Problem 44 - Chemistry and Chemical Reactivity

Question

Consider the following molecules: (a) \(\mathrm{CH}_{4}\) (c) \(\mathrm{BF}_{3}\) (b) \(\mathrm{NH}_{2} \mathrm{Cl}\) (d) \(\mathrm{CS}_{2}\) (i) Which compound has the most polar bonds? (ii) Which compounds in the list are not polar?

Step-by-Step Solution

Answer

(i) \(\mathrm{BF}_3\) has the most polar bonds; (ii) \(\mathrm{CH}_4\), \(\mathrm{BF}_3\), and \(\mathrm{CS}_2\) are not polar.

Step 1: Determine Electronegativity of Atoms

To evaluate bond polarity, start by looking at the electronegativity of the atoms involved in the bonds for each molecule. Electronegativity is the measure of how strongly atoms attract bonding electrons to themselves. Common electronegativity values are: C (2.55), H (2.20), N (3.04), Cl (3.16), B (2.04), F (3.98), S (2.58).

Step 2: Identify Polar Bonds

Calculate the difference in electronegativity between the atoms in each molecule. If the difference is greater than 0.5, the bond is considered polar:- For \(\mathrm{CH}_4\), the difference is 2.55 - 2.20 = 0.35.- For \(\mathrm{BF}_3\), the difference is 3.98 - 2.04 = 1.94.- For \(\mathrm{NH}_2\mathrm{Cl}\), the difference between N and Cl is 3.16 - 3.04 = 0.12.- For \(\mathrm{CS}_2\), the difference is 2.58 - 2.55 = 0.03.This identifies \(\mathrm{BF}_3\) as having the most polar bonds.

Step 3: Determine Molecular Polarity

Evaluate the molecular geometry to determine if any molecules are non-polar despite having polar bonds:- \(\mathrm{CH}_4\) is tetrahedral and symmetrical, making it non-polar.- \(\mathrm{BF}_3\) is trigonal planar and symmetrical, resulting in a non-polar molecule.- \(\mathrm{NH}_2\mathrm{Cl}\) has a trigonal pyramidal shape which makes it polar due to the lone pair on nitrogen and the difference in electronegativity.- \(\mathrm{CS}_2\) is linear and symmetrical, making it non-polar.

Key Concepts

Electronegativity

Electronegativity is an essential concept in chemistry. It's about how strongly an atom can attract electrons in a chemical bond. Think of it as the atom's ability to "pull" electrons towards itself. This "pull" varies among different elements because each element has a different number of protons, neutrons, and electrons. When comparing electronegativity values:

• Higher numbers mean stronger attraction.
• Lower numbers mean weaker attraction.

For instance, fluorine (\(F\)) has the highest electronegativity value of 3.98 among common elements. It really wants to attract electrons more than most other atoms! Meanwhile, carbon (\(C\)) has a lower electronegativity of 2.55, meaning it's less "hungry" for electrons compared to fluorine. The difference in electronegativity between two atoms in a bond determines how "polar" that bond will be. Understanding these differences helps in predicting how atoms will behave in molecules. It also reveals why some molecules are more reactive or stable in certain environments.

Molecular Polarity

Molecular polarity depends on two main things: the polarity of individual bonds and the shape of the molecule. A molecule might have polar bonds but still be non-polar overall. How does this happen? It’s all about symmetry and geometry.If a molecule is symmetrical, like a perfect circle, the individual bond dipoles can cancel each other out, making the molecule non-polar, despite having polar bonds.

• For example, \(\text{CH}_4\), methane, is symmetrical—it's like a balanced ball with no uneven "pull," hence it's non-polar.
• On the other hand, molecules that are asymmetrical—like \(\text{NH}_2\text{Cl}\)—have regions that don't cancel each other out, leading to an overall polarity.

In \(\text{NH}_2\text{Cl}\), the molecule is bent in such a way that creates a significant difference in charge distribution. The nitrogen atom and its lone pair of electrons make the molecule asymmetrical, contributing to its overall polarity. Thus, understanding molecular shape is crucial when determining molecular polarity. Symmetry plays a big role here, helping simplify whether a molecule is polar or non-polar.

Polar Bonds

Polar bonds are the result of differences in electronegativity between two atoms. When electrons in a bond aren't shared equally, one side of the bond becomes slightly negative, and the other slightly positive. This creates a dipole, which is a characteristic feature of polar bonds. To determine if a bond is polar, you can look at the electronegativity difference:

• If the difference is greater than 0.5, the bond is considered polar.
• If it's less than or around 0.5, the bond is generally non-polar.

Take \(\text{BF}_3\) as an example, where the electronegativity difference between boron and fluorine is 1.94. This indicates a very polar bond. However, despite these polar bonds, \(\text{BF}_3\) is non-polar as a molecule because of its symmetrical trigonal planar shape.Polar bonds can affect a molecule's properties significantly, including solubility, boiling points, and interactions with other molecules. By understanding which molecules have polar bonds and how they are arranged, we gain insight into the molecule’s behavior and properties in various environments.