If the attractions are large, then a lot of energy will have to be used to separate the ions - the lattice enthalpy will be large.
Forces of attraction are greatest if the distances between the ions are small. The size of the lattice enthalpy is governed by several factors, one of which is the distance between the centres of the positive and negative ions in the lattice.
If you think carefully about what happens to the value of the overall enthalpy change of the decomposition reaction, you will see that it gradually becomes more positive as you go down the Group.Įxplaining the relative falls in lattice enthalpy The oxide lattice enthalpy falls faster than the carbonate one. You can apply Hess's Law to this, and find two routes which will have an equal enthalpy change because they start and end in the same places.įor reasons we will look at shortly, the lattice enthalpies of both the oxides and carbonates fall as you go down the Group. The cycle we are interested in looks like this: The term we are using here should more accurately be called the "lattice dissociation enthalpy".
In that case, the lattice enthalpy for magnesium oxide would be -3889 kJ mol -1. Lattice enthalpy is more usually defined as the heat evolved when 1 mole of crystal is formed from its gaseous ions. Here's where things start to get difficult! If you aren't familiar with Hess's Law cycles (or with Born-Haber cycles) and with lattice enthalpies (lattice energies), you aren't going to understand the next bit.
0 kommentar(er)
