[oldmanatoxford]

Rates of chemical reactions usually slow down over time. However, how might a rate of reaction increase after an initial slow rate, even though nothing extra is added and the conditions in the environment are kept constant?

[Aldebaran]

Some reactions are self-catalysed. For example the oxidation of the ethanedioate ion by manganate(VII) ions. At room temperature is is quite slow to start with but speeds up as Mn²⁺ ions, which are formed, autocatalyse the reaction. It also goes a bit faster if warmed but your question indicated unchanged environmental conditions

[oldmanatoxford]

Makes sense. Thanks so much. It asks for 2 reasons tho. So my reason was the same as yours. Could you maybe also add taht the reaction itself created some heat allowing for the rate to increase?

[rjb]

An excellent example of a reaction which meets your criteria and produces a significant amount of heat is that of the thermal decomposition of some nitrate esters, the most well studied (because of its use as a propellant in small arms ammunition and artillery) perhaps being nitrocellulose (NC).

Because of the relatively low bond dissociation enthalpy of the nitrate ester group in NC, you tend to see very slow but continuous mass loss, resulting from homolytic cleavage of RO-NO2 bond. This yields *NO2 and other reactive or acidic byproducts which are formed as the glucose ring is destroyed. This process is known as intrinsic decomposition.

If NC does not incorporate stabilisers (such as DPA, EC, MC etc. which are able to mop up *NO2 produced by the intrinsic decomposition process), then a second and more rapid decomposition pathway known as autocatalytic decomposition may result. This pathways leads to the production of yet more *NO2 (effectively a positive feedback loop) as well as to an increase in temperature; conditions which progressively accelerate the rate of reaction leading to a situation known as accelerative self-heating. This certainly meets your criteria for a chemical reaction which accelerates over time!

These processes are certainly not just theoretical and it seems that accelerative self-heating was likely to be the initial cause of a series of explosions which took place in Tianjin China in 2015, leading to the tragic death of at least 173 people.

https://en.wikipedia.org/wiki/2015_Tianjin_explosions

R

[oldmanatoxford]

Thank you so much for this! This idea makes a lot more sense to me now too, thanks to your explanation how intrinsic decomposition and the idea of accelerative self-heating. To both who have helped me do this question, you guys have my utmost gratitude!