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I had a question on standard enthalpy of formation discussed in class. What exactly is it? Dr. Lavelle mentioned that for some molecules, the standard enthalpy of formation is 0 and thus we cannot find the value on a chart (ex: diatomic hydrogen; H2). Why exactly is this so? Thank you in advance!
The standard enthalpy of formation is related to standard reaction enthalpies. Since reactants and products can be in different states and reactions can occur at different pressures, the enthalpy can change. The standard reaction enthalpies found on pg. 17 of the course reader allow for uniformity during experiments. The standard enthalpy of formation is just the standard reaction enthalpy used to form one mole of a substance when the elements are most stable. Since the elements would be most stable as they are, the enthalpy is 0. It's like "making" water by pouring water into a container, there's no energy involved.
You won't always be given the ΔH of the reaction. It depends on the question and what kind of information the question provides. To provide an example, when we use the second method he described during lecture, you're given the bond enthalpies for what bonds are formed and broken. From that given information, we can figure out what the ΔH of the reaction. I guess the way I'd put it is that the ΔH of the reaction is the standard enthalpy for the given reaction (you don't have to modify the moles for anything in the original reaction) whereas in that example specifically you'd need to divide by 2 moles of ethanol to find out the standard enthalpy of formation for ethanol in terms of kJ/mol.
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