(a) Does the lattice energy of an ionic solid increase or decrease (i) as the charges of the ions increase, (ii) as the sizes of the ions increase? (b) Arrange the following substances not listed in Table 8.1 according to their expected lattice energies, listing them from lowest lattice energy to the highest: MgS, KI, GaN, LiBr.

Lattice energy is the energy required to separate one mole of an ionic solid into its constituent gaseous ions. It depends on the charges of the ions and the distance between them. The lattice energy can be calculated using the Born-Lande equation or the Born-Haber cycle, but for this exercise, we will stick to the basic concept. A key principle to keep in mind is that the larger the charge on the ions and the smaller the distance between the ions, the greater the lattice energy.

Now, let's address part (a) of the exercise. (i) As the charges of the ions increase, the electrostatic force between the ions increases, which results in a higher lattice energy. (ii) As the sizes of the ions increase, the distance between the ions also increases, which results in a lower lattice energy.

For part (b), we should consider the ion charges and sizes for each substance to rank their lattice energies. Let's analyze each substance: 1. MgS: Magnesium ion (Mg^(2+)) has a charge of +2 and sulfur ion (S^(2−)) has a charge of -2. Both Mg^(2+) and S^(2−) are relatively small ions. 2. KI: Potassium ion (K^+) has a charge of +1 and iodide ion (I^−) has a charge of -1. Both K^+ and I^− are larger ions than Mg^(2+) and S^(2−). 3. GaN: Gallium ion (Ga^(3+)) has a charge of +3 and nitride ion (N^(3−)) has a charge of -3. Both Ga^(3+) and N^(3−) are relatively small ions. 4. LiBr: Lithium ion (Li^+) has a charge of +1 and bromide ion (Br^−) has a charge of -1. The Li^+ ion is smaller than K^+, but the Br^− ion is larger than I^−. Considering the information above, we can arrange the substances in order of their expected lattice energies from lowest to highest: Lowest Lattice Energy: KI (Lowest charges and larger ions) < LiBr (Lowest charges and smaller ions) < MgS (Higher charges and larger ions) < GaN (Highest charges and smaller ions): Highest Lattice Energy So, the final order is: KI, LiBr, MgS, GaN.