Wikipedia:Reference desk/Archives/Science/2019 December 22

= December 22 =

Chemical reactions.
What are some examples of reactions that take place below room temperature? And if so, that would mean the colder the temp is, the faster the reaction? 67.175.224.138 (talk) 17:34, 22 December 2019 (UTC).
 * See reaction rate and Arrhenius equation
 * In general, reactions continue at any temperature, but their rate will increase with increasing temperature. They don't suddenly "stop" (although this dependency on temperature can be exponential, so it might look like it!). Also many reactions will be exothermic (or even endothermic) and so their reaction may itself change the ambient temperature. This leads to the fairly common situation of a reaction undergoing some form of thermal runaway. It's hard to start it, but once started it quickly accelerates under its own influence. Andy Dingley (talk) 17:47, 22 December 2019 (UTC)
 * Okay look at the Haber process, the probability of it happening at room temperature, is like less than 1%? So imagine the equilibrium being at really cold temperatures, doesn't that mean at room temperature, it is almost 0%, and therefore, as we get colder, the reaction speeds up? Say a equilibrium is at -50 C, so at -70 C, the rate is slow, and as it gets warmer, towards -50 C, the rate gets faster, but doesn't that mean as we go past -50 C towards room temp, the reaction slows down again? 67.175.224.138 (talk) 20:00, 22 December 2019 (UTC).
 * You're confusing chemical kinetics with chemical thermodynamics. In the Haber process, it is thermodynamically favored at low temperatures, but kinetically favored at higher temperatures.  -- Jayron 32 20:25, 22 December 2019 (UTC)
 * So can some reaction be both thermodynamically favored at low temperature and kinetically favored at cold temperature? As well as something for hot for both. 67.175.224.138 (talk) 02:27, 23 December 2019 (UTC).
 * Depends on your definition of "reaction". For instance, certain fuels exhibit a "negative temperature coefficient" domain where an increase in temperature causes an increase in the autoignition delay (example), but that is because the "reaction" of combustion is actually a multitude of elementary reactions (i.e. reactions that cannot be broken down any further and involve very few molecules), and some of these compete for reactants so that the whole process becomes slower (there is no notable change in the thermodynamic equilibrium state). I would expect that any elementary reaction must have its reaction rate increase with temperature (that is at least the case for any reaction that follows one of the two common models: Arrhenius equation with a constant prefactor or Eyring equation). Tigraan Click here to contact me 09:58, 23 December 2019 (UTC)
 * There are times where lowered temperatures are desired for a reaction to prevent your product from decomposing or engaging in a secondary reaction, or to achieve a higher yield of your product by limiting competing side reactions. Rmhermen (talk) 19:16, 22 December 2019 (UTC)