![]() We begin with the elements in their most common states, Cs( s) and F 2( g). Therefore, NaOH can easily dissolve in water, Mg(OH) 2 is sparingly soluble, and Al(OH) 3 is insoluble.\): The Born-Haber cycle shows the relative energies of each step involved in the formation of an ionic solid from the necessary elements in their reference states. For example, from the above table, one can find that sodium hydroxide (NaOH) has lower lattice energy than magnesium hydroxide (Mg(OH) 2), which is lower than aluminum hydroxide (Al(OH) 3). Salts with higher lattice energy are insoluble in water than lower ones. The lattice energy of ionic salts indicates their solubility in water because it represents the energy needed to separate ions in solution. For example, it can predict the solubility of ionic compounds. ![]() The lattice energy is related to the physical properties of the ionic compound. When magnesium ion (Mg 2+) combines with chloride ion (Cl –), magnesium chloride (MgCl 2) crystal forms, and 2450 kJ of energy is released. When magnesium ion (Mg 2+) combines with oxide ion (O 2-), magnesium oxide (MgO) crystal forms, and 3795 kJ of energy is released. When calcium ion (Ca 2+) combines with chloride ion (Cl –), calcium chloride (CaCl 2) crystal forms, and 2195 kJ of energy is released.Ĭa 2+ (g) + 2 Cl – (g) → CaCl 2 (s) ΔH lattice = -2195 kJ/mol When calcium ion (Ca 2+) combines with oxide ion (O 2-), calcium oxide (CaO) crystal forms, and 3414 kJ of energy is released.Ĭa 2+ (g) + O 2- (g) → CaO (s) ΔH lattice = -3414 kJ/mol When sodium ion (Na +) combines with chloride ion (Cl –), sodium chloride (NaCl) crystal forms, and 787.3 kJ of energy is released. Lattice Energy Trend Lattice Energy Examples Hence, the lattice energy increases from left to right across a period and decreases from top to bottom down a group. Across a period, the atomic charge increases, and down a group, the ionic radius increases. It is clear that the bond between Na + and OH – (NaOH) has the smallest lattice energy, and that between Al 3+ and O 2- (Al 2O 3) has the greatest.įrom the above tables, one can observe that the lattice energy increases with atomic charge and decreases with ionic radius. The following table shows the lattice energies for salts of OH – and O 2. The lattice energy is proportional to the product of the two ionic charges ( ΔH lattice∝ | Q 1Q 2|). In other words, the ionic bond becomes stronger as the charge on the ions becomes large. Atomic charge: As the atomic charge increases, the lattice energy increases. It is clear that the bond between Li + and F – (LiF) has the highest lattice energy and that between Cs + and I – (CsI) has the lowest.Ģ. Write the chemical equation that represents the process of lattice energy for the case of NaCl. The following table shows the lattice energy values (in kJ/mol) for the ionic bond formed between alkali metals and halogens. In other words, the bond between opposite ions is strongest when the ions are small. Ionic radius: As the ionic radius increases, the lattice energy decreases. Nonetheless, NaCl is said to dissolve in water, because evaporation of the solvent returns crystalline NaCl. Lattice energy can be calculated using electrostatics or estimated from the Born-Haber cycle. When, however, an ionic compound such as sodium chloride (NaCl) dissolves in water, the sodium chloride lattice dissociates into separate ions which are solvated (wrapped) with a coating of water molecules. The lattice energy magnitude of an ionic crystal can be determined from the following equation derived from Coulomb’s law.
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