# Reason for thermal motion

I've been brought up to believe that the higher the temperature of a material, the faster the "random motion" of the molecules in that material will be. Consequently I think of density (eg. Of uncontained gas) dropping with increased temperature because the molecules are banging against each other at higher speeds, forcing the molecules further apart etc.

Can someone explain a mechanical/physical reason as to why molecules should bounce around, and why temperature would make this physical motion more aggressive/forceful?

Take the example of an exothermic chemical reaction - two molecules react, generating heat. How did the chemical reaction increase the velocity of the molecules (without invoking the circular argument "the temperature increased")?

• Perhaps "temperature" is just a measurement of this fundamental kinetic motion (ie. they are the same thing)? Commented Aug 9, 2016 at 17:45
• Wouldn't you become more forceful/aggressive if someone kept hitting you :). Kinetic energy increases the momentum of the molecules, and if they are in a confined space...., well there ya go. en.m.wikipedia.org/wiki/Temperature the second half of the articles deals with k.e.
– user108787
Commented Aug 9, 2016 at 17:51
• Thanks! The article says "a freely moving particle has an average kinetic energy of kBT/2" - this explanation of kinetic theory already starts with the assumption that motion & temperature are one and the same, but doesn't elaborate as to why they are equated. Commented Aug 9, 2016 at 18:01
• @Demis , for an exothermic chemical reaction, the reactants must be supplied with enough energy to reach their activation energy. At that point, new molecular species are generated, and some of the chemical potential energy of the reactants gets converted to kinetic energy of the products. On a macroscopic scale, this shows up as a temperature increase in the reaction products. Commented Aug 9, 2016 at 18:08
• @DavidWhite So when PE ⇒ KE, a chemical bond breaks and a molecule consequently goes flying away at higher speed than before. Why did it leave with higher speed? I guess I'm thinking that the motion occurs because of something physical. Energy is our mathematical construct (as incredibly useful as it is), but the conversion between different types of energy is seemingly explained by the black-box of "Energy", when it seems likely that a mechanical/physical process is responsible at it's core. Perhaps this is similar to Einstein's dislike of probability "explaining" physical processes... Commented Aug 9, 2016 at 18:14

An increase in temperature does not necessarily reduce density. For that to happen the volume needs to increase. But an increase in volume, and decrease in density, can take place without any change in temperature. Rather, for a fixed mass and volume of an ideal gas, increased temperature is noticed as an increase in pressure :
$P=nkT$
where $n$ is number density of the gas molecules.

Your 1st question (why does an increase in temperature make atoms move more violently?) has a trivial answer. What we measure as temperature is the effect which is caused by the violent random motion of atoms. For example, we measure an increase in temperature by the increase in volume of some mercury or alcohol in a sealed tube, or of some gas in the above piston-chamber.

Your 2nd questions is more interesting and more difficult to answer : What starts this motion in the first place? How does chemical or nuclear energy turn into kinetic energy? Saying that mass is energy and pointing to the equation $E=mc^2$ does not explain the process(es) which are involved.

One possible answer is that the rearrangement of electrons in a reacting molecule releases photons with energies of a few $eV$ which is 2 orders of magnitude larger than typical kinetic energies of molecules, which are $\frac32kT = \frac{1}{40}eV$ at room temperature. This explanation is appealing because some reactions, such as the combustion of magnesium, release intense amounts of light. However, the momentum of a photon is orders of magnitude smaller than that of an atom. So this cannot be the explanation even for magnesium.

The answer must be that excess chemical energy is, in almost all cases, released as the kinetic energy of the products and/or catalysts. The difficulty with an exothermic reaction of the type $A+B \to AB$ is that an increase in kinetic energy defies the conservation of momentum. Interaction with a 3rd molecule is essential. The intermediate state is highly unstable but exists for a short time. If the gas is under high pressure then a collision with another molecule becomes likely within the lifetime of the excited state. Reactions of the type $A+B \to C+D$ do not have the same problem.

In nuclear fission, the electrostatic repulsion between daughter nuclei is held in check by the strong nuclear force. Once the nuclear force is overcome, it is easy to see how the electrostatic potential energy is converted into the kinetic energy of the fission products.

• the first link you provided links to a great video of a molecular vibration absorbing/releasing energy. It got me thinking: the absorption or emission of photons could significantly alter the spring constant of a molecule suddenly (via change in electron states), leading to a change in the molecular vibrational amplitude/state. Although the photon had little momentum, the resulting molecular vibrations could impart significant momentum/velocity to surrounding particles. Commented Aug 11, 2016 at 6:15
• Yes, my explanation does not quite get to the heart of the matter : how the bond energy turns into KE. It does (in the molecular case) hinge on the rearrangement of electrons. I was thinking about a stretched rubber band, holding 2 repulsive nuclei together, which is cut so they fly apart. From what I've read, shedding excess internal vibrational energy through collisions with other molecules during the lifetime of the excited state is an important part of the process. If it doesn't happen, the reactants fly apart again, reversing the reaction. Commented Aug 11, 2016 at 10:40

In the case of an ideal monatomic gas, random-motion kinetic energy per particle is the way we identify a temperature. A diatomic gas, or a chemically active gas will have a more complex thermal behavior (because there are other energy-containing degrees of freedom). When that hypothetical second gas comes into equilibrium with the simple monatomic gas, it is (by definition) at the same temperature.

There is no reason why molecules should bounce around freely. A solid, or liquid, can also come into thermal equilibrium with the hypothetical ideal gas, and when it does, it is being hammered by all the gas molecules that come into contact with it. So, it is moving (vibrating, quivering, ringing like a billion bells). That, in a nutshell, is why temperature is expected to make internal motions more energetic.

If the solid weren't jostling around, if it were just taking damage from the gas, it would COOL THE GAS. However, it is in equilibrium, so it has to be moving, bouncing gas molecules into higher velocity just as often as a gas molecule loses velocity by rebounding with a dull 'thud'.

"Can someone explain a mechanical/physical reason as to why molecules should bounce around, and why temperature would make this physical motion more aggressive/forceful?"

Temperature is a measure of the average kinetic energy of a group of atoms or molecules. The higher the temperature of, say, a container of air, the greater the average kinetic energy of the air molecules in the container.

What could increase the average kinetic energy of the air molecules in this container? Radiation could be one thing. If you could shine a "gamma ray flashlight" on those air molecules, the gamma rays would tend to increase the kinetic energy of the molecules they collide/interact with. Energy is being transferred from high to low, the way a pool shark breaks the starting triangle.

Again, temperature is just the "electronic readout" of how energetic the molecules/atoms are on average.