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Chapter 8: Problem 40

Consider the following molecules: (a) \(\mathrm{CH}_{4}\) (c) \(\mathrm{BF}_{3}\) (b) \(\mathrm{NH}_{2} \mathrm{Cl}\) (d) \(\mathrm{CS}_{2}\) (i) In which compound are the bonds most polar? (ii) Which compounds are not polar? (iii) Are the H atoms in \(\mathrm{NH}_{2} \mathrm{Cl}\) negative or positive?

Short Answer

Expert verified
(i) Most polar bonds are in \(\mathrm{BF}_3\). (ii) \(\mathrm{CH}_4\), \(\mathrm{BF}_3\), and \(\mathrm{CS}_2\) are not polar. (iii) H atoms in \(\mathrm{NH}_2 \mathrm{Cl}\) are positive.

Step by step solution

01

Determine Bond Polarity

Bond polarity is determined by the difference in electronegativity between two atoms. The greater the difference, the more polar the bond. - For \(\mathrm{CH}_4\), the difference between C (2.55) and H (2.20) is 0.35.- For \(\mathrm{BF}_3\), the difference between B (2.04) and F (3.98) is 1.94.- For \(\mathrm{NH}_2 \mathrm{Cl}\), the difference between N (3.04) and Cl (3.16) is 0.12, and between N and H (2.20) is 0.84.- For \(\mathrm{CS}_2\), the difference between C (2.55) and S (2.58) is 0.03.The most polar bonds are in \(\mathrm{BF}_3\).
02

Determine Molecular Polarity

Molecular polarity depends on both bond polarity and the symmetry of the molecule. A molecule that is symmetrical in 3D space is non-polar because its dipoles cancel out.- \(\mathrm{CH}_4\) is tetrahedral and symmetrical, making it non-polar.- \(\mathrm{BF}_3\) is trigonal planar and symmetrical, thus non-polar.- \(\mathrm{NH}_2 \mathrm{Cl}\) is asymmetrical and polar due to the presence of lone pairs and the different atoms attached to N.- \(\mathrm{CS}_2\) is linear and symmetrical, making it non-polar.Thus, \(\mathrm{CH}_4\), \(\mathrm{BF}_3\), and \(\mathrm{CS}_2\) are not polar.
03

Determine Charge on Hydrogen in \(\mathrm{NH}_2 \mathrm{Cl}\)

In \(\mathrm{NH}_2 \mathrm{Cl}\), we compare the electronegativities of H (2.20) and N (3.04). Since nitrogen is more electronegative than hydrogen, the N-H bond is polar with hydrogen carrying a partial positive charge.

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Key Concepts

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

Electronegativity
Electronegativity is the ability of an atom to attract electrons towards itself in a chemical bond. It plays a critical role in determining the polarity of bonds within molecules. Each element has a specific electronegativity value, often found in the periodic table.
The difference in electronegativity between two bonded atoms dictates how polar the bond is. A large difference indicates a highly polar bond, while a small difference points to a non-polar or less polar bond.
  • In the molecule \(\mathrm{CH}\_{4}\), Carbon has an electronegativity of 2.55, while Hydrogen's is 2.20. The difference (0.35) shows that the bonds are relatively non-polar.
  • In \(\mathrm{BF}\_{3}\), Boron has an electronegativity of 2.04, and Fluorine is much higher at 3.98. The large difference of 1.94 indicates very polar bonds.
  • The bond difference in \(\mathrm{NH}\_{2} \mathrm{Cl}\) between Nitrogen and Chlorine is low (0.12), suggesting weaker polarity.
  • Meanwhile, \(\mathrm{CS}\_{2}\) shows minimal difference between Carbon and Sulfur (0.03), indicating non-polar bonds.
Molecular Polarity
Molecular polarity refers to the distribution of electric charge across a molecule. This property is determined not just by individual bond polarities but also by the overall shape and symmetry of the molecule. A molecule can have polar bonds while being non-polar if it is symmetrical.
  • For \(\mathrm{CH}_{4}\), it's a tetrahedral shape. Despite having slightly polar bonds, the symmetry cancels out dipoles, making it non-polar overall.
  • In \(\mathrm{BF}_{3}\), the trigonal planar geometry also results in non-polarity, as the dipole moments cancel out.
  • On the other hand, \(\mathrm{NH}_{2} \mathrm{Cl}\) is asymmetrical due to the lone pair on nitrogen and different attached atoms, resulting in a net dipole moment. Thus, it is polar.
  • Finally, \(\mathrm{CS}_{2}\) has a linear shape, making it symmetrical and non-polar, despite having minor polar bonds.
Symmetry in Molecules
Symmetry in a molecule is when its molecular structure is such that it is balanced and identical across various axes. Symmetrical molecules tend to be non-polar because the dipoles from any polar bonds cancel each other out.
The symmetry of a molecule is crucial in determining its polarity, and here are some key examples:
  • \(\mathrm{CH}_{4}\) has a tetrahedral shape where hydrogen atoms are evenly spaced, making it symmetrical and non-polar.
  • \(\mathrm{BF}_{3}\) forms a trigonal planar shape; the equal angles between fluorine atoms create a balance that leads to non-polarity.
  • \(\mathrm{CS}_{2}\) displays a linear configuration, with sulfur atoms directly opposite each other, maintaining symmetry and non-polarity.
  • In contrast, \(\mathrm{NH}_{2} \mathrm{Cl}\) lacks symmetry due to differing attached atoms and a lone pair on nitrogen, making it polar.

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Most popular questions from this chapter

Show all possible resonance structures for each of the following molecules or ions: (a) nitrate ion, \(\mathrm{NO}_{3}\) (b) nitric acid, HNO_3 (c) dinitrogen monoxide (nitrous oxide, laughing gas), \(\mathrm{N}_{2} \mathrm{O}\) (where the bonding is in the order \(\mathrm{N}-\mathrm{N}-\)O)

Draw a Lewis structure for each of the following molecules: (a) chlorodifluoromethane, \(\mathrm{CHClF}_{2}\) (b) propanoic acid, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{H}\) (basic structure pictured below) (c) acetonitrile, \(\mathrm{CH}_{3} \mathrm{CN}\) (the framework is \(\left.\mathrm{H}_{3} \mathrm{C}-\mathrm{C}-\mathrm{N}\right)\) (d) allene, \(\mathrm{H}_{2} \mathrm{CCCH}_{2}\) EQUATION CANT COPY

A paper published in the research journal Science in 2007 (S. Vallina and R. Simo, Science, Vol. \(315,\) p. \(506,\) January 26,2007 ) reported studies of dimethylsulfide (DMS), an important greenhouse gas that is released by marine phytoplankton. This gas "represents the largest natural source of atmospheric sulfur and a major precursor of hygroscopic (i.e, cloud-forming) particles in clean air over the remote oceans, thereby acting to reduce the amount of solar radiation that crosses the atmosphere and is absorbed by the ocean." (a) Sketch the Lewis structure of dimethylsulfide, \(\mathrm{CH}_{3} \mathrm{SCH}_{3},\) and list the bond angles in the molecule. (b) Use electronegativities to decide where the positive and negative charges lie in the molecule. Is the molecule polar? (c) The mean seawater concentration of DMS in the ocean in the region between \(15^{\circ}\) north latitude and \(15^{\circ}\) south latitude is \(2.7 \mathrm{nM}\) (nanomolar). How many molecules of DMS are present in \(1.0 \mathrm{m}^{3}\) of seawater?

Draw the resonance structures for the formate ion, \(\mathrm{HCO}_{2}^{-},\) and find the formal charge on each atom. If an \(\mathrm{H}^{+}\) ion is attached to \(\mathrm{HCO}_{2}^{-}\) (to form formic acid), does it attach to \(\mathrm{C}\) or \(\mathrm{O} ?\)

Determine the formal charge on each atom in the following molecules or ions: (a) SCO (b) \(\mathrm{HCO}_{2}^{-}\) (formate ion) (c) \(\mathrm{CO}_{3}^{2-}\) (d) \(\mathrm{HCO}_{2} \mathrm{H}\) (formic acid)
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