Answer to Question #70972 in General Chemistry for Racquel Dillard

Question

why can't the change in enthalpy for the formation of magnesium oxide be measured directly in a coffee cup calorimeter

Answer 1

The heat absorbed when a mole of compound in its standard state is formed at a given temperature from the appropriate elements, also in their standard states, is the molar enthalpy of formation, ΔH0f, of the compound at that temperature. The molar enthalpy of formation is positive if heat is absorbed when the compound is formed under these conditions (an endothermic reaction) and negative if heat is released (an exothermic reaction).
The molar enthalpy of formation of MgO(s) is simply the enthalpy of the reaction
(1) Mg(s) + 1/2 O2(g, 1 atm) → MgO(s) ∆H1
While it is possible to tabulate enthalpies of formation for a variety of substances, the enthalpy cannot always be directly measured for some reactions. One of such reaction is the reaction of formation MgO(s), which proceeds slowly at room temperature. So to determine the enthalpy of reaction for the reaction given above, we need to use the enthalpies of reaction for other reactions, which are easier to carry out, and safer.