Chapter 8

Some chemists believe that satisfaction of the octet rule should be the top criterion for choosing the dominant Lewis structure of a molecule or ion. Other chemists believe that achieving the best formal charges should be the top criterion. Consider the dihydrogen phosphate ion, \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) , in which the \(\mathrm{H}\) atoms are bonded to \(\mathrm{O}\) atoms. (a) What is the predicted dominant Lewis structure if satisfying the octet rule is the top criterion? (b) What is the predicted dominant Lewis structure if achieving the best formal charges is the top criterion?

State whether each of these statements is true or false. (a) The longer the bond, the larger the bond enthalpy. (b) \(C-C\) bonds are stronger than \(C-H\) bonds. (c) A typical single bond length is in the \(5-10\) Ä range. (d) If you break a chemical bond, energy is released.(e) Energy is stored in chemical bonds.

State whether each of these statements is true or false. (a) A carbon-carbon triple bond is shorter than a carbon-carbon single bond. (b) There are exactly six bonding electrons in the \(\mathrm{O}_{2}\) molecule. (c) The \(\mathrm{C}-\mathrm{O}\) bond in carbon monoxide is longer than the \(\mathrm{C}-\mathrm{O}\) bond in carbon dioxide. (d) The O \(-\mathrm{O}\) bond in ozone is shorter than the \(\mathrm{O}-\) O bond in \(\mathrm{O}_{2}\) . (e) The more electronegative the atom, the more bonds it makes to other atoms.

We can define average bond enthalpies and bond lengths for ionic bonds, just like we have for covalent bonds. Which ionic bond is predicted to be stronger, \(\mathrm{Na}-\mathrm{Cl}\) or \(\mathrm{Ca}-\mathrm{O}\) ?

We can define average bond enthalpies and bond lengths for ionic bonds, just like we have for covalent bonds. Which ionic bond is predicted to have the smaller bond enthalpy, Li-F or \(\mathrm{Cs}-\mathrm{F}\) ?

A carbene is a compound that has a carbon bonded to two atoms and a lone pair remaining on the carbon. Many carbenes are very reactive. (a) Draw the Lewis structure for the simplest carbene, \(\mathrm{H}_{2} \mathrm{C}\) . ( b) Predict the length of the carbon-carbon bond you would expect if two \(\mathrm{H}_{2} \mathrm{C}\) molecules reacted with each other by a combination reaction.

Draw the Lewis structure for \(\mathrm{NO}^{+} .\) Is the nitrogen-oxygen bond in \(\mathrm{NO}^{+}\) longer, shorter, or the same length as the nitrogen- oxygen bond in NO? Explain.

A new compound is made that has a \(\mathrm{C}-\mathrm{C}\) bond length of 1.15 \(\mathrm{A} .\) Is this bond likely to be a single, double, or triple \(\mathrm{C}-\mathrm{Cbond} ?\)

Consider the lattice energies of the following Group 2 \(\mathrm{A}\) compounds: \(\mathrm{Be} \mathrm{H}_{2}, 3205 \mathrm{kJ} / \mathrm{mol} ; \mathrm{MgH}_{2}, 2791 \mathrm{kJ} / \mathrm{mol}\) \(\mathrm{CaH}_{2}, 2410 \mathrm{kJ} / \mathrm{mol} ; \mathrm{SrH}_{2}, 2250 \mathrm{kJ} / \mathrm{mol} ; \mathrm{BaH}_{2}, 2121 \mathrm{kJ} / \mathrm{mol}\) (a) What is the oxidation number of \(\mathrm{H}\) in these compounds? (b) Assuming that all of these compounds have the same three-dimensional arrangement of ions in the solid, which of these compounds has the shortest cation-anion distance? (c) Consider BeH \(_{2} .\) Does it require 3205 kJ of energy to break one mole of the solid into its ions, or does breaking up one mole of solid into its ions release 3205 \(\mathrm{kJ}\) of energy? (d) The lattice energy of \(\mathrm{ZnH}_{2}\) is 2870 \(\mathrm{kJ} / \mathrm{mol}\) . Considering the trend in lattice enthalpies in the Group 2 \(\mathrm{A}\) compounds, predict which Group 2 \(\mathrm{A}\) element is most similar in ionic radius to the \(\mathrm{Zn}^{2+}\) ion.

An ionic substance of formula MX has a lattice energy of \(6 \times 10^{3} \mathrm{k} / \mathrm{mol} .\) Is the charge on the ion M likely to be \(1+, 2+,\) or \(3+?\) Explain.

Construct a Born-Haber cycle for the formation of the hypothetical compound NaCl , where the sodium ion has a \(2+\) charge (the second ionization energy for sodium is given in Table 7.2 . (a) How large would the lattice energy need to be for the formation of \(\mathrm{NaCl}_{2}\) to be exothermic? (b) If we were to estimate the lattice energy of \(\mathrm{NaCl}_{2}\) to be roughly equal to that of \(\mathrm{MgCl}_{2}(2326 \mathrm{kJ} / \mathrm{mol}\) from Table 8.1\(),\) what value would you obtain for the standard enthalpy of formation, \(\Delta H_{f}^{\circ},\) of \(\mathrm{NaCl}_{2} ?\)

A classmate of yours is convinced that he knows everything about electronegativity. (a) In the case of atoms \(X\) and \(Y\) having different electronegativities, he says, the diatomic molecule \(X-Y\) must be polar. Is your classmate correct? (b) Your classmate says that the farther the two atoms are apart in a bond, the larger the dipole moment will be. Is your classmate correct?

Consider the collection of nonmetallic elements \(\mathrm{O}, \mathrm{P}\) , Te, \(\mathrm{I},\) and \(\mathrm{B} .\) (a) Which two would form the most polar single bond? (b) Which two would form the longest single bond? (c) Which two would be likely to form a compound of formula \(\mathrm{XY}_{2} ?(\mathbf{d})\) Which combinations of elements would likely yield a compound of empirical formula \(\mathrm{X}_{2} \mathrm{Y}_{3}\) ?

The substance chlorine monoxide, ClO(g), is important in atmospheric processes that lead to depletion of the ozone layer. The ClO molecule has an experimental dipole moment of \(1.24 \mathrm{D},\) and the \(\mathrm{Cl}-\) O bond length is 1.60 \(\mathrm{A}\) . (a) Determine the magnitude of the charges on the Cl and O atoms in units of the electronic charge, \(e\) (b) Based on the electronegativities of the elements, which atom would you expect to have a partial negative charge in the Clo molecule? (c) Using formal charges as a guide, propose the dominant Lewis structure for the molecule. (d) The anion \(\mathrm{ClO}^{-}\) exists. What is the formal charge on the Cl for the best Lewis structure for \(\mathrm{ClO}^{-}\) ?

Although \(\mathrm{I}_{3}\) is a known ion, \(\mathrm{F}_{3}^{-}\) is not. (a) Draw the Lewis structure for \(\mathrm{I}_{3}^{-}\) (it is linear, not a triangle). (b) One of your classmates says that \(\mathrm{F}_{3}^{-}\) does not exist because \(\mathrm{Fis}\) too electronegative to make bonds with another atom. Give an example that proves your classmate is wrong. (c) Another classmate says \(\mathrm{F}_{3}^{-}\) does not exist because it would violate the octet rule. Is this classmate possibly correct? (d) Yet another classmate says \(\mathrm{F}_{3}^{-}\) does not exist because \(\mathrm{F}\) is too small to make bonds to more than one atom. Is this classmate possibly correct?

Calculate the formal charge on the indicated atom in each of the following molecules or ions: (a) the central oxygen atom in \(\mathrm{O}_{3},\) (b) phosphorus in \(\mathrm{PF}_{6}^{-},(\mathbf{c})\) nitrogen in \(\mathrm{NO}_{2}\) (d) iodine in ICl_\(\mathrm{ICl}_{3},\) (e) chlorine in \(\mathrm{HClO}_{4}\) (hydrogen is bonded to \(\mathrm{O} )\)

The hypochlorite ion, \(\mathrm{ClO}^{-},\) is the active ingredient in bleach. The perchlorate ion, \(\mathrm{ClO}_{4}^{-},\) is a main component of rocket propellants. Draw Lewis structures for both ions (a) What is the formal charge of Cl in the hypochlorite ion? (b) What is the formal charge of Cl in the perchlorate ion, assuming the Cl-O bonds are all single bonds? (c) What is the oxidation number of Cl in the hypochlorite ion? (d) What is the oxidation number of \(C l\) in the perchlorate ion, assuming the \(C l-O\) bonds are all single bonds? (e) In a redox reaction, which ion would you expect to be more easily reduced?

(a) Triazine, \(\mathrm{C}_{3} \mathrm{H}_{3} \mathrm{N}_{3},\) is like benzene except that in triazine every other \(\mathrm{C}-\mathrm{H}\) group is replaced by a nitrogen atom. Draw the Lewis structure(s) for the triazine molecule. (b) Estimate the carbon-nitrogen bond distances in the ring.

Consider the hypothetical molecule \(\mathrm{B}-\mathrm{A}=\mathrm{B}\) . Are the following statements true or false? (a) This molecule cannot exist. (b) If resonance was important, the molecule would have identical \(A-B\) bond lengths.

The Ti \(\mathrm{Ti}^{2+}\) ion is isoelectronic with the Ca atom. (a) Write the electron configurations of \(\mathrm{Ti}^{2+}\) and (b) Calculate the number of unpaired electrons for Ca and for \(\mathrm{Ti}^{2+} .\) (c) What charge would Ti have to be isoelectronic with \(\mathrm{Ca}^{2}+?\)

(a) Draw the Lewis structure for hydrogen peroxide, \(\mathrm{H}_{2} \mathrm{O}_{2}\) . (b) What is the weakest bond in hydrogen peroxide? (c) Hydrogen peroxide is sold commercially as an aqueous solution in brown bottles to protect it from light. Calculate the longest wavelength of light that has sufficient energy to break the weakest bond in hydrogen peroxide.

You and a partner are asked to complete a lab entitled "Oxides of Ruthenium" that is scheduled to extend over two lab periods. The first lab, which is to be completed by your partner, is devoted to carrying out compositional analysis. In the second lab, you are to determine melting points. Upon going to lab you find two unlabeled vials, one containing a soft yellow substance and the other a black powder. You also find the following notes in your partner's notebook Compound \(1 : 76.0 \%\) Ru and 24.0\(\%\) O (by mass), Compound \(2 : 61.2 \%\) Ru and 38.8\(\%\) O (by mass). (a) What is the empirical formula for Compound 1\(?\) (b) What is the empirical formula for Compound 2\(?\) Upon determining the melting points of these two compounds, you find that the yellow compound melts at \(25^{\circ} \mathrm{C}\) , while the black powder does not melt up to the maximum temperature of your apparatus, \(1200^{\circ} \mathrm{C}\) . (c) What is the identity of the yellow compound? (d) What is the identity of the black compound? (e) Which compound is molecular? (f) Which compound is ionic?

The compound chloral hydrate, known in detective stories as knockout drops, is composed of \(14.52 \% \mathrm{C}, 1.83 \% \mathrm{H}\) , \(64.30 \% \mathrm{Cl},\) and 13.35\(\% \mathrm{O}\) by mass, and has a molar mass of 165.4 \(\mathrm{g} / \mathrm{mol}\) . (a) What is the empirical formula of this substance? (b) What is the molecular formula of this substance? (c) Draw the Lewis structure of the molecule, assuming that the Cl atoms bond to a single \(C\) atom and that there are a \(C-C\) bond and two \(C-O\) bonds in the compound.

Barium azide is 62.04\(\%\) Ba and 37.96\(\%\) N. Each azide ion has a net charge of \(1-\) (a) Determine the chemical formula of the azide ion. (b) Write three resonance structures for the azide ion. (c) Which structure is most important? (d) Predict the bond lengths in the ion.

Under special conditions, sulfur reacts with anhydrous liquid ammonia to form a binary compound of sulfur and nitrogen. The compound is found to consist of 69.6\(\% \mathrm{S}\) and 30.4\(\% \mathrm{N} .\) Measurements of its molecular mass yield a value of 184.3 \(\mathrm{g} / \mathrm{mol}\) . The compound occasionally detonates on being struck or when heated rapidly. The sulfur and nitrogen atoms of the molecule are joined in a ring. All the bonds in the ring are of the same length. (a) Calculate the empirical and molecular formulas for the substance. (b) Write Lewis structures for the molecule, based on the information you are given. (Hint: You should find a relatively small number of dominant Lewis structures.) (c) Predict the bond distances between the atoms in the ring. (Note: The \(S-S\) distance in the \(S_{8}\) ring is 2.05 A.) ( d.) The enthalpy of formation of the compound is estimated to be 480 \(\mathrm{kJ} / \mathrm{mol}^{-1} . \Delta H_{f}^{9}\) of \(\mathrm{S}(g)\) is 222.8 \(\mathrm{kJ} / \mathrm{mol} .\) Estimate the average bond enthalpy in the compound.

A common form of elemental phosphorus is the tetrahedral \(\mathrm{P}_{4}\) molecule, where all four phosphorus atoms are equivalent: At room temperature phosphorus is a solid. (a) Are there any lone pairs of electrons in the \(\mathrm{P}_{4}\) molecule? (b) How many \(\mathrm{p}-\mathrm{p}\) bonds are there in the molecule? (c) Draw a Lewis structure for a linear \(P_{4}\) molecule that satisfies the octet rule. Does this molecule have resonance structures? (d) On the basis of formal charges, which is more stable, the linear molecule or the tetrahedral molecule?

Formic acid has the chemical formula HCOOH. It is a colorless liquid that has a density of 1.220 \(\mathrm{g} / \mathrm{mL}\) . (a) The carbon atom in formic acid is bound to one \(\mathrm{H}\) and both \(\mathrm{O}^{\prime}\) 's. Draw the Lewis structure for formic acid, showing resonance if present. (b) Formic acid can react with NaOH in aqueous solution to produce the formate ion, HCOO- . Write the balanced chemical equation for this reaction. (c) Draw the Lewis structure of the formate ion, showing resonance if present. (d) How many milliliters of a 0.100 M solution of NaOH would it take to completely react with 0.785 \(\mathrm{mL}\) of formic acid?

Ammonia reacts with boron trifluoride to form a stable compound, as we saw in Section 8.7 . (a) Draw the Lewis structure of the ammonia-boron trifluoride reaction product. (b) The B-N bond is obviously more polar than the \(\mathrm{C}-\mathrm{C}\) bond. Draw the charge distribution you expect on the \(\mathrm{B}-\mathrm{N}\) bond within the molecule (using the delta plus and delta minus symbols mentioned in Section 8.4\()\) . ( ) Boron trichloride also reacts with ammonia in a similar way to the trifluoride. Predict whether the \(B-N\) bond in the trichloride reaction product would be more or less polar than the \(B-N\) bond in the trifluoride product, and justify your reasoning.

Ammonium chloride, \(\mathrm{NH}_{4} \mathrm{Cl},\) is a very soluble salt in water. (a) Draw the Lewis structures of the ammonium and chloride ions. (b) Is there an \(\mathrm{N}-\) Cl bond in solid ammonium chloride? (c) If you dissolve 14 gof ammonium chloride in 500.0 \(\mathrm{mL}\) of water, what is the molar concentration of the solution? (d) How many grams of silver nitrate do you need to add to the solution in part (c) to precipitate all of the chloride as silver chloride?