Problem 73 - Chemistry The Central Science

Question

We can define average bond enthalpies and bond lengths for ionic bonds, just like we have for covalent bonds. Which ionic bond is predicted to be stronger, \(\mathrm{Na}-\mathrm{Cl}\) or \(\mathrm{Ca}-\mathrm{O}\) ?

Step-by-Step Solution

Answer

The ionic bond in \(\mathrm{Ca-O}\) is predicted to be stronger than that in \(\mathrm{Na-Cl}\), as it involves ions with higher charges (+2 and -2 in \(\mathrm{Ca-O}\) compared to +1 and -1 in \(\mathrm{Na-Cl}\)) and smaller radii, leading to stronger electrostatic interactions and a higher lattice energy.

Step 1: Identify the type of bond

We're dealing with ionic bonds here, which involve a transfer of electrons from one atom to another, resulting in the formation of positively charged cations and negatively charged anions. Our task is to compare the strength of the ionic bonds in NaCl (sodium chloride) and CaO (calcium oxide).

Step 2: Consider the charges of the ions

The charge of an ion has a significant impact on the strength of an ionic bond. The greater the charge on the ions involved in the bond, the stronger the bond will be. For Na-Cl, Na loses one electron to become a +1 cation and Cl gains one electron to become a -1 anion. The charges are +1 and -1. For Ca-O, Ca loses two electrons to become a +2 cation, while O gains two electrons to form a -2 anion. The charges are +2 and -2. Since the charges on the ions in Ca-O are greater than those in Na-Cl, the ionic bond in Ca-O is likely to be stronger based on this factor.

Step 3: Consider the ion sizes and lattice energy

The sizes of the ions involved in the bond are also important because they affect the lattice energy. Lattice energy is defined as the energy required to break an ionic bond and convert the compound into gaseous ions. A higher lattice energy indicates a stronger bond. For Na-Cl, the atomic radius of Na+(cation) is about 95 pm and Cl-(anion) is about 181 pm. For Ca-O, the atomic radius of Ca2+(cation) is about 100 pm and O2-(anion) is about 140 pm. As the cation in Ca-O (Ca2+) has a higher charge density than the cation in Na-Cl (Na+), the electrostatic interactions between the ions in Ca-O would be stronger than those in Na-Cl.

Step 4: Compare the ionic bonds and strength

Considering the charges of the ions and ion sizes, Ca-O involves ions with higher charges and smaller radii resulting in stronger electrostatic interactions and a higher lattice energy. Therefore, the ionic bond in Ca-O is predicted to be stronger than that in Na-Cl.