[renzoms]

Hello!
I hope you can help.
While I was reading an astrochemistry article I came across something about how dust grains are the third body that absorb energy, allowing reactions to happen:

"The dust grains provide a third body to chemical reactions that which absorbs the energy released in exothermic bond formations (and e.g., allow for the formation of H2)."

I'm confused. I can't remember ever learning about a third body required in chemical reactions in General Chemistry 1 and 2.

Could you please try to explain this to me?

Thanks!!!!!!

[Borek]

No idea without context, does the article in any way suggests what are first two bodies? Reacting molecules perhaps?

[Corribus]

Collisions with neutral spectators to drive reactions is common in atmospheric reactions. For instance, kinetic energy from collision with a spectator molecule helps to get a reaction going. E.g., see the Lindemann-Hinshelwood mechanism for unimolecular reaction. In other cases, collisions help disperse excess energy to trap reactive intermediates in inescapable potential wells.  A reaction quite well-known for this is the reaction between hydroxyl radicals and carbon monoxide, which proceeds through the formation of an activated formyloxyl radical, HOCO. When HOCO collides with a neutral molecule, HOCO loses some of its energy, which can allow it to persist long enough in a lower energy state to participate in other atmospheric reactions: https://cen.acs.org/articles/94/i43/Long-sought-atmospheric-intermediate-detected.html.

One would guess this is quite important in space, where there is little thermal energy to drive reactions... and also few molecules around to disperse energy from products after they are formed. We take the latter for granted. Reactions proceed through a high energy transition state. In order to convert the transition state to final products, that excess energy has to go somewhere. In higher pressure systems, like on Earth, there are plenty of other molecules around to help soak up that excess energy and trap the system in the "product state". But in outer space this isn't the case. Short of potentially slow radiation of that excess energy (in the form of photons), you need a collision with something to disperse that energy and complete the chemistry. I guess dust particles could fit the bill.

[renzoms]

Dude! Thanks! Perfect explanation.