[ThatGuy]

Examine the following thermochemical equation:

H₂ + I₂ + 53 kJ --> 2 HI

a. What is the enthalpy change of the reaction?
b. What quantity of energy is required for 4.57 x 10²⁴ molecules of iodine to react with surplus hydrogen?

a. ΔH = 53 KJ / 2 = 26.5 kJ/mol

b. ( 4.57 x 10²⁴ ) / (6.022 x 10²³) = 7.6 mol Iodine

Therefore: 7.6 mol x 26.5 kJ/mol = 201.4 kJ ?

[Borek]

a. ΔH = 53 KJ / 2 = 26.5 kJ/mol

Question doesn't say anything about "enthalpy change related to what". 53 kJ (without dividing by two) is a perfectly correct answer "per mole of reaction as written", or "per mole of hydrogen/iodine reacting". What you calculated is a correct answer "per mole of HI" - but as the question didn't specify what it really wants, neither of these answers is more correct than the other.

However, question b asks about enthalpy per I₂ - think your answer over.

[ThatGuy]

In terms of enthalpy per I₂: 53 kJ/mol

( 4.57 x 1024 ) / (6.022 x 1023) = 7.6 mol Iodine * 53 kJ/mol = 402.8 kJ ?

Does the energy required change according to the referred compound?

[Borek]

Does the energy required change according to the referred compound?

It can, if the stoichiometric coefficients differ. Note that these are perfectly equivalent - 1 mole of I₂ reacting requires 53kJ, 1 mole of HI produced requires 26.5 kJ, but as there are two moles of HI produced per each mole of I₂, it is in fact the same number.

[ThatGuy]

Does the energy required change according to the referred compound?

It can, if the stoichiometric coefficients differ. Note that these are perfectly equivalent - 1 mole of I₂ reacting requires 53kJ, 1 mole of HI produced requires 26.5 kJ, but as there are two moles of HI produced per each mole of I₂, it is in fact the same number.

Ok. I understand. Thank you for your help.