Difference between types of explosives (contact vs spark/flame)?

[Ascendant78]

I have done some research online about this, but have not found a clear-cut answer. I am wondering what the difference is between different types of explosives? Like how nitroglycerin can explode simply through impact, while other explosives require a spark, and others electricity. I am assuming it is all about what is required to cause the activation for the explosion, but I am just trying to get a better grip of what we only briefly skimmed over in chemistry. Thanks.

 

[Borek]

I would say it is mainly a matter of the activation energy. Low activation energy and some rubbing is enough, high activation energy and you need a high temperature to start the reaction.

There can be some fine print to that.

 

[Ascendant78]

I would say it is mainly a matter of the activation energy. Low activation energy and some rubbing is enough, high activation energy and you need a high temperature to start the reaction.

There can be some fine print to that.

Thanks for the information. I guess one of the things I'm wondering about is exactly what causes a reaction from a spark or flame? Is it about the heat, is it about the reaction occurring when something is ignited, or does the activation depend more on exactly what is being reacted with that heat source? For example, are there some materials that don't necessarily need a flame or spark to ignite if the temperature of the air alone is high enough? I'm assuming the answer to this is yes, but since I'm not sure of the exact reaction, I'm really not sure?

 

[256bits]

Some reading
spontaneous combustion --> http://en.wikipedia.org/wiki/Spontaneous_combustion

flashover --> http://workingfire.net/misc3.htm
( deadly for firemen )

autoignition --> http://en.wikipedia.org/wiki/Autoignition_temperature

 

[bigfooted]

Adding energy, usually in the form of heat, is enough to ignite your mixture. When you only add heat to say a hydrogen-air mixture, you will reach the activation energy for the decomposition reaction of hydrogen: H2 -> H+H. This will then further react with oxygen, causing a chain branching reaction that will result in the complete oxidation of hydrogen (to water mostly).

With spark ignition, you also add energy, but you only add it in a small volume so the reaction starts locally. Also, sparks create plasma's (the local energy can be quite high) so some reactions in the chain branching are different because you immediately reach much higher activation energies.

 

[Ascendant78]

Finally got a chance to take a look at this (had finals). Great links 256bits, and also great information from you too bigfooted. After looking into all the information you two provided, it all makes much more sense to me. Thank you!