[Music] hi and welcome back to free science lessons. co.uk by the end of this two-part video you should be able to use Bond energies to calculate the energy change for a chemical reaction this can look quite tricky but I'll show you a method that always works we've seen that some reactions are exothermic remember that in exothermic reactions energy is transferred from the chemicals to the surroundings this means that in exothermic reactions energy changes are shown as negative that's because energy has been transferred from the chemicals in endothermic reactions the chemicals gain energy from the surroundings so endothermic energy changes are shown as positive to show that energy has been gained in this video we're going to see how we can calculate energy changes and to do this we need to look at the idea of bond energies when we break a chemical bond this requires energy and is endothermic making bonds releases energy and is therefore exothermic every chemical bond has an energy value and this tells us the energy required to break that Bond so we're going to start by looking at a sample question hydrogen plus chlorine produces hydrogen chloride here's the balanced symbol equation we're going to calculate the energy change for this reaction to do this we need two pieces of information and and both of these are given in the exam first we need the structures of the molecules involved that way we can see all the different bonds so here are the structures secondly we need to know the energy value for each Bond so here they are we now need to look at the reaction on the left hand side we're breaking chemical bonds so these bonds are being broken on the right hand side we're making chemical bonds so this bond is being made we now need to put the energy values in Breaking the Bond between the hydrogen atoms requires 436 K per mole remember that breaking bonds is endothermic so energy is gained that means we need to write a positive sign so here's the value for breaking that bond breaking the bond between the chlorine atoms requires 242 K per mole and again this needs a positive sign so here is that value if we add these values together we get plus 678 K this is the total amount of energy needed to break all of these chemical bonds on the right hand side we're making the bond between the hydrogen and the chlorine atoms this releases 431 KJ per mole so here's that value now you'll notice that we're actually making two molecules of hydrogen chloride which means that we're making two lots of this Bond Bond so we need to multiply this value by two this gives us a total value of 862 K of energy by making these bonds remember that making bonds releases energy in other words it's exothermic this means that energy leaves the reaction so we need to put a negative sign in front of this value and here it is now we're ready to calculate the final energy change for the reaction it's very straightforward simply move the right hand value beneath the left hand value like this and then carry out the sum so we've got 678 minus 862 which gives us a final energy change of minus 184 K this shows us that this reaction is exothermic hopefully you're getting the idea that this is actually fairly straightforward in the next video I'll give you a couple more examples to try yourself [Music]