Problem 54 - Chemistry Principles and Practice

Question

For each pair of bonds, indicate the more polar bond, and use an arrow to show the direction of polarity in each bond. (a) \(\mathrm{C}-\mathrm{O}\) and \(\mathrm{C}-\mathrm{N}\) (b) \(\mathrm{P}-\mathrm{Br}\) and \(\mathrm{P}-\mathrm{Cl}\) (c) \(\mathrm{B}-\mathrm{O}\) and \(\mathrm{B}-\mathrm{S}\) (d) \(\mathrm{B}-\mathrm{F}\) and \(\mathrm{B}-\mathrm{I}\)

Step-by-Step Solution

Answer

(a) C-O (C→O), (b) P-Cl (P→Cl), (c) B-O (B→O), (d) B-F (B→F) are more polar.

Step 1: Identify Electronegativity Values

To determine the polarity of a bond, you should compare the electronegativity values of the elements involved. Elements with higher electronegativity pull shared electrons closer, creating partial negative charges. Using a periodic table of electronegativity values: - Oxygen ( O ) = 3.44, Nitrogen ( N ) = 3.04 - Bromine ( Br ) = 2.96, Chlorine ( Cl ) = 3.16 - Oxygen ( O ) = 3.44, Sulfur ( S ) = 2.58 - Fluorine ( F ) = 3.98, Iodine ( I ) = 2.66 - Carbon ( C ), Phosphorus ( P ), and Boron ( B ) have lower electronegativities, making the other elements in these pairs more electronegative.

Step 2: Compare Bond Polarities

Calculate the difference in electronegativity for each bond to determine polarity. The bond with the greater difference is more polar. (a) C-O vs C-N : |3.44 - 2.55| = 0.89 for C-O ; |3.04 - 2.55| = 0.49 for C-N . The C-O bond is more polar. (b) P-Br vs P-Cl : |2.96 - 2.19| = 0.77 for P-Br ; |3.16 - 2.19| = 0.97 for P-Cl . The P-Cl bond is more polar. (c) B-O vs B-S : |3.44 - 2.04| = 1.40 for B-O ; |2.58 - 2.04| = 0.54 for B-S . The B-O bond is more polar. (d) B-F vs B-I : |3.98 - 2.04| = 1.94 for B-F ; |2.66 - 2.04| = 0.62 for B-I . The B-F bond is more polar.

Step 3: Determine Direction of Polarity

The direction of bond polarity is from the less electronegative element to the more electronegative element. Use an arrow ( → ) to show this direction, where the arrow points towards the more electronegative element which has a partial negative charge. (a) C-O: C→O C-N: C→N (b) P-Br: P→Br P-Cl: P→Cl (c) B-O: B→O B-S: B→S (d) B-F: B→F B-I: B→I

Key Concepts

Electronegativity Values

Electronegativity is a measure of an atom's ability to attract and hold onto electrons. This property varies among elements and is an essential factor in determining the polarity of chemical bonds. When comparing electronegativity values, elements with higher values have a greater tendency to pull electrons closer to themselves.

For example, in the original exercise, we are given the electronegativity values for specific elements. Oxygen has a value of 3.44, while nitrogen's is 3.04. Higher numbers, such as fluorine's 3.98, indicate a stronger ability to attract electrons.

• Electronegativity differences between bonded atoms determine how electrons are shared.
• The greater the difference, the more polar the bond will be.

Understanding electronegativity is crucial for predicting bond behavior in molecules. It helps dictate the type of bond that forms, ranging from nonpolar covalent to highly polar covalent bonds.

Polar Bonds

A polar bond occurs when two atoms in a molecule have different electronegativity values, resulting in an unequal sharing of electrons. The atom with the higher electronegativity value becomes slightly negative because it draws electrons closer, while the other atom becomes slightly positive. This charge separation creates a dipole moment, which is characteristic of polar bonds.

In the exercise solutions, bonds like B-F and C-O exhibit significant differences in electronegativity. For example, in the B-F bond, boron (B) has a lower electronegativity compared to fluorine (F), which stands at 3.98. The substantial difference in electronegativity values between these atoms (1.94, in this case) results in a highly polar bond.

• Polar bonds influence the physical properties of substances such as boiling points and solubility.
• The greater the electronegativity difference, the stronger the polarity of the bond.

Polar bonds are prevalent in many compounds and often impact how substances interact in biological and chemical contexts.

Direction of Polarity

Determining the direction of polarity in a bond is essential for understanding molecular interactions. The direction indicates where electron density is shifted in the bond.

To visualize this, you can use an arrow to point from the less electronegative atom to the more electronegative atom, indicating the flow of electron density towards the more electronegative side. This shift creates slight charges on the atoms, with the arrow pointing towards the atom with the partial negative charge.

In the solutions provided, for example, the arrow in a C-O bond is written as C→O. Here, the arrow points towards oxygen because it is more electronegative than carbon.

• The direction highlights regions of partial negative and positive charges within the bond.
• This information is vital for predicting molecular geometry and reactivity.

Understanding the direction of polarity helps in predicting the overall dipole moment of a molecule, which is crucial in determining the molecule's interaction with other molecules and solvents.