Profound question from my high school classes: “Mr. Canaff, are we going to blow stuff up?
Blowing stuff up involves chemical reactions. For the most part, these reactions need to be rapid, exothermic and have one or more products which are gaseous. What do all these terms and conditions mean?
Chemical reactions are not always rapid, to say the least. There is a subset, so to speak, of extremely rapid ones. All chemical reactions involve a gain or loss of energy. Reactions which release heat to the surroundings are exothermic, while those which do not are endothermic.
What’s with gaseous products? It involves the taking up of space, what physical scientists define as “volume”. Basically, all matter exists in one of three phases: solid. liquid and gas. Solids and liquids are often called “condensed states”, in that the atoms/molecules are touching each other. With gases, on the other hand, the atoms/molecules are not in contact. Let’s look at a familiar substance – water. The molecular weight of water is about 18 grams. In the logical setup of the Metric System. one gram of liquid takes up 1 milliliter of room, so to speak, so that 18 g of water would take up 18 ml. Barely a half a shot glass. At the boiling point, the gaseous mole of water would take in excess of 30 liters, about 17,000 times!
Of course, water doesn’t burn, Heating it to the boiling point represents a phase change from liquid to gas (steam). Let’s check out something which does burn: propane, C3H8. The balanced equation: C3H8(l) + 5 O2(g) —–> 3 CO2(g) + 4 H2O(g) (s,l and g are phases).
Both products are gaseous. For each mole of propane, 7 moles of gas are formed. The volume change is, to say the least, considerable. Gas volumes also increase with temperature. The reaction is strongly exothermic. You need to cook those hamburgers! If the reaction is carried out in the open (gas grill) where the volume change is dissipated, no problem. If, however, the reaction takes place in a closed environment, (you forget to leave the cover open) watch out!!
Then there is the “classic” gunpowder recipe:
4KClO3(s) + 3S(s) + 3C(s) —-> 3SO2(g) + 3CO2(g) + 4KCl(s)
Here again, we start with three solids and end up with six moles of gas. The huge volume of gas formed, having no place else to go, is funneled along a gun barrel and pushes bullets along. Some of the energy released causes the action/reaction bump along the shooter’s hand or shoulder. (Full disclosure: I HATE guns!).
In addition to the noise, one often feels a “shock wave”. This is caused by the sudden compression of the atmosphere in the vicinity of the explosion.
So,there you have it, boys and girls. Chemistry can be fun!!