Question

(a) The H-Cl bond length is \(136 \mathrm{pm}\). What would the dipole moment of HCl be if the molecule were \(100 \%\) ionic, \(\mathrm{H}^{+} \mathrm{Cl}^{-} ?\) (b) The actual dipole moment of HCl is \(1.08 \mathrm{D}\). What is the percent ionic character of the H-Cl bond?

Short answer

(a) Dipole moment if 100% ionic: 6.53 D. (b) Percent ionic character: 16.53%.

Step-by-step solution

Calculate Dipole Moment if 100% Ionic

To calculate the dipole moment for a 100% ionic bond, use the formula \[ \mu = q \times d \]where \( \mu \) is the dipole moment, \( q \) is the charge (charge of an electron \( = 1.602 \times 10^{-19} \) Coulombs), and \( d \) is the distance in meters. Convert the bond length from pm to meters: \[136 \text{ pm} = 136 \times 10^{-12} \text{ m} \]Then calculate the dipole moment:\[ \mu = (1.602 \times 10^{-19} \text{ C}) \times (136 \times 10^{-12} \text{ m}) \approx 2.1787 \times 10^{-29} \text{ Cm} \] Convert to Debye (1 D = \(3.336 \times 10^{-30} \text{ Cm})\):\[ \mu \approx \frac{2.1787 \times 10^{-29}}{3.336 \times 10^{-30}} \approx 6.53 \text{ D} \]

Calculate Percent Ionic Character

The percent ionic character is given by the ratio of the actual dipole moment to the dipole moment if the molecule were 100% ionic, multiplied by 100:\[ \% \text{ Ionic Character} = \left( \frac{\text{Actual Dipole Moment}}{\text{Dipole Moment if 100% Ionic}} \right) \times 100 \]Substitute the values:\[ \% \text{ Ionic Character} = \left( \frac{1.08 \text{ D}}{6.53 \text{ D}} \right) \times 100 \approx 16.53\% \]

Key concepts

Percent Ionic Character

The percent ionic character of a bond refers to how much of the bond's overall character resembles that of an ideal ionic bond. In a 100% ionic bond, one atom completely transfers an electron to another atom, resulting in fully charged ions. However, in reality, most bonds are not purely ionic or covalent, but somewhere in between.

To find the percent ionic character, we compare the actual dipole moment of a bond with the theoretical dipole moment if the bond was 100% ionic. The dipole moment is measured in Debye (D), a unit that reflects the separation of positive and negative charges within a molecule.

Here's the simple formula to calculate percent ionic character:

• Take the ratio of the actual dipole moment to the theoretical (100% ionic) dipole moment.
• Multiply by 100 to convert to a percentage.

In the case of an H-Cl bond, the measured dipole moment is 1.08 D, while the calculated dipole moment for a completely ionic H-Cl bond is around 6.53 D, showing a percent ionic character of approximately 16.53%. This indicates that there is some degree of ionic behavior in the bond, but it is not entirely ionic.

H-Cl Bond

The hydrogen-chlorine (H-Cl) bond is an interesting bond often discussed in the study of chemistry because it showcases a blend of ionic and covalent characteristics. H-Cl is commonly found in hydrochloric acid, a vital industrial chemical.

The bond length of H-Cl is 136 pm (or 0.136 nm), indicating the spatial distance between the nuclei of the hydrogen and chlorine atoms. Bond lengths provide valuable insight into the strength and nature of a chemical bond. Typically, shorter bonds are stronger and involve higher energy.

• In the H-Cl bond, the dipole moment arises due to the difference in electronegativity between hydrogen and chlorine.
• Chlorine is more electronegative, meaning it more tightly attracts the shared electrons, creating a polar covalent bond.

In theory, if the H-Cl bond were entirely ionic, forming ions like \(\text{H}^{+}\) and \(\text{Cl}^{-}\), it would produce a larger dipole moment. Yet in reality, the actual dipole moment measured is less, indicating the presence of covalent character as well.

Ionic and Covalent Bonds

Ionic and covalent bonds represent two ends of a spectrum describing how atoms hold together in compounds. Understanding the difference between these types of bonds helps explain the basic structure and functions of molecules.

**Ionic Bonds** An ionic bond forms between a metal and a non-metal when electrons are transferred from one atom to another. This transfer creates ions: metal atoms lose electrons to form positively charged cations, while non-metal atoms gain electrons to form negatively charged anions. The electrostatic attraction between these opposite charges binds the ions together.

• Characterized by high melting and boiling points.
• Typically form solid crystals at room temperature.
• Conduct electricity when dissolved in water.

**Covalent Bonds** In a covalent bond, electrons are shared between atoms, usually between non-metals. This sharing allows each atom to achieve a stable electron configuration akin to that of noble gases.

• Can form discrete molecules or extended networks.
• Generally have lower melting and boiling points compared to ionic compounds.
• Non-conductive as solids.

The H-Cl bond is an example of a polar covalent bond due to the different electronegativities of hydrogen and chlorine, which leads to unequal sharing of electrons and a dipole moment. This places the bond in between the purely ionic and purely covalent descriptions, illustrating the nuanced nature of chemical bonding.