In a previous video we used Kulom's law to predict the energy it would take to break or form an ionic bond. Remember that Kulom's bond says the larger the charge is or the smaller the distances that greater that energy would be. The energy is required to break or form Kovalev bonds are generally found experimentally. Remember if you're going to break a bond you have to put energy into the system that's endothermic and as you form new bonds energies released and that's exothermic. This chart in the text shows a variety of bond energies for various Kovalev bonds. Let's use this chart to predict the enthalpy change for a chemical reaction. Let's see if we can find out how much energy is involved when you combine hydrogen gas and bromine liquid to make hydrogen bromide gas. The reaction again takes hydrogen gas H2 and bromine liquid which is BR2 as a liquid and we're going to make HBR gas and to balance it I will need two HBRs. Now to find the energy change the first thing we have to do is we have to break the bonds. So the bonds that we're going to break is we're going to break a hydrogen hydrogen bond and then we are going to break a bromine bromine bond. Remember to break bonds you have to put energy in. I'm going to take 432 kilojoules per mole of hydrogen and there's only one mole of hydrogen here plus 193 kilojoules per mole and that's one mole of bromine so I'm okay there. The amount of energy that I'm putting in is positive 625 kilojoules per mole of each of my reactants but that's the energy being put in. I'm then going to release energy by forming new bonds and the new bond that I'm forming is this HBR bond right here. But now there are two moles of that so the energy that's being released is going to be negative 2 times 363 kilojoules per mole. Negative because we are releasing energy we're not absorbing energy. I'm going to be releasing negative 726 kilojoules of energy. That means the total energy for the process is well I'm absorbing 625 kilojoules of energy by breaking the bonds. I'm releasing 726 kilojoules by forming new bonds so for the change in enthalpy for the overall process I'm going to have negative 101 kilojoules of energy or I'm going to release 101 kilojoules of energy for this reaction. So this is an exothermic process. This value is an approximation. There's more to finding the delta H for a reaction than simply breaking and forming new bonds. You also have to account for changes in states of matter. But this gives you a good idea that this is an exothermic reaction and about how much energy you can expect to be released. In this reaction we're forming hydrogen bromide gas from its elements, hydrogen bromide. So that value we calculated is called a heat of formation. The amount of energy it takes to make a compound from its base elements. If you look up the heat of formation for a hydrogen bromide you get negative 36.45 kilojoules per mole. Well in our reaction we made two moles of hydrogen bromide so we could take that value and double it and get negative 72.9 kilojoules of energy. That's how much energy would be actually released in this reaction. We calculated the value of negative 101 which is close but not exact. And as I said before using those bond energies is just an approximation. It gives you an idea of whether a reaction is endothermic or exothermic and about how much energy would actually be released. One final thought to wrap up this video. The zoomed also show a relationship between the length of a bond, the distance between one nucleus and another nucleus when atoms are bonded together. And the strength of the bond, the values that we just looked at those tables. What you should see here is that there is an inverse relationship. The stronger the bond the shorter it gets. So for example we have a carbon-carbon single bond, a carbon-carbon double bond and a carbon-carbon triple bond. It should be no surprise that the single bond is weaker than the double bond which is weaker than the triple bond. And if you look at the length of the bond you'll see that the single bond is the longest bond, the double bond is in the middle, and the triple bond is the shortest of the bonds. So the stronger your covalent bond, the shorter the bond will be.