[thetada]

So, I am a teacher marking an AS question (16-17yrs) for which I cannot find a mark scheme.

The qu asks why highly exothermic reactions (such as hydrogen and oxygen eg given in qu) are more likely to explode.

In the context (rates of reaction) it seems as if they want students to consider heat and effects on rate. It suddenly made sense to me that the heat generated by the initial reaction would increase the rate of the next stage of the reaction and that would be the foundation of a full mark answer (two marks). But then I thought that this idea contradicts the idea that the rate of a reaction is always fastest initially and gradually slows to a stop, as shown by graphs of concentration of products against time in which the line is initially steep and gradually levels off. What does anyone else think?

Thanks!

[Irlanur]

the graphs you mention are normally "conducted" at a constant temperature. an explosion obviously isn't. but an explosion is more than only highly exothermic. in the reaction mechanisms, one reactive species produces normally 1,2 ore more reactive species etc...

[Corribus]

I don't think it's a particularly good question. Explosions have as much to do with kinetics as they do with thermodynamics, and in thermodynamics there are entropic as well as enthalpic factors to consider. Kinetic and thermodynamic stability are only partially related. Explosive materials tend to also produce large volumes of gasses. That often but not always goes hand in hand with large reaction enthalpies. The release of heat alone will not create an explosion - high reaction rates are also needed so that the heat is produced rapidly enough to rapidly expand gas and create large pressures. There are lots of fuels that produce great amounts of heat, but are not explosive because the reaction kinetics do not favor it.

Were I answering the question, I'd probably say something like - all things being equal, highly exothermic reactions are also usually highly exergonic, especially at high temperatures when the entropy is also large (as it would be for any reaction that produces a large amount of gas). Highly exergonic reactions also have - all things being equal, again - large reaction rates. Explosions are generally characterized by producing large amounts of heat, which contributes to the production of rapidly expanding gas. But things are rarely equal, so it is inappropriate to say that all highly exothermic reactions give rise to chemical explosions. It would probably be more appropriate to say that all reactions that give rise to chemical explosions are highly exothermic (need a Venn diagram?). This may not be always true, either, but I can't think of an exception off the top of my head...

[Borek]

This may not be always true, either, but I can't think of an exception off the top of my head...

To quote wikipedia article on the acetone peroxide:

Very little heat is created by the explosive decomposition of TCAP. Recent research describes TCAP decomposition as an entropic explosion.

[Corribus]

Well there you go. I figured there had to be an exception. There always is.

[thetada]

Found the mark scheme after all. This is "the answer".

1st mark: after the reaction has started the temperature goes up/energy increases/speed of the reactant molecules increases

2nd mark: allowing many molecules to have E>Ea, and thus speeding up the reaction

Thanks everyone for your contributions. I particularly enjoyed the insight that explosions are often promoted by the diversity of possible reaction intermediates.