Problem 25 - Chemistry: The Central Science

Question

Energy is required to remove two electrons from Ca to form \(\mathrm{Ca}^{2+}\) and is required to add two electrons to \(\mathrm{O}\) to form \(\mathrm{O}^{2-}\). Why, then, is \(\mathrm{CaO}\) stable relative to the free elements?

Step-by-Step Solution

Answer

CaO is stable relative to its free elements, Ca and O, because the overall energy change in its formation is exothermic (negative). Although energy is required for ionization of Ca and electron affinity of O, the lattice energy released when forming the ionic bond in CaO is greater than the energy required for these processes. This results in a net release of energy, making CaO more stable than its constituent elements.

Step 1: Discuss Ionization Energy

Ionization energy is the energy required to remove an electron from an atom or ion. The first ionization energy is the energy required to remove the first electron, the second ionization energy is the energy required to remove the second electron, and so on. Generally, ionization energy increases as we move across a period from left to right in the periodic table and decreases as we move down a group. In this case, we have to remove two electrons from Ca to form Ca²⁺.

Step 2: Discuss Electron Affinity

Electron affinity is the energy change when an electron is added to an atom or ion. Ionization energy and electron affinity are related in the sense that when an electron is added to an atom, it may release energy (negative electron affinity), or it may require energy (positive electron affinity). In general, electron affinity becomes more negative (exothermic) as we move across a period from left to right in the periodic table, and it tends to become less negative (less exothermic) as we move down a group. In this case, we have to add two electrons to O to form O²⁻.

Step 3: Discuss Lattice Energy

Lattice energy is the energy required to separate one mole of an ionic crystal into its gaseous ions. It is a measure of the strength of the ionic bond in the crystal lattice. Lattice energy is always exothermic (negative). The stronger the ionic bond, the more negative the lattice energy, and the more stable the compound is relative to its free elements. Here, the lattice energy plays a key role in stabilizing CaO.

Step 4: Explain the Stability of CaO

In order to form CaO, energy is required to remove two electrons from Ca (ionization energy) and to add two electrons to O (electron affinity). However, when these ions come together and form an ionic bond in the crystal lattice of CaO, a significant amount of lattice energy is released. The overall energy change in the formation of CaO involves the sum of ionization energy, electron affinity, and lattice energy. Although energy is required to remove electrons from Ca and add electrons to O, the lattice energy released when CaO is formed is greater than the energy required for these processes. Hence, the overall energy change in the formation of CaO is exothermic (negative). This makes CaO stable relative to its free elements, Ca and O.