Answer:
ΔH₍reaction₎ = ΔH₍ hydrogen gas bond-breakage₎ + ΔH₍fluorine gas bond-breakage₎ + 2.ΔH₍hydrogen fluoride bond-formation₎ .
Step-by-step 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₎.