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I think, BC, that EvilFrog is actually after a slightly more in-depth explanation! Presumably it's the reactivity of the Mg relative to the N2 bond strength?Chris
Why do these metals form nitrides on heating in air?Nitrogen is often thought of as being fairly unreactive, and yet all these metals combine with it to produce nitrides, X3N2, containing X2+ and N3- ions.Nitrogen is fairly unreactive because of the very large amount of energy needed to break the triple bond joining the two atoms in the nitrogen molecule, N2.When something like magnesium nitride forms, you have to supply all the energy needed to form the magnesium ions as well as breaking the nitrogen-nitrogen bonds and then forming N3- ions. All of these processes absorb energy.This energy has to be recovered from somewhere to give an overall exothermic reaction - if the energy can't be recovered, the overall change will be endothermic and won't happen. Note: This is a bit of a simplification! In order to find out whether a reaction is feasible, you have to consider free energy changes and not just whether the reaction is exothermic or endothermic. If you don't know anything about free energy changes, don't worry about it. The simplification is valid in this particular case.Energy is evolved when the ions come together to produce the crystal lattice. This energy is known as lattice energy or lattice enthalpy.The size of the lattice energy depends on the attractions between the ions. The lattice energy is greatest if the ions are small and highly charged - the ions will be close together with very strong attractions. In the whole of Group 2, the attractions between the 2+ metal ions and the 3- nitride ions are big enough to produce very high lattice energies.When the crystal lattices form, so much energy is released that it more than compensates for the energy needed to produce the various ions in the first place. The excess energy evolved makes the overall process exothermic.This is in contrast to what happens in Group 1 of the Periodic Table (lithium, sodium, potassium, rubidium and caesium). Their ions only carry one positive charge, and so the lattice energies of their nitrides will be much less.Lithium is the only metal in Group 1 to form a nitride. Lithium has by far the smallest ion in the Group, and so lithium nitride has the largest lattice energy of any possible Group 1 nitride. Only in lithium's case is enough energy released to compensate for the energy needed to ionise the metal and the nitrogen - and so produce an exothermic reaction overall.In all the other cases in Group 1, the overall reaction would be endothermic. Those reactions don't happen, and the nitrides of sodium and the rest aren't formed.