[helenee]

I am a bit confused here because if there was N2(g) within the vessel, then shouldn't it have reacted with NaHCO3(s)? Why does the correct answer only show how NaHCO3 decomposes?

Also, I don't get why the question even mentions vessel 2 — none of the answer choices deal with NaBr(s).

[chenbeier]

The picture is given for question 7 to 9. The first quesetion (7) asked only the comparison between vessel 1 and 2. At 127 ° C there is no much change in vessel 2, but in vessel one bicarbonate decomposes. That is the reason.
In vessel 5 aslo something happen., but this is not asked.

[Enthalpy]

N₂ serves as unreactive gas: cheaper than Ar, suffices often. At 127°C with not too exotic reactants, expect it to do nothing.