in today's video we're going to look at reversible reactions and see what we mean by the terms equilibrium and the position of equilibrium now most chemical reactions look something like this with the arrow in the middle pointing to the right to indicate that the reactants can turn into the products this means that it's a one-way reaction once the carbon dioxide is formed it won't break back down into carbon and oxygen in other reactions though like this one here we have a double arrow which tells us that the reaction is reversible the top arrow tells us that ammonium chloride can break down into ammonia and hydrogen chloride which we call the forward reaction while the bottom arrow tells us that the ammonia and hydrogen chloride can also combine to reform the ammonium chloride which we call the backward reaction so because both of these can happen we call it a reversible reaction now the important thing to understand is that the forward and backward reactions can take place at different rates at the beginning of the reaction there will be loads of ammonium chloride but no ammonia or hydrogen chloride so the forward reaction will be really fast but the backward reaction won't have started yet as the reactants turn into products though the forward reaction will slow down and the backward reaction will speed up after a while the rate will even out and the forward and backward reactions will be going at exactly the same speed at this point the concentrations of the reactants and products won't change anymore so we can say that the reaction is at equilibrium so to clarify at equilibrium both reactions are still happening but they effectively cancel each other out so there's no overall change in the concentrations of the reactants or products of course in reality the molecules are still breaking down and reforming but because they do so at the same rate there's no overall change in the concentrations now something to point out here is that just because the concentrations of the reactants and products are constant while at equilibrium does not mean that they're the same as each other for example there could be lots of ammonium chloride and only a small amount of ammonia and hydrogen chloride or there could be lots of ammonia and hydrogen chloride but only a small amount of ammonium chloride in either case though as long as the forward and backward rates are the same then the reaction would still be at equilibrium however what would change when we have these different concentrations is the position of the equilibrium when there were more products like this we would say that the equilibrium lies to the right but if we had more reactants and fewer products again then we'd say that the equilibrium lies the left this position of equilibrium can change depending on the conditions for example adding heat to this reaction encourages the forward reaction meaning we'll have relatively more products as the position of equilibrium moves to the right on the other hand if we cool the conditions of our reaction it would push the position of equilibrium back to the left so it would have more ammonium chloride again regardless of its position though equilibrium can only be reached if the reversible reaction is done in a closed system which means some kind of sealed environment from which none of the reactants or products can escape because if the products kept escaping it would never reach equilibrium the last thing we need to cover is that reversible reactions are always exothermic in one direction and endothermic in the other for example in this reaction which shows the thermal decomposition of hydrated copper sulfate to anhydrous copper sulfate and water the forward reaction is endothermic and so the backward reaction is exothermic if you're not familiar with these terms hydrated just means that water is present and anhydrous means that there's no water now because the forward reaction is endothermic it requires heat energy from the surroundings to work so if we were to heat some hydrated copper sulfate which exists as blue crystals it will drive the reaction to the right which evaporates off the water and leaves white anhydrous copper sulfate powder if we then took away the heat and added some water to this powder though it would drive the backwood reaction so the equilibrium would shift to the left and reform our blue crystals of copper sulfate and because this reaction is exothermic it would re-release all of that energy it had just absorbed in the forward reaction so to quickly recap everything reversible reactions are those with a double arrow in the middle which shows that they can react in both forward and backward directions one of which will have to be exothermic and the other endothermic and if these two rates are the same then we say that the reaction is at equilibrium and the concentrations of the reactants and products will remain constant the position of equilibrium can change though and will shift to the left foot or the right depending on the conditions anyway that's all for today so hope you enjoyed it and we'll see you next time