You have a series of four chemical reactions (1→2→3→4).
How do you calculate the ΔH2 if you know the other ΔH values?

ΔH2=ΔH1+ΔH3 + ΔH4

ΔH2=(ΔH1)(ΔH3)/ (ΔH4)

ΔH2=ΔH4−(ΔH1 + ΔH3)

ΔH2=ΔH1+ΔH3 − ΔH4

Explanation:

The question must be restated because you can calculate ΔH₂ from the other reactions only if you know the global ΔH, ΔH₁, and ΔH₃.

As stated by Hess’s law, the overall change in the enthalpy, ΔH, of a reaction does not depend of the path followed.  Thus, when you have a series of reactions,  the net change from the first reaction to the last reaction is equal to the sum of the ΔH values for the individdual reactions:

If you name ΔH₄, the overal change in the enthalpy to convert 1 to 4, but ΔH₁, ΔH₂, and ΔH₃ are the changes of the intermediate reactions that transform 1 in 2, 2 in 3, and 3 in 4, respectively, then, you can write, accordingly to Hess's law, you can write:

Then, assuming you know ΔH₄, ΔH₁, and ΔH₃, you can calculate ΔH₂ clearing from the previous equation:

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