I was interested in an application of Le Chatelier's principle, where an increase in pressure could result in a decrease in the moles of gas.

In particular, a situation where when the pressure increased, it drove the reaction in one direction (towards fewer moles), but when the pressure was lower, then the reaction went the other direction.

What pairs of two reactive room-temperature gases can both combine to make a third room-temperature gas where the change in chemical energy can be used to store the energy from the work done on the system by compression?

So far, I've found these possible candidates:

$$ \begin{align} \ce{2 SO3 &<=> 2 SO2 + O2}\\ \ce{2 CO2 &<=> 2 CO + O2}\\ \ce{2 NH3 &<=> N2 + 3 H2} \end{align} $$

I'm imagining a situation where a mixture of gasses could be compressed to a significant fraction of its original volume without too big of an increase in temperature.

Maybe a catalyst might be critical to make the activation energy less of an issue.

I don't know if this is possible, though.


I'm teaching a module on energy conversion.

We have 5 common types of energy: mechanical, electrical, thermal, chemical and light. And we are discussing conversions between them.

The example of changing mechanical energy to chemical energy I had been using was the compression of graphite to make diamonds. I wasn't happy with this one, though.

Then I half-remembered something from my college chemistry class where the direction of a chemical reaction could be shifted by changing the pressure or the temperature, and I wanted to investigate that idea.

That's the inspiration for this question. If this process is useful, all the better. If it's not, then it's a way to introduce some important concepts and something they'll see later in their college chemistry class.

Second Additional

If we have a cylinder of gas, a piston compressing that gas acts a bit like a spring. But compressed gasses heat up and lose thermal energy to the environment.

Could a chemical system like this be interesting because the energy of compression is partially stored in the chemical bonds and not completely in the motion of the molecules, and so might not lose as much energy to the environment?

  • $\begingroup$ It seems to me to be rather an expensive and complicated way to decrease efficiency of the already deploying standard gas compression technology. In contrary to the gas compression, there would be bigger difference between compressing and expanding pressure, and between compressing and expanding work. $\endgroup$ – Poutnik Nov 3 '19 at 10:57
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    $\begingroup$ I think you would want a reaction that was endothermic in the direction towards the lower-volume side. That way, as you compress the gas, the thermal energy from compression could be better converted to chemical energy. Unfortunately, that is not the case for any of the three reactions you've listed in your post--all of them are endothermic in the forward direction (i.e., forward as you've written them). I'd suggest consulting tables of enthalpies of formation to see if you can find a reaction that meets this criterion (unless someone here on Chem SE can suggest such a reaction). $\endgroup$ – theorist Nov 4 '19 at 7:21
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    $\begingroup$ I'll add it could be a challenge to find a reaction that is endothermic in the lower-volume direction, since lower-volume means fewer gas molecules, and collecting the same number of atoms into fewer molecules means, on average, more bonds, and more bonds typically means exothermic. You can see this if you compare the total no. of bonds on the LHS vs. RHS of your reactions. $\endgroup$ – theorist Nov 4 '19 at 7:30

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