[Music] I'm welcome back to free size lessons cold UK by the end of this video you should be able to describe the effective change in the pressure on a reversible reaction at equilibrium and this is for higher-tier students only in the last couple of videos we've looked at what happens to reversible reactions are at equilibrium when we change the conditions we saw that if a system is at equilibrium and the changes made to the conditions then the system responds to counteract the change and we call this lush ateliers principle in this video we're going to look at the effect of pressure on reversible reactions now the first key factors are pressure effects reactions involving gases I'm showing you here two gases these are at the same temperature and in the same volume container as you can see the container on the right has twice the number of molecules as a container on the left this brings us to the second key fact the pressure of a gas depends on the number of molecules so that means that the gas on the right will have twice the pressure as the gas on the left now the pressure of a gas does depend on other factors but in this video were just focusing on the number of molecules take a look out of this reaction we've got nitrogen reacting with hydrogen to make the chemical ammonia and as you can see this is a reversible reaction if we look at the large numbers we can see the number of molecules in this reaction on the left hand side we've got one molecule of nitrogen and three molecules of hydrogen in other words we've got four molecules in total on the left hand side on the right hand side we've got two molecules of ammonia so how does pressure affect reactions like this you need to learn these rules if we increase the pressure on a reversible reaction our equilibrium the position of the equilibrium shifts to the side with the smaller number of molecules and if we reduce the pressure then the position of the equilibrium shifts to the side with the larger number of molecules so if we increase the pressure on this reaction then the equilibrium will shift to the right-hand side that's because the right-hand side has fewer molecules than the left and if we reduce the pressure on this reaction then the equilibrium will shift to the left-hand side and again that's because the left-hand side has more molecules than the right I'm showing you another reaction here in this case the chemicals iodine and hydrogen are reacting to form hydrogen iodide and again this is a reversible reaction if we look at the number of molecules we can see that we've got two molecules on the left hand side and two molecules on the right hand side now because the number of molecules is the same on both sides changing pressure has no effect on the position of equilibrium remember you'll find plenty of questions on reversible reactions in my revision workbook and you can get that by clicking on the link above [Music]