Standard Enthalpy of Formation

[Sasha Gladkikh 2A]

For the standard enthalpy of formation of a substance, should its chemical equation always represent the formation of 1 mole of that substance?

For example, for the standard enthalpy of formation of ethanol (CH₃CH₂OH_(l)), should its chemical equation be written as:

(1) 4C_(s) + 6H_2(g) + O_2(g) → 2CH₃CH₂OH_(l)

OR

(2) 2C_(s) + 3H_2(g) + ½O_2(g) → CH₃CH₂OH_(l)

[Caden Ciraulo 1J]

The standard enthalpy of formation does account for the formation of 1 mole, but the reaction will be the empirical formula. Therefore, 1/2 O₂ will not be written. The reaction will be the first reaction you wrote, but the enthalpy of formation will still be half of the enthalpy of the reaction because the standard enthalpy of formation will be for one mole of the molecule.

[Rhona_McChesney_1F]

Yes, for the standard enthalpy of formation of a substance, the chemical equation you use to calculate this enthalpy should always represent the formation of 1 mole of that substance. It may not be given to you as such and you may have to multiply or divide the formula by some factor so that it describes the formation of 1 mole of the the substance. Most of the time I don't think these types of problems will be graded on whether or not you write the correct empirical formula with whole number coefficients, but in whether or not you used the chemical formula in its form that represents 1 mole of the substance in question.

[Hannah_Pon_1F]

Yes, the standard enthalpy of formation should always be considered as per 1 mole of the product. The "standard" indicates that either it is the enthalpy value of either 1 mol or 1 Molar.

[kylanjin]

It doesn't matter too much as long as you use the ratios of the equation, since it works out the same either way. E.g., from the example you provided which was from lecture, the enthalpy change of the reaction is -555.38 kJ. to calculate the enthlapy of formation for 1 mole of ethanol, you could just divide the reaction enthalpy by the coefficient of 2 on ethanol.

[Amanda Pineda 3H]

It will be the first equation you wrote, however when you calculate the standard enthalpy of formation for that equation, you will divide the answer by 2 since it is per one mole and the equation has 2 moles of ethanol. Hope this helps!