 to enthalpy. A directly analogous relationship applies to enthalpy. The enthalpy of a reaction is just like a price per unit. It tells you how much energy is released or absorbed per mole of reactant. And in fact we can express it as per mole of product but we'll get to that at the latest stage. So the relationship looks like this. The enthalpy in kilojoules per mole equals the energy that's released or absorbed in kilojoules divided by the amount of reactant in moles. So let's take ethanol for instance. Ethanol is the alcohol that's in wine and spirits but in its pure form it makes a great fuel. So say you wanted to work out the enthalpy of ethanol burning in oxygen and you set up an experiment where you bought burn 0.780 moles of ethanol and you measure that that releases 1070 kilojoules. Well you can now work out the enthalpy of reaction by dividing the energy by the moles of reactant. That's 1070 kilojoules divided by 0.780 moles and that gives you 1,370 once you've rounded off your significant figures. Now when you burnt that ethanol you noticed that the heat was released so the surroundings became hotter. So that value of the enthalpy 1,370 kilojoules per mole has to be made negative minus 1,370 kilojoules per mole. So what this means is when one mole of ethanol burns in sufficient oxygen it releases 1,370 kilojoules to the surroundings. So here's another case. Now you know the enthalpy of combustion for ethanol and you set up another experiment where you know that you've just burnt 2.40 moles of ethanol. You can rearrange that equation and use it to calculate the amount of energy that should be released when you burn that much ethanol. So we rearrange it and we get that the energy equals the enthalpy times the moles of reactant which equals minus 1,370 kilojoules per mole times 2.40 moles which equals minus 3,290 kilojoules. That means 3,290 kilojoules of energy should be released when 2.40 moles of ethanol are burnt. Okay the third possibility is say you know the enthalpy as we do and we could measure the energy that was released during a reaction then that would allow us to calculate how much ethanol had been burnt. For instance you run the experiment say you measure that 15.3 kilojoules of energy has been released then we can rearrange the equation so that the amount of reactant is the subject and we get the moles of reactant equals the energy released over the enthalpy of the reaction and that equals minus 15.3 kilojoules minus because it's been released divided by minus 1,370 kilojoules per mole and that equals 0.0112 moles of ethanol must have been burnt. So as with a unit price of cherries as long as you have two of the three quantities in this relationship you can work out the last one. We can now take this further by combining this enthalpy relationship with what we know about stoichiometry. So where you know the mass of a reactant or the mass of a product and the chemical equation that governs the reaction you can do all sorts of calculations that relate to the amount of reactants and products that are used or produced and the energy that is absorbed or released and in the next video we'll look at a couple of calculations of that sort.