Exothermic reactions are energy releasing. A great deal of heat is released during such reactions. However this does not make an explosive condition.
Explosive reactions need to:
- release heat energy quickly so as to accelerate the gas particles to high speeds;
- have products that are gases;
- have close to 100% of the reactants react quickly.
Click to see the result of an explosive chemical reaction. Notice the destructive force of the expanding gases inside the plane.
Not all exothermic reactions are explosive. This depends on how fast the reaction takes place. Some exothermic reactions react slowly enough to be used in heat pack and to warm up food. Others take place fast enough to be used as fuels or to propel bullets.
Matter exists in three states solids, liquids and gases. Gas particles are very energetic and can be accelerated easily by absorbing heat energy. It is this sudden increase in kinetic energy (moving energy) that causes the destructive pressure wave to form.
This building was destroyed by the pressure wave created by a truck bomb made of several drums of petrol covered with ammonium nitrate, a common fertiliser.
Such devastating pressure waves are visible when observing an exploding bomb from above. The video on the right shows B-52 bombers dropping explosives during the Vietnam war. Notice the pressure waves created by each explosion.
A psychological advantage for an army operating in remote conditions and traveling too fast for supply lines to keep up is the provision of hot meals. Here chemistry can play a major role.
The heat source for the self heated can comes from a chemical exothermic reaction. This reaction starts when the user presses on the bottom of the can to break a seal and mix the reactants in a separate compartment.. The can consists of three chambers. A chamber for the beverage surrounds a central chamber of calcium oxide (quicklime) that is separated from a container of water by a thin breakable membrane. When the user pushes on the bottom of the can, a membrane is broken and mixing of water and calcium oxide begin. The resulting chemical reaction releases heat and warms the beverage surrounding it.
The chemical reaction is represented by the equation below.
Other reactions produce more heat than the hydration of quicklime. Why is CaO used instead of, say, anhydrous copper sulfate? View the video on the right of the heat released when water is added to anhydrous copper sulfate.
When warming up food using chemical reactions, is the production of a gaseous product of concern? Explain.
Particles in solids are
Particles in liquids are
Particles in gases are
When a gas is heated the kinetic energy of most gas particles increases. This is obvious when
Why is it important for an explosive chemical reaction to release heat energy quickly?
Why is the formation of gas important for such an explosive reaction?
