in today's video we're going to take a look at bond energies and see how we can use them to determine whether a reaction will be exothermic or endothermic when we say bond energy what we're actually referring to is the amount of energy needed to break one mole of a particular covalent bond so if i had an atom of hydrogen bonded to an atom of chlorine and said that the bond energy was positive 431 kilojoules per mole that means we'd have to supply 431 kilojoules of energy to break one mole of these bonds and remember one mole would be 6.02 times 10 to the 23 bonds which is avogadro's constant so breaking bonds is actually an endothermic process because it requires energy from the surroundings on the other hand bond formation is exothermic so if we were to form one mole of hydrogen chloride bonds it would release 431 kilojoules of energy to the surroundings so if we took a simple reaction like hydrogen plus chlorine goes to form two hydrogen chloride and we were to think about it in terms of molecules then we could see that for the reaction to work we'd have to break this hydrogen hydrogen bond and this chlorine chlorine bond which would both require energy and be endothermic processes and then would form two of these hydrogen chlorine bonds which would release energy and so be exothermic processes to determine if the overall reaction is exothermic or endothermic though we'd have to compare the total amount of energy required to break these bonds to the total amount of energy released when these bonds form in an exam you'd only be given a list or a table with all the bond energy values that you'll need and to work out the overall energy change we just do the energy required to break bonds minus the energy released by forming bonds which in our case would be 436 for the hydrogen hydrogen bond plus 242 for the chlorine chlorine bond minus 2 times 431 for the bonds within the two hydrogen chloride molecules which simplifies to 678 minus 862 or negative 184 kilojoules per mole and if you get a negative value like this it means the reaction must be exothermic as the chemicals are losing 184 kilojoules of energy to their surroundings before we finish let's try one more example nitrogen and hydrogen react together in the following reaction calculate the overall energy change for this reaction now although you don't have to i think it's definitely easier if the first thing you do is draw out all the molecules using their displayed formula so that you can actually see the bonds the left we can see that we'll need to break one nitrogen nitrogen triple bond and three hydrogen hydrogen bonds while on the right we'll have to form six nitrogen hydrogen bonds so if you recall our equation which stated that the overall energy change equals the energy required for break bonds minus the energy released by forming bonds we can just plug in our numbers so 1 times 941 plus 3 times 436 which we can put in one big bracket minus 6 times 391 which simplifies to 2 330 minus 2 346 giving us a minus 16 kilojoules per mole which makes it an exothermic reaction anyway that's all for today so if you enjoyed it then give us a like and subscribe and we'll see you next time