Problem 39 - Chemistry: An Atoms-Focused Approach

Question

In which of the following compounds do the molecules experience dipole-dipole interactions? (a) \(\mathrm{CF}_{4} ;\) (b) \(\mathrm{CF}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{CCl}_{4} ;\) (d) \(\mathrm{CFCl}_{3}\)

Step-by-Step Solution

Answer

Answer: Only compound (d) CFCl3 experiences dipole-dipole interactions due to its net molecular dipole moment resulting from the unequal bond polarities in its tetrahedral geometry.

Step 1: Compound (a): CF4

In tetrafluoromethane (CF4), the carbon atom is bonded to 4 fluorine atoms in a tetrahedral shape. Each carbon-fluorine bond is polar due to the difference in electronegativity. However, because the molecule has a symmetrical tetrahedral shape, these polar bonds cancel each other out, making the molecule nonpolar. Therefore, CF4 does not experience dipole-dipole interactions.

Step 2: Compound (b): CF2Cl2

In dichlorodifluoromethane (CF2Cl2), the carbon atom is bonded to 2 fluorine atoms and 2 chlorine atoms. The molecule's shape is also tetrahedral, much like compound (a). The carbon-fluorine and carbon-chlorine bonds are both polar due to differences in electronegativity. However, the geometry of the molecule allows for the polar bond vectors to cancel each other out, resulting in a nonpolar molecule. Therefore, CF2Cl2 does not experience dipole-dipole interactions.

Step 3: Compound (c): CCl4

In carbon tetrachloride (CCl4), the carbon atom is bonded to 4 chlorine atoms in a tetrahedral shape. Similar to compounds (a) and (b), the polar carbon-chlorine bonds cancel each other out due to the molecular geometry, making the molecule nonpolar. Therefore, CCl4 does not experience dipole-dipole interactions.

Step 4: Compound (d): CFCl3

In chlorotrifluoromethane (CFCl3), the carbon atom is bonded to 1 chlorine atom and 3 fluorine atoms. The molecular geometry is tetrahedral. Unlike the other compounds, the carbon-fluorine and carbon-chlorine polar bonds do not cancel each other out due to their unequal bond polarities. Thus, the molecule has a net molecular dipole moment. This makes the molecule polar, so it experiences dipole-dipole interactions. Based on the analysis of the given compounds, only compound (d) \(\mathrm{CFCl}_{3}\) experiences dipole-dipole interactions.

Key Concepts

Molecular Geometry

Molecular geometry is like the shape of a molecule made by the arrangement of its atoms. It plays a crucial role in determining whether a molecule will be polar or nonpolar. Imagine each atom as a dot, and the bonds between atoms as lines connecting these dots. The arrangement of these dots and lines influences the molecule’s properties.

• Different molecular shapes include linear, bent, tetrahedral, trigonal planar, and more.
• The shape of a molecule affects how its electrons are spread, which in turn affects chemical behavior.

In the context of our exercise, we looked at the tetrahedral shape for compounds like CF₄ and CCl₄, where four atoms surround a central atom. This shape allows the polar bonds to cancel out, creating a nonpolar molecule.

Polar Molecules

Polar molecules have a separation of electric charge leading to a molecule having a dipole moment. This happens when there is an uneven distribution of electrons in the bonds of a molecule. Simply put, one end of the molecule is slightly positive, and the other end is slightly negative. In our solved exercise, polar molecules are the ones experiencing dipole-dipole interactions.

• Molecules like CFCl₃ are polar because the bonds don't cancel each other out due to unequal bond polarities.
• Other examples of polar molecules include water (H₂O) and ammonia (NH₃).

A polar molecule will align itself in an electric field, with the negative end pointing towards the positive field and vice versa.

Chemical Bonding

Chemical bonding is the interaction that holds atoms together in a molecule. There are different kinds of bonds, but the focus here is on covalent bonds, where atoms share electrons. Depending on how these electrons are shared, the overall property and behavior of the molecule can change.

• Polar covalent bonds occur when electrons are shared unequally, leading to a partial electric charge.
• Nonpolar covalent bonds happen when the sharing of electrons is equal, with no charged ends.

In the case of CF₂Cl₂, even though polar covalent bonds are present, the tetrahedral shape ensures that the dipoles cancel out, rendering the molecule nonpolar.

Electronegativity Differences

Electronegativity is the tendency of an atom to attract electrons towards itself in a bond. The difference in electronegativity between two atoms dictates the type of bond formed:

• If the difference is large, the bond is ionic; one atom takes electrons completely from another.
• If the difference is small, the bond is covalent; atoms share electrons more equally.
• Intermediate differences lead to polar covalent bonds, where electrons are shared unequally.

In our examples, the electronegativity differences between carbon and its bonded atoms (like fluorine and chlorine) create polar covalent bonds. However, due to molecular geometry, only in CFCl₃ do they not cancel each other, resulting in a polar molecule.