[Music] hi and welcome back to frees 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 in the last video we saw how to calculate the energy change for a reaction using Bond energies so in this video I'm going to give you two examples to try yourself here's the first one you have to calculate the energy change for this reaction we've got two molecules of hydrogen reacting with one molecule of oxygen to make two molecules of water I'm showing you the structures of these molecules and the bond energies here so pause the video and calculate the energy change for this reaction okay so you can see that on the left hand side we've got to break these bonds and on the right hand side we're going to make these bonds The energy needed to break the bond between two hydrogen atoms is 43 6 K per mole however we've got two molecules of hydrogen so we've got two of these bonds we need to put in 872 K remember that the positive sign shows us that we're putting energy in to break these bonds the energy required to break the oxygen double bond is 495 KJ per mole and again the plus sign shows that we're putting energy in adding these values together gives us a total energy input of Plus 1,36 67 K the bond energy for an oxygen to hydrogen bond is 463 K per mole we need to make this negative because we're making this Bond and making bonds is exothermic each water molecule has two of these bonds and we've got two molecules of water so we've got four of these Bonds in total multiplying - 463 by 4 gives us a total energy value ofus 1,852 k to calculate the overall energy change we subtract 1,852 K from 1,367 K giving us a total of minus 485 k for the reaction this shows that the reaction is exothermic as energy has been released here's one final example for you to try this shows the complete combustion of methane and oxygen pause the video and work out the energy change okay so on the left hand side we can see that we're going to break all of these bonds and on the right hand side we're going to make these bonds so first we've got four carbon de hydrogen bonds in the methane molecule the bond energy of a carbon de hydrogen bond is 413 K per mole multiplying this number by four gives us a value of 1,652 this needs to be positive as breaking bonds is endothermic we've also got to break the double bond in the oxygen molecule the bond energy of this is 495 K per mole however as you can see we've got two molecules of oxygen so we've got to multiply this number by two this gives us 990 and remember that this also has to be positive adding together 1,652 and 990 gives us a total bond energy of Plus 2,642 on the right hand side we're going to make these Bonds in the carbon dioxide molecule the energy of the carbon to oxygen bond is 799 KJ per mole however each molecule contains two carbon to oxygen bonds so we need to multiply this by two giving us 1,598 we also need to make this negative as we're making these bonds okay next we're going to make the oxygen to hydrogen bonds in water the bond energy of an oxygen to hydrogen bond is 463 K per mole and we've got two of these Bonds in each molecule of water however we've got two water molecules so we've got four of these Bonds in total multiplying 463 by 4 gives us a total of 1,852 remember this has to be a negative number as we're making these bonds adding together- 1,598 and - 1,852 gives us a total of - 3,450 to work out the total energy change for the reaction we subtract 3,450 from 2 , 642 this gives us a value of- 88 K and that's our final answer showing that this reaction is exothermic you'll find plenty more examples of bond energy calculations in my vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to use Bond energies to calculate the energy change for a chemical reaction [Music]