# Enthalpy change of thermal decomposition

For benzene, carbon and hydrogen, these are:.

## How to calculate enthalpy change

So this is all abstract and everything, and I've told you about Hess's Law. Unless otherwise specified, all reactions in this material are assumed to take place at constant pressure. And we know what those are. Or essentially, how much heat is absorbed or released by this reaction. And water, on the other hand, it releases energy when you form it from its elemental form. In the combustion of methane example, the enthalpy change is negative because heat is being released by the system. Use the decomposition of ammonium nitrate as an example of an enthalpy calculation. I had some propane, and I'm going to combust it. And it would also be minus the same thing for B. You can do calculations by setting them out as enthalpy diagrams as above, but there is a much simpler way of doing it which needs virtually no thought. So Hess's Law tells us that delta H of this reaction, the change in enthalpy of this reaction, is essentially going to be the sum of what it takes to decompose these guys, which is the minus heat of formations of these guys, plus what it takes to reform these guys over here. That doesn't make it any harder! Use Table 3 for the next two problems. And it's ammonia gas.

And am I starting in the gaseous or the aqueous state? It's exothermic. Hopefully you found that helpful. See Table 2.

## Enthalpy change of thermal decomposition

So let me just do that. If more energy is produced in bond formation than that needed for bond breaking, the reaction is exothermic and the enthalpy is negative. So let me write that down. So we have to multiply this times the number of moles. I have labelled the vertical scale on this particular diagram as enthalpy rather than energy, because we are specifically thinking about enthalpy changes. Unless otherwise specified, all reactions in this material are assumed to take place at constant pressure. So if I were to give you 1 mole of propane, and I were to combust it with enough oxygen, I'll produce 2, kilojoules that's released from the system. Because if I combust one mole of propane, I get this much heat.

That's just for one mole of ammonia, the heat of formation. A thermochemical equation is a chemical equation that includes the enthalpy change of the reaction. And you get 1, It's minus Refer again to the combustion reaction of methane. And water. The reaction is and the enthalpies of the three compounds are given in Table 1.

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