![]() There are two different ways of defining lattice enthalpy which directly contradict each other, and you will find both in common use. Two different ways of defining lattice enthalpy Just don't assume that any bit of data you are given (even by me) is necessarily "right"! It doesn't affect the principles in any way. Values from this now fairly old book often differ slightly from more recent sources.ĭon't worry about this. These came from the Chemistry Data Book edited by Stark and Wallace, published by John Murray. If you use my chemistry calculations book, you will find a slightly different set of numbers. If you are doing a course for 16 - 18 year olds, none of this really matters - you just use the numbers you are given. In the Born-Haber cycles below, I have used numbers which give a consistent answer, but please don't assume that they are necessarily the most accurate ones. ![]() Note: While I have been writing this section, the different values for the same piece of data from different data sources has driven me crazy, because there is no easy way of knowing which is the most recent or most accurate data. Unless you go on to do chemistry at degree level, the difference between the two terms isn't likely to worry you. However, the difference is small, and negligible compared with the differing values for lattice enthalpy that you will find from different data sources. In fact, there is a difference between them which relates to the conditions under which they are calculated. Lattice enthalpy and lattice energy are commonly used as if they mean exactly the same thing - you will often find both terms used within the same textbook article or web site, including on university sites. The explanation is that silver chloride actually has a significant amount of covalent bonding between the silver and the chlorine, because there isn't enough electronegativity difference between the two to allow for complete transfer of an electron from the silver to the chlorine.This page introduces lattice enthalpies (lattice energies) and Born-Haber cycles. In other words, treating the AgCl as 100% ionic underestimates its lattice enthalpy by quite a lot. Depending on where you get your data from, the theoretical value for lattice enthalpy for AgCl is anywhere from about 50 to 150 kJ mol -1 less than the value that comes from a Born-Haber cycle. The experimental and theoretical values don't agree.Ī commonly quoted example of this is silver chloride, AgCl.That means that for sodium chloride, the assumptions about the solid being ionic are fairly good. Sodium chloride is a case like this – the theoretical and experimental values agree to within a few percent. There is reasonable agreement between the experimental value (calculated from a Born-Haber cycle) and the theoretical value.You won't be expected to be able to do these calculations at this level, but you might be expected to comment on the results of them. There are several different equations, of various degrees of complication, for calculating lattice energy in this way. If you know how to do it, you can then fairly easily convert between the two. ![]() By doing physics-style calculations, it is possible to calculate a theoretical value for what you would expect the lattice energy to be.Īnd no – I am not being careless about this! Calculations of this sort end up with values of lattice energy, and not lattice enthalpy. Let's also assume that the ions are point charges – in other words that the charge is concentrated at the centre of the ion. Let's assume that a compound is fully ionic.
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