This means we can usually assume that ions with greater magnitude charges will result in greater lattice energies, and without having to take into consideration the atomic radii. Generally the charges of the ions have more bearing than the distance between them when determining lattice energies. If the goal is to maximize the lattice energy then you'd want ions with larger magnitude charges and/or small in size ions. So both the magnitude of the ion's charges and their atomic radii effect the lattice energy. And the denominator consists of of the distance between the two ions, squared. If we observe the equation of Coulomb's Law the numerator consists of the product of the absolute value of the charges of the ions. So lattice energy is measuring how attracted both the ions are to each other in an ionic compound. In ionic compounds the force is always attractive since the ions have different charges. Whether the force is attractive or repulsive depends on whether the charges have the same sign or not. Coulomb's Law describes the force of attraction (or repulsion) between two point charges. Therefore, NaOH can easily dissolve in water, Mg(OH) 2 is sparingly soluble, and Al(OH) 3 is insoluble.Lattice energies of ionic compounds broadly correspond with Coulomb's Law which Sal provided in the video. 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. 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. Lattice energy can be calculated using electrostatics or estimated from the Born-Haber cycle. The lattice energy magnitude of an ionic crystal can be determined from the following equation derived from Coulomb’s law.
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