Chapter 6

\(\mathrm{CO}_{2}\) Answer: First draw a dot diagram, then examine it via VSEPR: Draw a dot diagram: Apply VSEPR: Determine the molecule's shape: There are two electron The molecule groups \(180^{\circ}\) apart around is linear. the central \(C\) atom. \((S N=2)\)

$$ \mathrm{C}_{2} \mathrm{Cl}_{2} \text { , which is connected } \mathrm{Cl}-\mathrm{C}-\mathrm{C}-\mathrm{Cl} . $$

$$ \mathrm{COCl}_{2} \text { , in which all atoms are connected to } \mathrm{C} \text { , and there is a carbon-oxygen double bond. } $$

Sulfur dioxide, \(\mathrm{SO}_{2}\), a pollutant that comes from burning coal contaminated with sulfur.

\(\mathrm{HNO}_{3}\), in which hydrogen is attached to one of the oxygens and all oxygens are attached to nitrogen. To simplify, ignore the hydrogen when you name the molecule's shape.

Is phosphorus trichloride, \(P C l_{3}\), a polar molecule? If it is, draw the dipole moment vector for the entire molecule and show where the \(\delta+\) and \(\delta-\) regions of the molecule are.

Is chloroform, \(\mathrm{CHCl}_{3}\), a polar molecule? If it is, draw the dipole moment vector for the entire molecule and show where the \(\delta+\) and \(\delta-\) regions of the molecule are.

How do you think a bacterium species might evolve to develop resistance to a drug like sulfanilamide that is designed to kill it?

Why is the theory that governs the shape of molecules called VSEPR and not just EPR?

Consider the methane molecule. (a) Draw the methane molecule showing its tetrahedral shape, using lines, solid wedges, and dashed wedges to show three-dimensionality. (b) Draw the methane molecule inscribed inside ? tetrahedron such that the \(\mathrm{H}\) atoms touch the vertices of the tetrahedron. (c) Why is it better for methane to have \(109.5^{\circ}\) bond angles rather than \(90^{\circ}\) bond angles?

Shown below are dot diagrams for some simple molecules and polyatomic ions: \(: \ddot{B r}-\dot{S}_{\cdots}^{\prime \prime}-\vec{B}_{r}^{\prime \prime}\) (a) Draw the three-dimensional shape for each molecule. Use lines, solid wedges, and dashed wedges as necessary. Indicate the numeric value of all bond angles. (b) For each species, name both the electrongroup geometry around the central atom and the molecular shape.

Why is it reasonable to treat a multiple bond as a single electron group in VSEPR shape determination?

Two molecules may both be correctly described as bent, even though one has a bond angle of approximately \(118^{\circ}\) and the other has a bond angle of approximately \(105^{\circ} .\) How is this possible?

Consider the \(\mathrm{CH}_{3}\) anion. (a) Draw and name its three-dimensional shape and indicate the predicted values of its bond angles. (b) Explain how the structure of this anion would compare with that of \(\mathrm{NH}_{3}\) and why both would have bond angles that are less than ideal.

Ammonia has four pairs of electrons around the central nitrogen atom, and yet we don't call it a tetrahedral molecule. Why not? What is the shape of this molecule?

Consider \(\mathrm{CH}_{3}^{+}\) and \(\mathrm{C} \mathrm{H}_{3}^{-}\). Using these as examples, explain why it is necessary to draw a correct Lewis dot diagram before trying to predict the shape of a molecule. Use the concept of steric number in your explanation.

In terms of an operational definition, when is a molecule considered to be polar?

Is it possible to predict whether a molecule will be polar? Explain completely what you would need to know about the molecule.

True or false? All molecules that contain polar bonds must be polar. Explain your answer.

Consider the molecules \(\mathrm{HCl}\) and \(\mathrm{HBr}\). (a) Which molecule has the larger bond dipole moment? Explain why. (b) Which molecule is more polar? Explain why.

What does the magnitude of a bond dipole moment (the length of the arrow) tell you about the bond?

Why do we use an arrow to represent a bond dipole moment? Why not just use a number?

Consider an individual bond dipole moment. (a) What does electronegativity have to do with an individual bond dipole moment? (b) Explain how an individual bond dipole moment is drawn.

Which of these molecules is the least polar? (a) \(\mathrm{CH}_{2} \mathrm{O}\) (b) \(\mathrm{F}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{~F}\) (d) \(\mathrm{CH}_{3} \mathrm{Cl}\)

Consider \(\mathrm{SO}_{2}\) and \(\mathrm{CO}_{2}\). Both have polar covalent bonds. One of these molecules is polar and the other is nonpolar. Which is which and why?

Which of each pair is most polar? Explain. (a) Chlorine and phosphorus trichloride (b) Carbon disulfide and sulfur difluoride (c) Nitrogen trifluoride and phosphorus trifluoride (d) Silicon tetrabromide and hydrogen cyanide \((\mathrm{HCN})\) (e) Nitrogen trichloride and oxygen dichloride

Consider all the hydrogen halide molecules \(\mathrm{HX}\), where \(X\) is a group VIIA atom. (a) Which is the most polar? Why? (b) Which is the least polar? Why? (c) Draw all these molecules, showing their relative bond dipole moments.

Consider the two molecules \(\mathrm{CO}\) and \(\mathrm{CO}_{2}\). They are both made of the same elements, and yet only one is polar. Why is this so?

Consider the following molecules. For those that are polar, draw the molecular dipole moment. (a) \(\mathrm{CHBr}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{Br}\) (c) \(\mathrm{H}_{2} \mathrm{~S}\) (d) \(\mathrm{NOC}\) (e) \(\mathrm{C}_{2} \mathrm{Cl}_{2}\) (connected \(\left.\mathrm{Cl}-\mathrm{C}-\mathrm{C}-\mathrm{Cl}\right)\)

What is a dipole-dipole force? Give an example.

Are dipole-dipole forces between molecules as strong as the forces between oppositely charged ions? Explain.

What do we mean by intermolecular forces? What evidence is there that they exist?

Draw two HCl molecules and show how they would be attracted to each other. Show the partial charges and dipole moment vectors for both molecules, and orient the molecules properly with respect to each other.

Why is it important to always show the lone pairs in a Lewis dot diagram?

The dot diagram \(\ddot{\mathrm{Q}}=\mathrm{S}=\ddot{\mathrm{O}}\) for \(\mathrm{SO}_{2}\) is incorrect. (a) Draw the correct dot diagram. (b) How do the correct and incorrect dot diagrams differ in their prediction of molecular shape and polarity?

Consider the molecule \(\mathrm{SiCl}_{4}\). (a) Draw the dot diagram. (b) Draw the molecule's three-dimensional shape, and label the numeric value of all bond angles. (c) What is the shape of this molecule? (d) Draw in the individual bond dipole moments. (e) Is the molecule polar? If yes, draw the molecular dipole moment vector.

Consider the molecule \(\mathrm{AsF}_{3}\) (a) Draw the dot diagram. (b) Draw the molecule's three-dimensional shape, and label the numeric value of all bond angles. (c) What is the shape of this molecule? (d) Draw in the individual bond dipole moments. (e) Is the molecule polar? If yes, draw the molecular dipole moment vector.

Consider the molecule \(\mathrm{SO}_{3}\). (a) Draw the dot diagram. (b) Draw the molecule's three-dimensional shape, and label the numeric value of all bond angles. (c) What is the shape of this molecule? (d) Draw in the individual bond dipole moments. (e) Is the molecule polar? If yes, draw the molecular dipole moment vector.

The atoms in the molecule HSCN are connected in the order given in the formula. (a) Draw the dot diagram. (b) Draw the molecule's three-dimensional shape, and label the numeric value of all bond angles. (c) Draw in the individual bond dipole moments. (d) Is the molecule polar? If yes, draw the molecular dipole moment vector.

Consider the molecule \(\mathrm{HNF}_{2}(\mathrm{~N}\) is the central atom in the molecule). (a) Draw the dot diagram. (b) Draw the molecule's three-dimensional shape, and label the numeric value of all bond angles. (c) What is the shape of this molecule? (d) Draw in the individual bond dipole moments. (e) Is the molecule polar? If yes, draw the molecular dipole moment vector.

Consider the molecule \(\mathrm{N}_{2} \mathrm{O}\) (connected \(\mathrm{N}-\mathrm{N}-\mathrm{O})\) (a) Draw the dot diagram. (b) Draw the molecule's three-dimensional shape, and label the numeric value of all bond angles. (c) What is the shape of this molecule? (d) Draw in the individual bond dipole moments. (e) Is the molecule polar? If yes, draw the molecular dipole moment vector.

Some molecules pose special challenges to the rules for obtaining the correct Lewis dot diagram. Consider the molecule \(\mathrm{NO}_{2}(\mathrm{~N}\) is the central atom). (a) What challenge does it present? (b) Suppose you were allowed to violate the octet rule for the \(\mathrm{N}\) atom. What would the dot diagram look like? (c) Based on your answer to part (b), what is the shape of such a molecule? Is the molecule polar?

Consider the phosphonium ion, \(\mathrm{PH}_{4}^{+}\). (a) Draw the dot diagram. (b) Draw the ion's three-dimensional shape, and label the numeric value of all bond angles. (c) What is the shape of this polyatomic ion? (d) Draw in the individual bond dipole moments.

Imagine that you could vary the magnitude of the molecular dipole moment of all molecules in a substance. What would happen to the strength of the intermolecular forces between a group of molecules as the dipole moment of each molecule is increased? Explain.

Consider the \(\mathrm{PX}_{3}\) molecule, where \(\mathrm{X}\) is either \(\mathrm{H}\) or \(\mathrm{F}\). (a) For \(\mathrm{X}=\mathrm{H}\), the entire molecule is nonpolar. Why is this so? (Hint: Consider electronegativities.) (b) For \(\mathrm{X}=\mathrm{F}\), the entire molecule is polar. Draw two molecules next to one another in proper orientation so as to yield a dipolar intermolecular attraction, and explain why the attraction occurs.

An atom has no lone pairs of electrons on it and four other atoms bound to it. Why is \(109.5^{\circ}\) the bond angle adopted by this molecule?

Using lines, solid wedges, and dashed wedges, draw the threedimensional shape of ethane, \(\mathrm{C}_{2} \mathrm{H}_{6}\). Indicate the numeric value of all bond angles.

Using lines, solid wedges, and dashed wedges, draw the three-dimensional shape of acetaldehyde, \(\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}\). Indicate the numeric value of all bond angles.

Consider the molecules \(\mathrm{CH}_{4}, \mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{2} \mathrm{Cl}_{2}, \mathrm{CHCl}_{3}\) and \(\mathrm{CCl}_{4}\). Which are polar and which are nonpolar?

Using lines, solid wedges, and dashed wedges, draw the three-dimensional shape of chloromethylacetylene, \(\mathrm{C}_{3} \mathrm{H}_{3} \mathrm{Cl}\). Indicate the numeric value of all bond angles.