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Chapter 1: Problem 22

Which of the following compounds would you expect to have a dipole moment? If the molecule has a dipole moment, specify its direction. (a) \(\mathrm{BF}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{CH}_{4}\) (d) \(\mathrm{CH}_{3} \mathrm{Cl}\) (e) \(\mathrm{CH}_{2} \mathrm{O}\) (f) \(\mathrm{HCN}\)

Short Answer

Expert verified
Molecules with a dipole moment are H2O (towards O), CH3Cl (towards Cl), CH2O (towards O), and HCN (towards N).

Step by step solution

01

Analyze BF3

The compound \( \mathrm{BF}_{3} \) has a trigonal planar geometry. The three \(\mathrm{B-F}\) bonds are equivalent and symmetrically arranged around the central boron atom. Due to symmetry, the polarities of these bonds cancel each other out. Thus, \( \mathrm{BF}_{3} \) does not have a dipole moment.
02

Analyze H2O

The compound \( \mathrm{H}_{2} \mathrm{O} \) has a bent molecular shape with an angle of about 104.5 degrees between the hydrogen and oxygen atoms. The \( \mathrm{O-H} \) bonds are polar, and their vector sum does not cancel out. Therefore, \( \mathrm{H}_{2} \mathrm{O} \) does have a dipole moment directed from the hydrogen atoms towards the oxygen atom.
03

Analyze CH4

\( \mathrm{CH}_{4} \) (methane) has a tetrahedral geometry, where the hydrogen atoms are symmetrically positioned around the carbon atom. The \( \mathrm{C-H} \) bonds are slightly polar but symmetrically arranged, leading to the cancellation of any dipole. Thus, \( \mathrm{CH}_{4} \) has no dipole moment.
04

Analyze CH3Cl

\( \mathrm{CH}_{3} \mathrm{Cl} \) exhibits a tetrahedral shape with one of the \( \mathrm{H} \) atoms replaced by \( \mathrm{Cl} \). The \( \mathrm{C-Cl} \) bond is significantly polar due to the difference in electronegativity, and due to the asymmetric shape of the molecule, it creates a net dipole moment directed towards the chlorine atom.
05

Analyze CH2O

\( \mathrm{CH}_{2} \mathrm{O} \) (formaldehyde) exhibits a trigonal planar shape around the carbon. The \( \mathrm{C=O} \) bond is more polar than the \( \mathrm{C-H} \) bonds, resulting in a net dipole moment directed from the hydrogen atoms to the oxygen atom.
06

Analyze HCN

\( \mathrm{HCN} \) has a linear geometry. Upon consideration of electronegativities, the \( \mathrm{C\equiv N} \) triple bond is considerably polar, creating a dipole moment directed from the hydrogen to the nitrogen.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Molecular Geometry
Molecular geometry is fundamentally the 3D arrangement of atoms within a molecule. This geometry plays a crucial role in determining the physical and chemical properties of a compound such as its reactivity, color, phase, and polarity. The shape of the molecule is defined largely by electron pair repulsions, as per the VSEPR theory (Valence Shell Electron Pair Repulsion theory). For instance, in the compound \( \mathrm{BF}_{3} \), the geometry is trigonal planar since the bonds are symmetrically arranged around the central boron atom.
  • Trigonal Planar: Seen in \( \mathrm{BF}_{3} \), with 120-degree bond angles around the Boron atom.
  • Bent: Found in \( \mathrm{H}_{2} \mathrm{O} \), where the \( \mathrm{O-H} \) bonds create a 104.5-degree angle.
  • Tetrahedral: Exhibited by \( \mathrm{CH}_{4} \), where four hydrogen atoms form 109.5-degree angles.
  • Other geometries include linear, as in \( \mathrm{HCN} \), where all the atoms align in a straight line.
Understanding molecular geometry is integral to predict a molecule鈥檚 behavior in various chemical contexts.
Polarity of Bonds
Bond polarity arises due to differences in electronegativity between bonded atoms. Electronegativity is the atom's tendency to attract a bonding pair of electrons towards itself. A bond becomes polar when there is a significant difference in electronegativity between two bonded atoms. For example, in \( \mathrm{H}_{2} \mathrm{O} \), the \( \mathrm{O-H} \) bonds are polar because oxygen is more electronegative than hydrogen, creating an uneven electron distribution.
  • Polar Bonds: Exist when there is a difference in electronegativity, for example, \( \mathrm{C=O} \) in formaldehyde and \( \mathrm{C-Cl} \) in \( \mathrm{CH}_{3} \mathrm{Cl} \).
  • Non-polar Bonds: Occur when electronegativities are equal or very similar, as seen in \( \mathrm{C-H} \) bonds in \( \mathrm{CH}_{4} \).
In essence, for a molecule to be considered polar, it must contain polar bonds that do not cancel out by symmetry.
Electronegativity
Electronegativity is essentially the ability of an atom to attract electrons in a bond. It plays a vital role in determining the nature and polarity of chemical bonds. Higher electronegative atoms pull electrons toward themselves more effectively than less electronegative ones. For instance, chlorine in \( \mathrm{CH}_{3} \mathrm{Cl} \) is much more electronegative than hydrogen, causing a significant dipole moment.
  • Highly electronegative elements include Fluorine, Oxygen, and Nitrogen.
  • Trends indicate that electronegativity increases across a period and decreases down a group.
This characteristic underpins the behavior of molecules in reactions and their interactions, such as hydrogen bonding.
Symmetry in Molecules
Symmetry in molecules can often determine their dipole moment. In symmetrical molecules, the dipole moments of individual bonds can cancel each other out, resulting in a non-polar molecule. For instance, \( \mathrm{BF}_{3} \) and \( \mathrm{CH}_{4} \) are symmetrical, causing the dipole moments to cancel, resulting in no net dipole moment. On the contrary, asymmetrical molecules like \( \mathrm{H}_{2} \mathrm{O} \) and \( \mathrm{CH}_{3} \mathrm{Cl} \) are polar due to their shape not allowing complete cancellation.
  • Symmetrical: Means the molecular geometry is balanced and often non-polar, such as \( \mathrm{BF}_{3} \).
  • Asymmetrical: Implies an uneven shape and usually signals a polar molecule, like \( \mathrm{H}_{2} \mathrm{O} \).
By understanding symmetry, one can predict the polarity and resultant physical properties of molecules.

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Most popular questions from this chapter

Which compound in each of the following pairs would you expect to have the greater dipole moment \(\mu\) ? Why? (a) HF or \(\mathrm{HCl}\) (d) \(\mathrm{CHCl}_{3}\) or \(\mathrm{CCl}_{3} \mathrm{~F}\) (b) \(\mathrm{HF}\) or \(\mathrm{BF}_{3}\) (e) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) or \(\mathrm{CH}_{3} \mathrm{OH}\) (c) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CH}\) or \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) (f) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) or \(\mathrm{CH}_{3} \mathrm{NO}_{2}\)

Write Lewis formulas, including unshared pairs, for each of the following. Carbon has four bonds in each compound. (a) Propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8}\right)\) (b) Methanol \(\left(\mathrm{CH}_{4} \mathrm{O}\right)\) (c) Methyl fluoride \(\left(\mathrm{CH}_{3} \mathrm{~F}\right)\) (d) Ethyl fluoride \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{~F}\right)\)

Is 3-chlorobutanoic acid more or less acidic than 2-chlorobutanoic acid? Explain your reasoning.

With a \(\mathrm{p} K_{\mathrm{a}}\) of \(11.6\), hydrogen peroxide is a stronger acid than water. Why?

Potassium hydride \((\mathrm{KH})\) is a source of the strongly basic hydride ion \(\left(: \mathrm{H}^{-}\right)\). Using curved arrows to track electron movement, write an equation for the reaction of hydride ion with water. What is the conjugate acid of hydride ion?
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