Based on bond energies, what will be the heat of reaction for the formation of 2 moles of hydrogen fluoride from one mole each of hydrogen gas and fluorine gas?

ΔH₍reaction₎ = ΔH₍ hydrogen gas bond-breakage₎ + ΔH₍fluorine gas bond-breakage₎ + 2.ΔH₍hydrogen fluoride bond-formation₎ .

Explanation:

Reaction:

H₂ + F₂ --> 2.HF      ΔH₍reaction₎

In this reaction, bonds are broken in the hydrogen gas and fluorine gas molecules which then lead to the new bond formation, that is, bond between hydrogen and fluorine atoms which explains the formation of hydrogen fluoride.

ΔH₍reaction₎ = ΔH₍bond-breakage₎ + ΔH₍bond-formation₎

ΔH₍bond-breakage₎ is just ΔH₍ hydrogen gas bond-breakage₎ + ΔH₍fluorine gas bond-breakage₎.

ΔH₍bond-formation₎ is 2.ΔH₍hydrogen fluoride bond-formation₎ .Since the ΔH₍bond-formation₎ is in the units kJ/mol, a factor of 2 comes in the picture

Therefore,

ΔH₍reaction₎ = ΔH₍ hydrogen gas bond-breakage₎ + ΔH₍fluorine gas bond-breakage₎ + 2.ΔH₍hydrogen fluoride bond-formation₎ .

Note: Care should be taken while assigning opposite signs to ΔH₍bond-breakage₎ and ΔH₍bond-formation₎.

we can conclude that the melting point of table salt is above room temperature (it's actually 801 deg c). the melting point of mercury must be below room temperature (- 39 deg c).

the answer is b & c i believe : )