I photographed this helium balloon at my cousin's 40th wedding anniversary last summer. I can remember the wedding. Must be getting old... Anyway, it reminded me of an important rule in science about gases. Equal volumes of different gases always contain the same number of gas molecules.
Now consider this reaction:
All 3 of these chemicals are gases. Now, there is 3 hydrogen molecules for every nitrogen molecule, so hydrogen gas will have a volume 3 times the nitrogen. In fact, there are 4 molecules on the left hand side of the equation (one molecule of nitrogen and three molecules of hydrogen) but there are only two molecules on the right hand side. So if we are making ammonia, the reactants will take up twice as much space as the product. If we increase the pressure around the reaction, there will be less space available and that will favour the forward reaction that makes ammonia, because in the end the ammonia takes up less space than the nitrogen and hydrogen. We can also make the reaction go faster by heating it up, but high temperature tend to make the ammonia fall apart back into nitrogen and hydrogen. This reaction is complicated because you can make ammonia but it can also fall apart again. So we need to choose conditions that make as much ammonia as quickly as possible with as little as possible falling apart, and we need to do it as cheaply as possible.
