[Music] hi and welcome back to free science lessons. co.uk by the end of this video you should be able to describe what's meant by a reversible reaction and by equilibrium you should then be able to describe the energy changes taking place in reversible reactions now we've seen a lot of reactions so far in GCSE chemistry I'm showing you an example here in this reaction magnesium is reacting with oxygen to make the compound magnesium oxide now you'll notice that the arrow points in One Direction only in other words from left to right and we can see that here this tells us that this reaction only goes forward in other words magnesium oxide will not turn back to form magnesium and oxygen and in fact most of the reactions that you see in gcsc chemistry will look just like this one I'm showing you a different reaction here as you can see in this reaction when we heat the compound ammonium chloride it reacts to form ammonia and hydrogen chloride now if we take the products in other words the ammonia and the hydrogen chloride and cool them down they now react together to reform the ammonium chloride like this so as you can see this reaction has a different kind of Arrow showing that the products can react to reform the original reactants re reactions like this are called reversible reactions we can change the direction of reversible reactions by changing the conditions so in this case we make the reaction go forwards by heating it and we make the reaction go backwards by cooling it down now I should point out that each reversible reaction is different for example some reactions move forwards if we cool them down okay we're going to look now at the energy changes taking place in reversible reactions this is quite a straightforward topic I'm showing you here a chemical called hydrated copper sulfate which is blue if we heat hydrated copper sulfate then it reacts to form anhydrous copper sulfate which is white the reaction also produces water now because we're heating this we're putting energy in this tells us that the forward reaction is endothermic okay now if we take our anhydrous copper sulfate and add the water back then the reaction reverses like this now the key Point here is that in the reverse reaction energy is released in fact the reaction gets hot this tells us that the reverse reaction is exothermic so here are the key facts if a reversible reaction is exothermic in One Direction it's endothermic in the opposite direction and the same amount of energy is transferred in each case now I should point out that you need to learn the copper sulfate reaction as it's often asked in exams okay there is one final point about reversible reactions and that's the idea of equilibrium imagine that we're carrying out this reaction but now the reaction is taking place in a sealed container the container stops any reactants or products from escaping at some point the forward and the reverse reactions will take place at exactly the same rate scientist call this point equilibrium and it's worth learning that definition remember you'll find plenty of questions on reversible reactions in my vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to describe what's meant by a reversible reaction and by equilibrium you should then be able to describe the energy changes taking place in reversible reactions [Music]