Textbook Question
(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?
Ch.8 - Basic Concepts of Chemical Bonding
Chapter 8, Problem 29
Energy is required to remove two electrons from Ca to form Ca2+, and energy is required to add two electrons to O to form O2 - . Yet CaO is stable relative to the free elements. Which statement is the best explanation? (a) The lattice energy of CaO is large enough to overcome these processes. (b) CaO is a covalent compound, and these processes are irrelevant. (c) CaO has a higher molar mass than either Ca or O. (d) The enthalpy of formation of CaO is small. (e) CaO is stable to atmospheric conditions.
Verified step by step guidance1
Identify the type of bonding in CaO. Calcium oxide (CaO) is an ionic compound formed from the metal calcium (Ca) and the nonmetal oxygen (O).
Understand the process of ion formation for Ca and O. Calcium loses two electrons to form Ca2+ (calcium ion), and oxygen gains two electrons to form O2- (oxide ion).
Consider the energy changes involved in the formation of ions. The process of removing electrons from calcium (ionization energy) and adding electrons to oxygen (electron affinity) requires energy.
Examine the concept of lattice energy. Lattice energy is the energy released when the positive and negative ions in an ionic compound form a solid lattice. It is a measure of the compound's stability.
Evaluate the statements given in the problem in the context of ionic bonding and lattice energy. The correct statement should explain why the formation of CaO is energetically favorable despite the initial energy required to form ions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Lattice Energy
Lattice energy is the energy released when gaseous ions combine to form an ionic solid. It is a crucial factor in determining the stability of ionic compounds like CaO. A large lattice energy indicates that the electrostatic forces between the ions are strong enough to compensate for the energy required to remove or add electrons, thus stabilizing the compound.
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Lattice Energy
Ionization Energy and Electron Affinity
Ionization energy is the energy required to remove an electron from an atom, while electron affinity is the energy change when an electron is added to an atom. In the case of Ca and O, the energy needed to ionize Ca and add electrons to O must be considered when evaluating the overall stability of the resulting ionic compound, CaO.
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Enthalpy of Formation
The enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. A small enthalpy of formation for CaO suggests that the formation process is energetically favorable, contributing to the stability of the compound despite the energy costs associated with ionization and electron addition.
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Related Practice
Textbook Question
Consider the ionic compounds KF, NaCl, NaBr, and LiCl. (a) Use ionic radii (Figure 7.8) to estimate the cation–anion distance for each compound.
Textbook Question
Which of the following trends in lattice energy is due to differences in ionic radii: a. NaCl > RbBr > CsBr, b. BaO > KF, c. SrO > SrCl2?
Textbook Question
List the individual steps used in constructing a Born–Haber cycle for the formation of BaI2 from the elements. Which of the steps would you expect to be exothermic?
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Textbook Question
Use data from Appendix C, Figure 7.11, and Figure 7.13 to calculate the lattice energy of KI.
Textbook Question
(a) Based on the lattice energies of MgCl2 and SrCl2 given in Table 8.1, what is the range of values that you would expect for the lattice energy of CaCl2?