Question

Why would it be unreasonable to study the enthalpy of formation of Mg0(s) by directly using your Styrofoam calorimeter? Why is Hess's law a good way to study this reaction? Give one other example of a reaction that could not be studied using a Styrofoam container. (Hint: What solvents are used dissolve polystyrene?)

Answer 1

It is unreasonable to study enthalpy of formation of MgO using styrofoam calorimeter because of the reaction evolves too much heat and toxic substances. However enthalpy of formation we can find using indirect way by calculating a change in enthalpy of related series of reactions, that is using Hess's Law.

Hess's law is a good way to study this reaction because it will be calculated based on the change in enthalpy related to a series of reactions as discussed earlier.

Hess Law states that enthalpy of formation of MgO can be calculated by summation of the enthalpy changes for the reaction of Mg & MgO with diluted acid solution and enthalpy of liquid water formation.

MgO + 2H+ --> Mg2+ +H2O           .......................

Mg + 2H+ --> Mg2+ +H2                   ....................

H2 + 1/2 O2---->H2O                        .....................

Mg +1/2 O2----> MgO                    -------------------------

from this

from knowing enthalpy of first 3 reactions we can find the enthalpy of reaction 4.

example of a reaction that could not be studied using a Styrofoam calorimeter

2C(s)+H2(g) ---->C2H2 (g)