hot and cold

[kenny1999]

I think many substances, if not all, can be degraded or destroyed when temperature is high enough because heat facilitates many chemical reactions. On the other hand, is there any chemical reaction that would be accelerated when temperature drops? Or is there almost never any chemical reaction that would be favored at low temperature while inactive at high temperature?

[Phi for All]

https://pubs.acs.org/doi/pdf/10.1021/ja4034439

Quote

We present new reaction pathways relevant to low-temperature oxidation in gaseous and condensed phases. The new pathways originate from γ-ketohydroperoxides (KHP), which are well-known products in low-temperature oxidation and are assumed to react only via homolytic O−O dissociation in existing kinetic models. Our ab initio calculations identify new exothermic reactions of KHP forming a cyclic peroxide isomer, which decomposes via novel concerted reactions into carbonyl and carboxylic acid products.

This is one of the newest reactions I take it, discovered while experimenting with engine oil breakdown. Here's an article on it: https://news.mit.edu/2013/new-low-temperature-chemical-reaction-explained-0904

[Sensei]

1 hour ago, kenny1999 said:

I think many substances, if not all, can be degraded or destroyed when temperature is high enough because heat facilitates many chemical reactions. On the other hand, is there any chemical reaction that would be accelerated when temperature drops? Or is there almost never any chemical reaction that would be favored at low temperature while inactive at high temperature?

Temperature is not the only factor to consider. There is also pressure. e.g. water boils below 100 C when the pressure is reduced.

Edited September 6, 2022 by Sensei

[exchemist]

8 hours ago, Phi for All said:

https://pubs.acs.org/doi/pdf/10.1021/ja4034439

This is one of the newest reactions I take it, discovered while experimenting with engine oil breakdown. Here's an article on it: https://news.mit.edu/2013/new-low-temperature-chemical-reaction-explained-0904

This is interesting (especially to some like me with some background in oil chemistry).  However as far as a I can see it is not an example of a reaction that is accelerated by a drop in temperature.

9 hours ago, kenny1999 said:

I think many substances, if not all, can be degraded or destroyed when temperature is high enough because heat facilitates many chemical reactions. On the other hand, is there any chemical reaction that would be accelerated when temperature drops? Or is there almost never any chemical reaction that would be favored at low temperature while inactive at high temperature?

As far as I can recall, it isn't possible for a chemical reaction rate to increase with a fall in temperature. Reaction rate is governed by the Arrhenius equation: k = A exp(-E/kT) in which E is the activation energy for the reaction. The only way you can get a -ve temperature dependence is if E is -ve. But if that were so, instead of an activation energy you would have an energy minimum at the transition state, so that would be a new stable compound and the reaction would stop there, with the rate being diffusion limited (effectively a zero activation energy). But since the rate of diffusion also increases with temperature, even then it would go faster as the temperature increases.

However there are plenty of examples where the thermodynamics are favoured by low temperature. A famous example is the Haber process for ammonia synthesis. The reaction is N2 + 3H2 <->2NH3. The free energy change for this is -ve, making the right hand (ammonia) side favoured at room temperature. However the rate is extremely slow as almost none of the molecules have enough energy to break the N-N triple bond. Raising the temperature overcomes that and speeds it up, BUT that shifts the equilibrium towards the left, so there is less tendency to form ammonia. So to get over that problem, 2 things are done. One is to use a catalyst that makes it easier to break the N-N bond (lowering the activation energy) so not such a high temperature is needed. The other is to increase the pressure, which favours the right hand side because 4 molecules are replaced by only 2.