Gunpowder is a mixture of potassium nitrate (KNO3), sulfur (S), and charcoal (C). The substance that makes the gunpowder explode is the KNO3. The nitrate (NO3) unit is a molecule made out of nitrogen and oxygen and the atoms in this molecule are unhappy in their state. They would rather be pure nitrogen (N2) and pure oxygen (O2). However, there is an energetic barrier that prevents them from achieving their preferred state.
When setting the gunpowder on fire, the heat from the fire supplies the energy needed to overcome this barrier and the NO3 molecule can release its nitrogen and oxygen. The nitrogen likes to stay alone and becomes (N2), the oxygen further reacts with the carbon (C) and forms CO2. The potassium from the KNO3 reacts with the sulfur (S) to K2S.
The chemical reactions is therefore:
2 KNO3 + S + 3 C -> K2S + N2 + 3 CO2
After the combustion, the molecules in the gunpowder reacted and formed molecules that have a lower energy. The energy difference between the initial state (gunpowder before it is set on fire) and the final state (the products after the gunpowder was set on fire) is released during the explosion. If we would want to bring the gunpowder back to its original form, we would have to bring this energy back into the powder in order to transform the molecules back. However, there are two problems with this. First, the concept of "Entropy" or also known as the "Second Law of Thermodynamics".
When we set the gunpowder on fire, besides releasing energy, we are also increasing the entropy of the system. This means that during the explosion we converted more valuable energy (in the gunpowder) into less valuable energy (heat). A process like this is called irreversible - it can not be brought back to its starting point.
Therefore, the gunpowder can not be brought back to its original form. This concept can be imagined as the melting of an ice cube. If we take an ice cube out of the freezer, it will melt. This means that it will take up heat from its surrounding air and melt. The melted water will not transform back into the ice cube by just releasing its heat to the surrounding air, cooling down, and freezing again. It will not do that by itself and that is what is described by entropy. However, you could now say that you will just put the water back in the freezer and it will form the ice cube again and this is correct. But, this means that it won't use the same energy to freeze again, but rather new energy from the freezer.
We could apply the same idea to our gunpowder and just use "new" energy to bring it back to its original form rather than the exact same energy released by the explosion. This brings us to the second problem. There is no synthetic chemical reaction that can transform the K2S, the N2, and the CO2 back into KNO3, S, and C.
All this means that the gunpowder can not be brought back to its original form after being set on fire.
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