When magnesium metal is burned in air (Figure 3.5), two products are produced.
One is magnesium oxide, \(\mathrm{MgO}\). The other is the product of the
reaction of \(\mathrm{Mg}\) with molecular nitrogen, magnesium nitride. When
water is added to magnesium nitride, it reacts to form magnesium oxide and
ammonia gas. (a) Based on the charge of the nitride ion (Table 2.5), predict
the formula of magnesium nitride. (b) Write a balanced equation for the
reaction of magnesium nitride with water. What is the driving force for this
reaction? (c) In an experiment a piece of magnesium ribbon is burned in air in
a crucible. The mass of the mixture of \(\mathrm{MgO}\) and magnesium nitride
after burning is \(0.470 \mathrm{~g}\). Water is added to the crucible, further
reaction occurs, and the crucible is heated to dryness until the final product
is \(0.486 \mathrm{~g}\) of \(\mathrm{MgO}\). What was the mass percentage of
magnesium nitride in the mixture obtained after the initial burning? (d)
Magnesium nitride can also be formed by reaction of the metal with ammonia at
high temperature. Write a balanced equation for this reaction. If a
\(6.3-\mathrm{g} \mathrm{Mg}\) ribbon reacts with \(2.57 \mathrm{~g}
\mathrm{NH}_{3}(\mathrm{~g})\) and the reaction goes to \(\mathrm{com}\) pletion,
which component is the limiting reactant? What mass of
\(\mathrm{H}_{2}(\mathrm{~g})\) is formed in the reaction? (e) The standard
enthalpy of formation of solid magnesium nitride is \(-461.08 \mathrm{~kJ} /
\mathrm{mol}\). Calculate the standard enthalpy change for the reaction between
magnesium metal and ammonia gas.
