Lattice Energy - Online Tutor, Practice Problems & Exam Prep

Lattice energy is a crucial concept in understanding ionic solids and their properties. It refers to the change in energy when separate gaseous ions combine to form an ionic solid, resulting in an exothermic reaction where energy is released to create new bonds. The more negative the lattice energy, the more exothermic the reaction. Conversely, lattice dissociation energy describes the endothermic process where energy is absorbed to break an ionic solid into gaseous ions, indicated by a positive lattice energy value. The lattice energy formula, derived from Coulomb's law, is used to determine the strength of ionic bonds and is represented as the product of the charges of the cation and anion divided by the sum of their radii. Higher lattice energy correlates with stronger ionic bonds, leading to higher boiling and melting points but lower solubility. This relationship is pivotal in predicting the physical properties of ionic compounds.

Lattice Energy

Here we're going to say that lattice formation energy is the change in energy when separated gaseous ions combined to form an ionic solid. So here we're going to have sodium ion in its gaseous state, Chloride ion in its gaseous state. Opposite charges causes them to combine together to give us NaCl solid.

Realize here that we're forming bonds, so it is an exothermic reaction. That's why ΔH here is negative. Now in an exothermic reaction, we release energy in order to create this new bond and realize here that the more negative the lattice energy value than the more exothermic the reaction becomes.

So just remember, when we're dealing with lattice formation energy, we have gaseous ions as reactants and they combine together to give us a mole of an ionic solid.

Lattice Energy Example 1

So here it says the lattice formation of magnesium bromide is the energy change for which of the following processes. So remember in a lattice formation reaction we have the ionic gases combining to form an ionic solid.

So here magnesium is in Group 2A so it's two plus boards charge so this is good. Bromine's in Group 7/8 so it's -1 so this is good. And then remember they combine together to form an ionic solid, not a gas. So this is out the answer.

The only answer that can work is option three. None of the others work because they don't have ions to begin with that combine to form an ionic solid. So here our answer would be choice C.

Lattice Energy

Under lattice formation energy, we talked about ions forming an ionic solid, but in lattice dissociation energy we're doing the opposite. Here it's the change in energy of 1 mole of solid crystal as it's scattered into gaseous ions.

So here we would have our sodium chloride solid and what would happen is we would split it up into its ions here, so gaseous sodium ion and gaseous chloride ion. This reaction is endothermic. Here we absorb energy in order to break the bond in order to split it up into its ions, and because we're absorbing energy.

That's why ΔH now is positive in terms of sine. Now we're going to say here, the more positive your lattice energy value, then the more endothermic the reaction will be.

Lattice Energy Example 2

Here it says which of the following reactions is associated with the latticed association of lithium oxide. Now with lattice association, we start out with a solid ionic compound. So we're starting out with lithium oxide solid. That means that these three are out.

And remember it's an endothermic process, so we absorb energy to break this ionic solid into gaseous ions. So they have to have charges and they have to be in their gaseous state. That means option 4, which corresponds to choice D, would be the right answer.

Lattice Energy

Here we're going to take a look at the lattice energy formula. We're going to say by simplifying Coulomb's law, a general formula for lattice energy can be used to determine ionic bond strength. We're going to say here, the higher the lattice energy than the stronger the ionic bond.

Now the formula is lattice energy equals |Q1|×|Q2|r. This translates into the absolute value. So that means the answer's always going to be positive on top of the cation charge times the anion charge, and R here is radius. But when we simplify it, it means that we're adding the radius of the cation and the radius of the anion.

Now realize here that the radius of the ion equals the period or row of the periodic table. So using this simplified lattice energy formula, we can compare different ionic bonds.

Lattice Energy Example 3

So here it's saying which compound possesses the strongest ionic bond. We have magnesium bromide versus potassium chloride. So for magnesium bromide we break it up into its ions, so that breaks up into magnesium 2 plus and BR -1 for their lattice energy. Remember it's their charges in absolute terms, so plus 2 * -1 divided by their period numbers added together.

So magnesium, if you look on the periodic table is in row 3, Bromine is in row 4, so that is 2 substance for its lattice energy. Potassium chloride made-up of potassium ion and chloride ion. Look and see where they are on the periodic table. So remember we multiply their charges in absolute terms. Now look on the periodic table. Potassium is in the 4th row of the periodic table, so. 4. Chlorine is in the third row, so here lattice energy equals 17.

So if we compare the two we see that magnesium bromide has a larger lattice energy and therefore it has a stronger ionic bond. Remember, the stronger your your lattice energy or higher lattice energy than the stronger your ionic bond will be.

The lattice energy for ionic crystals decreases as the charge of the ions _____ and the size of the ions _____.

• A. Increases, increases
• B. Increases, decreases
• C. Decreases, increases
• D. Decreases, decreases

Lattice Energy

Now the lattice energy of an ionic solid has a direct impact on its physical properties. Here we can say recall and increase in the lattice energy causes an increase in the strength of an ionic bond. And we can say here as we increase the lattice energy our ionic bond gets stronger, so the ions are held more tightly together.

That would result in a higher boiling point, higher melting point, but then a lower solubility. Solubility is how easily things break up and separate. But if you have a lot of lattice energy, you're held more together tightly. Your ionic bond is stronger.

So just remember the direct relationship between lattice energy, ionic strength, boiling point and melting point. But the inverse relationship, lattice energy has two solubility.

Lattice Energy Example 4

Here it says choose the compound below that should have the highest melting point according to the ionic bonding model. So remember, highest melting point means highest lattice energy. When you break these all up into their ions, based on the groups that they're found in, we'll be able to figure out the charge. Let's see here. Remember group number and their charges.

Now we're going to say lattice energy for all of them. In absolute brackets, we have their charges multiplied together. On the bottom we're going to have the periods that they're found in. So here all I'm doing is I'm inputting the charges for each one O. Remember that's the first part we need for lattice energy, O. Here we're going to multiply. There are charges in absolute terms, so for all of them the number will be positive.

Now take the periods or rows that they're found on the periodic table and they're going to go on the bottom, but they're added together. So this would be 3+2. This would be 3+2, three plus 2 and 3+3. Now we're going to bring everything down. So this is 95. This is 15, this is 45. And finally this one here is 16.

The one with the largest lattice energy is aluminum nitride, and because of that it'll have the highest melting point. So just remember, the higher lattice energy, the higher your melting point, the higher your boiling point, and remember the lower your solubility. I know the question doesn't talk about boiling point and solubility, but it's important to remember these relationships.

Which of the following compounds would you expect to have the highest boiling point?

• A. MgCl₂
• B. SrO
• C. SrCl₂
• D. CsI
• E. LiBr

The solubilities of CaCrO₄ and PbCrO₄ in water at 25°C are approximately 0.111 g/L and 0.0905 g/L in H₂O respectively. Based on this information, which compound do you think has the smaller lattice energy?

• A. CaCrO₄
• B. PbCrO₄

Here’s what students ask on this topic:

What is lattice energy?

Lattice energy is the amount of energy that is released when ions in the gaseous state come together to form an ionic solid. In other words, it's the energy required to fully separate a mole of a solid ionic compound into its constituent ions in the gas phase. It's a measure of the strength of the bonds that hold the ionic solid together, which is why compounds with high lattice energy are typically more stable and have higher melting points.

Lattice energy is influenced by the charge on the ions and the size of the ions. The greater the charge and the smaller the ion size, the stronger the force of attraction between the ions, and thus the higher the lattice energy. It's an important concept in chemistry because it helps explain the properties of ionic compounds, including their stability, solubility, and melting points. Calculating lattice energy can be complex, but it often involves using the Born-Haber cycle, which relates lattice energy to other measurable energies involved in the formation of an ionic compound.

How do you calculate lattice energy?

Lattice energy is the energy released when ions in the gaseous state come together to form an ionic solid. It's a measure of the strength of the bonds in that ionic compound. The Born-Haber cycle is a common method used to calculate lattice energy indirectly, as direct measurement is not feasible.

Here's a simplified explanation of the process:

1. Write the formation equation of the ionic compound from its elements.
2. Determine the enthalpy change for each step of the Born-Haber cycle:

• Sublimation energy of the solid metal to gaseous atoms.
• Ionization energy of the gaseous metal atoms to form cations.
• Bond dissociation energy for diatomic non-metal molecules if necessary.
• Electron affinity for the non-metal to form anions.
• Heat of formation for the ionic compound from its elements.

1. Apply Hess's Law, which states that the total enthalpy change for a reaction is the same, no matter how many steps the reaction is carried out in.

The lattice energy can be calculated using the equation:

Lattice Energy = ( Sum of enthalpies of steps 1 to 4 ) - Heat of formation

Remember, the lattice energy can be either a positive or negative value, but it is often expressed as a positive value to represent the energy.

How can one find lattice energy?

Lattice energy is a measure of the strength of the forces between the ions in an ionic solid. It is defined as the energy required to separate a mole of an ionic compound into its constituent ions in the gas phase. To find the lattice energy, you typically use the Born-Haber cycle, which is an application of Hess's law. This cycle combines several steps, including the ionization energy of the metal, the electron affinity of the non-metal, the sublimation energy of the metal, and the enthalpy change of the reaction to form the ionic compound from its elements.

Since direct measurement is challenging, lattice energy can also be estimated using theoretical calculations. One such method is by using the Kapustinskii equation, which provides an approximation based on the charges of the ions, the interionic distance, and a constant that applies to all ionic compounds. These calculations often require knowledge of physical constants and properties of the elements involved, as well as access to relevant chemical data tables or computational chemistry software.