[Music] hi and welcome back to three science lessons cold okay by the end of this video you should be able to describe the effective change in concentration on a reversible reaction at equilibrium and this is for higher-tier students only in the last video we saw that some reactions are reversible and I'm showing an example here we saw that if we carry out a reversible reaction in a sealed container none of the reactants or product can escape at some point the forward and reverse reactions occur at the same rate and scientists call this point equilibrium so in this video we're looking at what happens to equilibrium if we change the conditions just a first key fact if a system is at equilibrium and the changes made to the conditions then the system responds to counteract the change now we're using the word system here but it's okay to simply say the reaction scientists call this louche ateliers principle now the best way to show this is to use examples take a look out of this reaction in this reaction the compound no.2 is reacting to form n 2 O 4 now this is a reversible reaction so the n2o4 can also react to form no.2 so the question is what will happen if we increase the concentration of no.2 in other words add more no.2 into the system remember that at equilibrium the system will counteract any changes that we make so if we change the concentration of the reactants or the products then the system is no longer at equilibrium in other words the forward and reverse reactions are not taking place at the same rate this means that the concentrations of all the substances will change until equilibrium is reached so in this case because we've increased the concentration of no.2 more n2o4 will be formed until equilibrium is reached again here's a question for you to try what would happen if we decrease the concentration of n2o4 pause the video and try this yourself okay so in this case because we've decreased the concentration of n2o4 this system is no longer at equilibrium therefore more no.2 will react to form n2o4 until equilibrium is reached again here's another question for you what would happen if we increase the concentration of n2o4 again pause the video and try this yourself okay again because we've increased the concentration of n2o4 the system is no longer our equilibrium so now more n2o4 will turn into no.2 until equilibrium is reached again we're gonna find plenty of questions on the effect of concentration on reversible reactions in my revision world walk and you can get that by click on the link above okay so hopefully now you should be able to describe the effect of changing concentration on a reversible reaction at equilibrium [Music]