![]() : Roughly, somewhere around half of the lattice enthalpy of a salt comes solely from the binding of the smallest electrically neutral agglomerate that is, it takes about as much energy to break a macroscopic solid NaCl crystal into a gas of ion pairs as it does to break all the ion pairs and create a true plasma. If you look up the atomic distance of $NaCl(g)$, you could estimate the enthalpy of (2) pretty accurately and get a numeric answer. It should be obvious that process (1) cost much more energy than what you get out of process (2) because you are going from 6 nearest neighbors to just one. ![]() So to get the required energy, you could first separate all ions (lattice energy) and then calculate the energy of forming individual pairs of sodium and chloride ions (assuming the interaction is purely ionic). If you are interested in this process, you could write it as: what is the product? If we boil sodium chloride at 1 atm pressure, it produces mostly NaCl monomers in the gas phase, see. What are the initial and final temperatures, what is the pressure? What does it mean to vaporize solid NaCl or any ionic compound, i.e. Source: diagram adapted from the cited textbook.īut do we need to add this energy again when we are trying to vaporize an ionic compound? All these processes are experimentally accessible except for the last, so you can determine the lattice energy this way. We can get the same energy in a thought experiment (path 2 in the diagram), first turning all species into gas, then into atoms, then transferring the electron and - finally - forming the lattice from the individual anions and cations. As the diagram below shows, we know the enthalpy of reaction for: This would be difficult to do experimentally, but the value may be determined by using a Born Haber cycle. Here is the textbook explanation ( Fleming: Physical Chemistry): The lattice energy is the energy required to separate the ions in an ionic lattice so that they are at infinite distance (but still ions). Now, according to wikipedia, NaCl has a lattice energy of −756 kJ/mol.įirst, we have to understand the term lattice energy.
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