Hess's Law

by Jared Rovny

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    00:01 We can use this enthalpy change very usefully in a given reaction and it goes like this: suppose we have some reaction that goes some products, some chemicals, from A to B, and then goes from that new state and another chemical reaction to state C.

    00:18 In this chemical reaction, it would be the case that the energy to go from A to B and then from B to C would be the same change in enthalpy to go all the way from A to C directly.

    00:31 So we would say that the change in enthalpy from A to B plus the change in enthalpy from B to C is exactly the same as the change in enthalpy to go from A directly to C.

    00:43 Here's the reason this is useful.

    00:45 Suppose we have some chemical reaction that might be very complicated.

    00:48 In this case we have CH4 and a couple of oxygen molecules.

    00:52 And these react and form instead carbon dioxide into water molecules.

    00:57 It might be very tricky when you're just looking at this reaction to know is this endorthermic, is it exothermic, how much enthalpy change is there.

    01:05 So one way to think about this is that to cause this reaction since as we just discussed, we can go from A to B to C, We can insert sort of an intermediary step just for ourselves to think about.

    01:16 So this intermediary step goes like this: We first try to break apart all of our bits into separate components.

    01:23 So we sort of take everything apart, dissassemble everything in our entire reaction.

    01:27 In this case, the only thing we have to this to dissassemble is the CH4, and the reason for this is that we have to be careful the enthalpy, the standard enthalpies of formation are again referring to the standard elemental forms of these molecules as find we them in nature.

    01:43 So for example, oxygen, we don't need to worry about trying to break apart into two different oxygens because in nature we never just see individual single oxygen atoms floating around.

    01:53 The standard form for oxygen is O2.

    01:56 So the only thing we actually have to break apart is the carbon with the four hydrogens.

    02:01 So breaking apart the carbon and the four hydrogens in this intermediate step.

    02:06 We have the standard enthalpy formation for this CH4 molecule.

    02:12 So then, what we would do is now that we've broken everything into all its little bits, we put all those little bits back together and assemble it in the other way.

    02:20 The way that we have the CO2 and the two water molecules.

    02:23 So, why is this a useful way to think about things? Why would we take this extra route through this lower path here? The reason this is useful is that this standard enthalpies of formation, the enthalpy required to create certain types of compounds from their standard elemental forms of nature are well tabulated; these have been studied and studied, and we have simple tables for all of these standard enthalpies of formation.

    02:46 So for a complicated chemical reaction like this one, I don't have to go and try to find how CH4 goes to CO2.

    02:53 I can simply say that the CH4 with the two oxygen molecules will break apart into its different components and then I can put them back together all sort of conceptually in my head.

    03:03 And then I can find what the total change of the enthalpy of the entire reaction is.

    03:08 I simply say that for this reaction, first of all, I had a change in enthalpy to break everything apart.

    03:15 And since we're breaking it apart, we're adding this enthalpy.

    03:17 This would be a minus term which you'll see on the far right of our equation, the standard change in enthalpy for the CH4.

    03:24 And then to put everything back together I would have the standard enthalpies of formation for both, the CO2 molecule and the two H2O molecules.

    03:33 So I can very easily, from a table of enthalpies, find the total change in enthalpy of this reaction by thinking in my head of this whole system breaking apart into small bits, and then putting the whole system back together just using the standard enthalpies of formation for each step from A to B to C.

    About the Lecture

    The lecture Hess's Law by Jared Rovny is from the course Thermodynamics and Thermochemistry.

    Included Quiz Questions

    1. The total enthalpy change in a reaction is independent of which steps or pathway is taken to go from the initial state to the final state.
    2. Zero of entropy is to take place at the absolute zero of temperature.
    3. If two systems are in thermal equilibrium with a third system, then these two systems are in thermal equilibrium with each other.
    4. Energy in a closed system is conserved.
    5. The entropy of a system will always increase.

    Author of lecture Hess's Law

     Jared Rovny

    Jared Rovny

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