Question 14

[bcapper22053]

My understanding is that only II is correct since the heat of formation is of one mole of product and in I there are 2 moles of product formed. Thus the heat of reaction would need to be divided by 2 in order to get the heat of formation.

[emilie.maz5603]

No, please see explanation below:

In a formation equation, it is defined that 1 mole of a compound forms from its elements. Thus, answer A (I and II) is correct, as for equation II (II. Mg(s) + C(s) + (3/2)O2(g) --> MgCO3(s)) you are correct there is only one mole of MgCO3 (s) formed and that is therefore justified. However, for equation I (2Mg(s) + O2(g) --> 2MgO(s)), there are 2 moles MgO(s) product formed (CORRECT) however you may also divide the complete equation by 2 to obtain: 1Mg(s) + ½ O2(g) --> 1MgO(s) which gives one mole of MgO and the heat of formation is defined for that equation. Thus, when 1 mol of a compound is produced from its elements, the heat of reaction (ΔH) is called the heat of formation (ΔHf) regardless of the rearrangement of the equation divided by 2 or whatever, it still remains the same as the heat of formation remains the same.

So, as for the explanation given above cited as follows (ie. ΔHformation is the enthalpy associated with the reaction that forms a compound from its elements in their most thermodynamically stable states. Reaction III is the only one in which the reactants are not all in their most thermodynamically stable elemental states: the reactants are not elements. In other words, the reactants could be oxygen, magnesium or carbon but not in combination, only as separate elements (part of the definition of heat or enthalpy of formation)) IT IS CORRECT! Thus, answer A is fine.