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Chapter 10: Problem 81

Predict the shapes of the following molecules, and then predict which would have resultant dipolemoments: (a) \(\mathrm{SO}_{2} ;\) (b) \(\mathrm{NH}_{3} ;\) (c) \(\mathrm{H}_{2} \mathrm{S} ;\) (d) \(\mathrm{C}_{2} \mathrm{H}_{4} ;\) (e) \(\mathrm{SF}_{6}\); (f) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\).

Short Answer

Expert verified
\( \mathrm{SO}_{2} \), \( \mathrm{NH}_{3} \), \( \mathrm{H}_{2} \mathrm{S} \) and \( \mathrm{CH}_{2} \mathrm{Cl}_{2} \) have a net dipole moment. \( \mathrm{C}_{2} \mathrm{H}_{4} \) and \( \mathrm{SF}_{6} \) do not have a dipole moment.

Step by step solution

01

Identify the Molecular Shapes

Based on the VSEPR theory, the shapes of molecules can be predicted. For \( \mathrm{SO}_{2} \), there are 2 bond pairs and 1 lone pair around the central atom, so its shape is bent or V-shaped. For \( \mathrm{NH}_{3} \), there are 3 bond pairs and 1 lone pair around the central atom, giving it a trigonal pyramidal shape. \( \mathrm{H}_{2} \mathrm{S} \) has 2 bond pairs and 2 lone pairs around the central atom, also providing a bent or V-shaped form. \( \mathrm{C}_{2} \mathrm{H}_{4} \) is a planar molecule since both carbon atoms are sp2 hybridized, forming a trigonal planar geometry. For \( \mathrm{SF}_{6} \), there are 6 bond pairs around the central atom, giving an octahedral shape. Lastly, \( \mathrm{CH}_{2} \mathrm{Cl}_{2} \) has a tetrahedral shape since there are 4 bond pairs around the central carbon atom.
02

Predict the Dipole Moment

A molecule will have a net dipole moment if its structure is unsymmetrical, which creates an imbalance of electron charge. Hence, \( \mathrm{SO}_{2} \), \( \mathrm{NH}_{3} \) and \( \mathrm{H}_{2} \mathrm{S} \) being V-shaped and trigonal pyramidal, will have a net dipole moment. \( \mathrm{C}_{2} \mathrm{H}_{4} \), being a symmetrical planar molecule, will not have a dipole moment. \( \mathrm{SF}_{6} \), being a symmetrical octahedral molecule, will not have a dipole moment. \( \mathrm{CH}_{2} \mathrm{Cl}_{2} \), although is a tetrahedral, it will have a net dipole moment due to the different atoms (H and Cl) connected to the central atom leading to a difference in electronegativities.

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Key Concepts

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

VSEPR Theory
The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the shape of individual molecules. It is based on the premise that electron pairs around a central atom will arrange themselves as far apart as possible to minimize the repulsive forces between them.
This model is incredibly useful because the shape of a molecule significantly affects its chemical properties and reactivity. For VSEPR, we consider both bonding pairs (shared between atoms) and lone pairs (non-shared) of electrons around the central atom.
  • Bent or V-shaped geometry occurs when there are two bonding pairs and one or more lone pairs, like in \((\mathrm{SO}_{2})\).
  • Trigonal pyramidal shapes, found in molecules like \((\mathrm{NH}_{3})\), arise from three bonding pairs and one lone pair.
  • When molecules have a planar shape, like \((\mathrm{C}_{2}\mathrm{H}_{4})\), the atoms are organized such that their electron pairs are in the same plane, often as a result of \(sp^2\) hybridization.
  • Octahedral shapes, such as in \((\mathrm{SF}_{6})\), occur with six bonding pairs symmetrically arranged around the central atom.
Understanding VSEPR theory helps in predicting molecular geometry, which is crucial for understanding molecular interactions and properties.
Dipole Moment
Dipole moment is a measure of the separation of positive and negative charges in a molecule. It arises from differences in electronegativity between atoms that create polar bonds within a molecule.
The presence and magnitude of a dipole moment can greatly affect a molecule's behavior and interactions, such as solubility and boiling point. Notably, a molecule will only have a net dipole moment if it is asymmetrical.
  • In a bent or V-shaped molecule like \((\mathrm{SO}_{2})\), the asymmetry leads to a net dipole moment despite symmetrical polar bonds.
  • \(\mathrm{NH}_{3},\) with its trigonal pyramidal shape and non-symmetrical distribution of polar bonds, exhibits a dipole moment as well.
  • \(\mathrm{CH}_{2}\mathrm{Cl}_{2}\) has a dipole moment because its tetrahedral shape is asymmetrical due to the differences in electronegativity between hydrogen and chlorine atoms.
  • For symmetric molecules, such as \(\mathrm{C}_{2}\mathrm{H}_{4}\) and \(\mathrm{SF}_{6},\) there is no dipole moment as the polarities cancel each other out.
The concept of dipole moment plays a vital role in understanding molecular interactions in solution and in the prediction of molecule behavior under external electric fields.
Molecular Geometry
Molecular Geometry refers to the three-dimensional arrangement of atoms in a molecule. It is a key factor that determines many physical and chemical properties of a substance.
Each geometry type gives unique characteristics to the molecule in terms of spatial configuration. VSEPR theory helps predict these geometries by considering electron pairs around a central atom.
  • For example, \(\mathrm{SO}_{2}\) has a bent or V-shaped geometry due to the influence of lone pairs on the central sulfur atom.
  • In \(\mathrm{NH}_{3},\) the trigonal pyramidal geometry is influenced by a lone pair on the nitrogen atom, deviating the hydrogen bonds from a perfect tetrahedral layout.
  • \(\mathrm{C}_{2}\mathrm{H}_{4}\) is planar due to \(sp^2\) hybridization in carbon atoms, leading to a single plane arrangement of atoms.
  • \(\mathrm{SF}_{6},\) on the other hand, exhibits an octahedral geometry with no lone pairs, giving it a highly symmetrical spatial configuration.
Correctly understanding molecular geometry helps to predict molecular reactivity, polarity, phase of matter, color, magnetism, biological activity, and chemical properties.

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Most popular questions from this chapter

Draw Lewis structures for two different molecules with the formula \(\mathrm{C}_{3} \mathrm{H}_{4}\). Is either of these molecules linear? Explain.

In certain polar solvents, \(\mathrm{PCl}_{5}\) undergoes an ionization reaction in which a \(\mathrm{Cl}^{-}\) ion leaves one \(\mathrm{PCl}_{5}\) molecule and attaches itself to another. The products of the ionization are \(\mathrm{PCl}_{4}^{+}\) and \(\mathrm{PCl}_{6}^{-}\). Draw a sketch showing the changes in geometric shapes that occur in this ionization (that is, give the shapes of \(\mathrm{PCl}_{5}\), \(\mathrm{PCl}_{4}^{+},\) and \(\mathrm{PCl}_{6}^{-}\) ). $$2 \mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{4}^{+}+\mathrm{PCl}_{6}^{-}$$

A few years ago the synthesis of a salt containing the \(\mathrm{N}_{5}^{+}\) ion was reported. What is the likely shape of this ion-linear, bent, zigzag, tetrahedral, seesaw, or square-planar? Explain your choice.

Alternative strategies to the one used in this chapter have been proposed for applying the VSEPR theory to molecules or ions with a single central atom. In general, these strategies do not require writing Lewis structures. In one strategy, we write (1) the total number of electron pairs \(=[\) (number of valence electrons) \(\pm\) (electrons required for ionic charge) \(] / 2\) (2) the number of bonding electron pairs \(=\) (number of atoms) -1 (3) the number of electron pairs around central atom \(=\) total number of electron pairs \(-3 \times[\) number of terminal atoms (excluding \(\mathrm{H}\) )] (4) the number of lone-pair electrons = number of central atom pairs - number of bonding pairs After evaluating items \(2,3,\) and \(4,\) establish the VSEPR notation and determine the molecular shape. Use this method to predict the geometrical shapes of the following: (a) \(\mathrm{PCl}_{5} ;\) (b) \(\mathrm{NH}_{3} ;\) (c) \(\mathrm{ClF}_{3} ;\) (d) \(\mathrm{SO}_{2} ;\) (e) \(\mathrm{ClF}_{4}^{-}\); (f) \(\mathrm{PCl}_{4}^{+}\). Justify each of the steps in the strategy, and explain why it yields the same results as the VSEPR method based on Lewis structures. How does the strategy deal with multiple bonds?

The following statements are not made as carefully as they might be. Criticize each one. (a) Lewis structures with formal charges are incorrect. (b) Triatomic molecules have a planar shape. (c) Molecules in which there is an electronegativity difference between the bonded atoms are polar.
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