You will see that I have arbitrarily decided to draw this for lattice formation enthalpy.However, the difference is small, and negligible compared with the differing values for lattice enthalpy that you will find from different data sources.
If you are doing a course for 16 - 18 year olds, none of this really matters - you just use the numbers you are given. These came from the Chemistry Data Book edited by Stark and Wallace, published by John Murray. Values from this now fairly old book often differ slightly from more recent sources. Just dont assume that any bit of data you are given (even by me) is necessarily right. Remember that energy (in this case heat energy) is given out when bonds are made, and is needed to break bonds. In other words, you are looking at a downward arrow on the diagram. In other words, you are looking at an upward arrow on the diagram. ![]() The lattice dissociation enthalpy is the enthalpy change needed to convert 1 mole of solid crystal into its scattered gaseous ions. The lattice formation enthalpy is the enthalpy change when 1 mole of solid crystal is formed from its scattered gaseous ions. If the sign is positive, for example, it must refer to breaking bonds, and therefore to a lattice dissociation enthalpy. Strictly speaking, because I havent added a sign to the vertical axis, the values are for lattice dissociation enthalpies. If you prefer lattice formation enthalpies, just mentally put a negative sign in front of each number. Thats because in magnesium oxide, 2 ions are attracting 2- ions; in sodium chloride, the attraction is only between 1 and 1- ions. The next bar chart shows the lattice enthalpies of the Group 1 chlorides. Caesium chloride has a different packing arrangement of ions in its crystal, and that has a small effect on the lattice enthalpy. The effect is small enough that it doesnt actually affect the trend. It is even more difficult to imagine how you could do the reverse - start with scattered gaseous ions and measure the enthalpy change when these convert to a solid crystal. Lattice enthalpies calculated in this way are described as experimental values. In fact, in this case, what you are actually calculating are properly described as lattice energies. ![]() You will quite commonly have to write fractions into the left-hand side of the equation.
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