Which would have the larger lattice energy—an ionic compound consisting of a large cation and a large anion or one consisting of a large anion and a small cation? Explain your answer and any assumptions you made. How would the lattice energy of an ionic compound consisting of a monovalent cation and a divalent anion compare with the lattice energy of an ionic compound containing a monovalent cation and a monovalent anion, if the internuclear distance was the same in both compounds?
Explain your answer. Which would have the larger lattice energy—CrCl 2 or CrCl 3 —assuming similar arrangements of ions in the lattice? Which cation in each pair would be expected to form a chloride salt with the larger lattice energy, assuming similar arrangements of ions in the lattice?
Explain your reasoning. Which cation in each pair would be expected to form an oxide with the higher melting point, assuming similar arrangements of ions in the lattice?
Learning Objectives To understand the relationship between the lattice energy and physical properties of an ionic compound. Note the Pattern An ionic lattice is more stable than a system consisting of separate ion pairs.
Calculating Lattice Energies The lattice energy of nearly any ionic solid can be calculated rather accurately using a modified form of Equation 4. Note the Pattern Lattice energies are highest for substances with small, highly charged ions. Given: four compounds Asked for: order of increasing lattice energy Strategy: Using Equation 4. The Relationship between Lattice Energies and Physical Properties The magnitude of the forces that hold an ionic substance together has a dramatic effect on many of its properties.
Note the Pattern High lattice energies lead to hard, insoluble compounds with high melting points. Summary Ionic compounds have strong electrostatic attractions between oppositely charged ions in a regular array. Key Takeaway The lattice energy is usually the most important energy factor in determining the stability of an ionic compound.
JoVE Core Chemistry. Previous Video Next Video. Next Video 9. Embed Share. Lattice energy is associated with the formation or separation of an ionic lattice.
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Please click here to activate your free hour trial. If you do not wish to begin your trial now, you can log back into JoVE at any time to begin. Save to playlist. Filter by:. Get cutting-edge science videos from J o VE sent straight to your inbox every month. The data in the table below show that the lattice energies for salts of the OH - and O 2- ions increase rapidly as the charge on the ion becomes larger.
When a salt, such as NaCl dissolves in water, the crystals disappear on the macroscopic scale. The lattice energy of a salt therefore gives a rough indication of the solubility of the salt in water because it reflects the energy needed to separate the positive and negative ions in a salt.
Born-Haber Cycle There are several important concept to understand before the Born-Haber Cycle can be applied to determine the lattice energy of an ionic solid; ionization energy, electron affinity, dissociation energy, sublimation energy, heat of formation, and Hess's Law.
Ionization Energy is the energy required to remove an electron from a neutral atom or an ion. This process always requires an input of energy, and thus will always have a positive value.
In general, ionization energy increases across the periodic table from left to right, and decreases from top to bottom. There are some excepts, usually due to the stability of half-filled and completely filled orbitals. Electron Affinity is the energy released when an electron is added to a neutral atom or an ion. Usually, energy released would have a negative value, but due to the definition of electron affinity, it is written as a positive value in most tables. Therefore, when used in calculating the lattice energy, we must remember to subtract the electron affinity, not add it.
In general, electron affinity increases from left to right across the periodic table and decreases from top to bottom. Dissociation energy is the energy required to break apart a compound. The dissociation of a compound is always an endothermic process, meaning it will always require an input of energy. Therefore, the change in energy is always positive. The magnitude of the dissociation energy depends on the electronegativity of the atoms involved.
Sublimation energy is the energy required to cause a change of phase from solid to gas, bypassing the liquid phase. This is an input of energy, and thus has a positive value. It may also be referred to as the energy of atomization. The heat of formation is the change in energy when forming a compound from its elements. This may be positive or negative, depending on the atoms involved and how they interact. Hess's Law states that the overall change in energy of a process can be determined by breaking the process down into steps, then adding the changes in energy of each step.
Using the Born-Haber Cycle The values used in the Born-Haber Cycle are all predetermined changes in enthalpy for the processes described in the section above. Step 1 Determine the energy of the metal and nonmetal in their elemental forms. Step 2 The Born-Haber Cycle requires that the elements involved in the reaction are in their gaseous forms. Step 3 Metals exist in nature as single atoms and thus no dissociation energy needs to be added for this element.
Step 4 Both the metal and nonmetal now need to be changed into their ionic forms, as they would exist in the ionic solid. Step 5 Now the metal and nonmetal will be combined to form the ionic solid.
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