[Music] hi and welcome back to free science lessons by the end of this video you should be able to use lelia's principle to predict the effect of changing pressure on reversible reactions at equilibrium in the last video we looked at Leer's principle Leer's principle states that when an external change is applied to a system at equilibrium the equilibrium moves in the direction that reduces the effect of that change in this video we're looking at how lelia principle can be applied to reversible reactions involving gases I'm showing you here the reaction for the harbor process which is a reversible reaction in this reaction the gas's nitrogen and hydrogen are reacted to form the gas ammonia because this is a reversible reaction the ammonia can also convert back to nitrogen and hydrogen with reactions involving gases we can change the position of the equilibrium by changing the pressure now there's a key idea here that you need to understand gas pressure is proportional to the number of moles of gas present the equation shows us that one mole of nitrogen reacts with three moles of hydrogen to make 2 moles of ammonia we have a total of four moles on the reactant side and two moles on the product side so what that means is that if the equilibrium was entirely on the left in other words just reactants the pressure would be twice as great as if the equilibrium was entirely on the right in other words just products okay so imagine that this reaction has reached equilibrium what would happen to the position of the equilibrium if we increase the pressure well going back to luchia's principle we know that when an external change is applied to a system with equilibrium the equilibrium moves in the direction that reduces the effect of that change we're increasing the pressure so this means that the equilibrium will move in the direction that reduces the pressure as we've seen the right hand side of the equation has fewer moles than the left hand side so that means that in order to reduce the pressure the equilibrium will move towards the right okay now I'd like you to use lelia's principle to describe what would happen to the equilibrium if we were reduce the pressure of this reaction so pause the video and try this yourself okay if we reduce the pressure then the equilibrium will move in the direction that increases the pressure in this reaction because we have more moles on the left of the equation the equilibrium will move towards the left here's another reaction involving gases in this reaction the gas D nitrogen tetroxide which is colorless forms the gas nitrogen dioxide which is brown I'd like you to use lelia's principle to work out what will happen to the position of the equilibrium if we increase the pressure so pause the video now and try this yourself okay if we increase the pressure then the equilibrium will move in the direction that reduces the pressure on the left hand side we have one mole of D nitrogen tetroxide however on the right hand side we have two moles of nitrogen dioxide so that means that if we increase the pressure the equilibrium will move towards the left to reduce the pressure the reaction mix will become less brown as nitrogen dioxide converts to dinitrogen tetroxide okay now there is one final point that you need to understand the position of equilibrium is only affected by pressure if the total number of moles is different on either side of the equation in the reaction I'm showing here hydrogen and BR brine react to form hydrogen bromide on the left we have two moles of gas and on the right we also have 2 moles of gas so this means that changing the pressure has no effect on the position of equilibrium of this reaction in the next video we look at the effect of temperature on reversible reactions [Music]