Energy changes and bonding
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Which is greater for ammonium chloride, the lattice energy or the hydration energy?
This is a tricky one because to compare the two we would have to envisage the existence of the ammonium ion in the gaseous state. This is an impossibility because ammonium ions dissociate on heating to ammonia and hydrogen ions. NH4+(g) is therefore an abstraction and any information about it would have to be found indirectly.
One indirect indication is given by the experimental observation that ammonium chloride dissolves endothermically. This would suggest that the lattice energy is numerically greater than the sum of the two hydration energies for the ions. The difference is not very great and is more than compensated for by the increase in entropy of the system on dissolving, which is why ammonium chloride dissolves although purely energetic considerations are against it.
Ulex
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updated: 24 October 2003
