[INeedSerotonin]

Among the alternatives below, find the one which contains the reaction whose enthalpy variation should be, necessarily, measured by indirect ways, using the law of Hess and using experimental values of formation enthalpy and combustion enthalpy.



Could you guys please give me a hint here? The answer is (D). I think that it is because only CH₄ can suffer combustion among all of the alternatives and is actually alone in one of the sides of the reaction.

[AWK]

https://www.khanacademy.org/science/chemistry/thermodynamics-chemistry/enthalpy-chemistry-sal/v/hess-s-law-example

[INeedSerotonin]

Thank you! So, in order to solve this exercise, I already had to know this specific reaction? Is it a classical example of Hess's law and a prerequisite for this exercise? Or perhaps I could solve it without previously knowing this reaction, but knowing Hess's law?

[AWK]

If it is not possible to carry out the synthesis reaction of a compound directly from elements or compounds with known enthalpies of formation, one must find a way that, using Hess's law, will lead to this.

[mjc123]

I think b should read H₂ +1/2O₂  H₂O, otherwise B would be a candidate. In any case it doesn't balance.

[Enthalpy]

D would be inconvenient to carry out and to measure, so the heat of this reaction is best computed from the heat of formation of the reactants and products.

Though, this reaction does happen. If you put hot graphite, say at 2500K or 3000K, in hydrogen, the graphite disappears, and rather quickly. Sad enough, because otherwise it's a good refractory material.