For a mixture to be good explosive, it's components should react violently. In other words it should release more energy in less time. Thus enthalpy change of the reaction should be very high and rate constants for the forward reaction should be very high. I think a good reducing agent ($\ce{LiAlH4}$) and good oxidizing ($\ce{KMnO4}$) agent will form a very good explosive.

While discussing about explosives, only good oxidizing agent is put in the criteria but I have rarely seen reducing agent in the criteria. Is there any other factor apart from enthalpy change and rate constant that makes the role of oxidizing agent superior? Also I want to know what happens if we use a mixture of lithium aluminium hydride and potassium permanganate as an explosive?

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First, many explosives in use are not mixtures. Nitroglycerine, pentaerythritol tetranitrate and TNT are not, but gunpowder is a mixture.

It's not only the total energy released but also the speed of the reaction that makes an explosive. Also, release of a gas is essential.

The thermite (Goldschmidt) aluminothermic reaction, $\ce{Al + Fe2O3 -> Al2O3 + Fe}$, releases an immense amount of energy, but is not explosive, because the reaction does not proceed rapidly enough (seconds as opposed to microseconds) and all products take up about the same space (ignoring the expansion due to heat), so there is no shock-wave, or even over-pressure.

Nitrogen triiodide, $\ce{NI3}$ or $\ce{NI3.NH3}$, is an incredibly sensitive explosive, with a rapidly propagating shock-wave, a detonation. However, it releases ~300 kJ/mol, which is only about a fifth of the energy released by nitroglycerin. $\ce{NI3}$ is ineffective as an explosive because of the low energy yield, though it does release gaseous products. (It's also difficult to accumulate large quantities of $\ce{NI3}$, since it detonates when the chemist looks at it ;-)

  • $\begingroup$ Well explained . Let me summarize the criterion:- 1) It should produce large amount of energy and at very rapid rate. 2) It should produce gaseous products. 3)It should create shock wave and over-pressure rapidly. May be 2) and 3) are related $\endgroup$
    – Users
    Commented Jun 10 at 18:35
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    $\begingroup$ May I add a further criterion for a GOOD explosive - it should go off when you want not when it feels like it $\endgroup$
    – Waylander
    Commented Jun 10 at 18:53
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    $\begingroup$ the part of what Waylander wrote is the explosive stability and low reactivity. As decomposition products in long term storage may cause unpredictable behaviour of old explosive. The typical case was picric acid in contact with metal casing. $\endgroup$
    – Poutnik
    Commented Jun 10 at 19:36
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    $\begingroup$ Some examples of good and bad: TNT cannot easily be set off by burning or hitting with a hammer. The above-mentioned NI3 is so unstable that it's hard to accumulate more than a gram or two, before BANG... $\endgroup$ Commented Jun 10 at 20:46

While the answer from drMoishe Pippik gets the essence of the issue, it is worth covering some more details about what "reaction speed" means for practical explosives.

The key criteria of reaction speed is not enough. It is important to understand the material science and physics of that speed.

The critical criteria that matter are not just the speed of the reaction but how fast the reaction propagates in the substance (usually but not always a solid). This speed is a property of the physical substance not just the chemistry.

And the speed also helps classify explosives into two categories which is helpful when finding the right explosive for the right task. In some compounds the reaction propagates faster than the speed of sound in the material. Explosives like this detonate. These are often called "high explosives". But there are other explosives where the propagation is slower than the speed of sound and these are said to deflagrate.

In a high explosive military shell, for example, we need both types. The propellant that pushes the shell from the gun needs to deflagrate; the explosive in the shell designed to cause damage to the target needs to detonate.

If the propellant detonated it would destroy the gun barrel rather than the target. The slower reaction of the propellant ensures that the shell can accelerate (relatively) slowly while in the barrel maximising range without destroying the gun.

Well known deflagrating explosives are gunpowder or guncotton. Well known high explosives include TNT, RDX or semtex.

TNT was once the favourite high explosive in shells for two good reasons. It is hard to set off and needs a detonator. This means it won't blow up from the shock of being fired. It is easy and relatively safe to handle. And it melts below 100°C so can be liquefied and poured into shell casings.

The important point is that speed of reaction propagation in the substance is an important physiochemical property of useful explosives. And the science of making better propellants or high explosives is largely an issue of engineering the right mix of speed and energy released.


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