pointing out that the ratio of forces can really be anything at all.

The key reason gravity will never be relevant to individual chemical processes, and hence never affect the rate, is that the external gravitational field will affect the molecules equally once they are close enough to interact electromagnetically. And once they are interacting electromagnetically, it is guaranteed this force will be much, much stronger than the gravitational interaction between them because both follows the same $1/r^2$ law, so it is just a matter of comparing the proportionality constants which go in front of the law, and inputting some masses and charges characteristic to the situation. The article I linked does provide a context in which the gravitational force dominates the electromagnetic force, so this is possible.

So, no individual rates will not be affected. It's fun to imagine how gravity could affect macroscopic equilibria though, so let's think about that.

Given sufficient time, gravity is very good at organizing things in a particular way. For instance, if you imagine two planets which have atmospheres that at some time are exactly identical, but one planet is much more massive than the other one, I don't think anyone would contend that these atmospheres will look exactly the same after a long time. For instance, the escape velocity of an object is $\sqrt{2rg}$, $r$ being the radius of the planet and $g$ the gravitational constant of acceleration. If $g$ is increased significantly, then you would not lose nearly as much hydrogen from the atmosphere, which could affect the location of certain chemical equilibria in the atmosphere. The atmosphere as a whole would be kept much closer to the planet, which could also be relevant.

All of this is kind of contrived, however. The point is that gravity will only really affect chemistry in how it organizes materials, but cannot affect the individual rates.

Answer 2

Whether gravity can direct affect the rate of homogeneous chemical reactions seems unlikely, but there are a lot of inhomogeneous reactions which will clearly behave differently in the absence of gravity.

Many real world reactions involve phase separation either because there are two miscible liquids involved or because there is a precipitation when some solid products drop out of solution. One of the forces that makes separation happen in such reactions is gravity (less dense liquids float of more dense liquids; dense solids drop to the bottom of the reaction vessel). Such reactions may be very different in microgravity. This is the mirror image of some things chemists already know (some separations don't occur very efficiently even under gravity so we enhance the forces by using a centrifuge to create better separation).

There are practical experiments where the results are different. One example is the production of some specialist optical fibre strands (see this wikipedia article) where the defects in the fibres seem to be much lower when produced in zero gravity. Crystallisation may behave differently in microgravity and this too has been tested with promising results (see this NASA link). Flames are basically chemical reactions but are complicated to understand because the reaction heat also induces gravity driven convection when the warm gases involved interact with the colder surrounding air. Flames behave very differently in microgravity as this NASA link explains. Interestingly, the "cool" flames produced by combustion in zero gravity have radically different chemistry to the ones we are used to seeing on the ground.

So while individual homogeneous chemical reactions might not be influenced by gravity, in practice real reactions and their result are influenced and can be very different in low-gravity conditions. This is new science and we don't yet know whether there will be big economic impacts.

Answer 3

At the gravitational forces we experience on earth, the kinetic energy of molecules and the electrostatic forces between molecules are the dominant factors in determining their average orientations. If you exerted enough gravity on a molecule though perhaps it would behave more like the macroscopic objects we deal with every day in that it would assume positions that minimize its gravitational potential energy. If molecules then had selective orientations in a strong gravitational field it's possible the reaction rates could change since molecules must collide with the right amount of energy in the right orientation to react and you just changed their average orientation.

This is all speculation.