Chemical Explosions : How they Work
In chemistry, a chemical substance is "...a form of matter that has constant chemical composition and characteristic properties" (wiki). Thus it can not be physically broken down and a chemical reaction must occur for it to change. Therefore, a chemical explosion must begin with a chemical reaction.
Since the term "Chemical" can mean a ton of different thing, there is no specific equation or reaction for this type of explosion.
A common explosive chemical reaction is Nitroglycerin. This compound is represented with the molecular formula CH5(ONO2)
The following equation represents the explosive reaction that occurs when Nitroglycerin has decomposed.
(now Balance it)
This equation shows the chemical reaction required for Nitroglycerin to explode.
Although Nitroglycerin doesn't combust it still has the ability to explode.
It is able to explode because it has a one of the major properties an explosion needs.
It can expand very rapidly
When the Nitroglycerin is agitated (by heating it or shaking it, the most common way is lighting it with fire) it begins to decompose. In the equation above all the products are gases. Nitroglycerin is a liquid at room temperature and since all of its products are gases, the volume must expand very rapidly to accommodate for 7.25 times as many moles of the gas. This rapid expansion of this gas, causes the fluid to "combust" and explode very violently.
Not all chemical reactions are this way. Most reactions that combust require a direct and constant heat source to be running (until the substance combusts, then the heat will be self generated and there will be no need for an initial heating source), one example would be a lighter.