CHEMISTRY 1E03-01 TUTORIAL #7

Dr. J. D. Laposa November 2 - 6, 1998

_________________________________________________________ __
NOTE: No Chem 1E03-01 students will have a formal tutorial during the week of November 2. You should try the problems on your own. Solutions will be posted.

1.a) Draw resonance forms for

SO32- 26 valence electrons

CO32- 24 valence electrons

NO3- 24 valence electrons

ClO3- 26 valence electrons

ClO2- 20 valence electrons

b) What are the bond orders for the central-atom-to-oxygen bonds in these species?

For SO32-, the bond order is 4/3.

For CO32-, the bond order is 4/3.

For NO3-, the bond order is 4/3.

For ClO3-, the bond order is 5/3.

For ClO2-, the bond order is 3/2.br>

c) What is the average formal charge on the oxygen atoms in each of these species?

The average formal charge on oxygen in SO32- is -2/3.

The average formal charge on oxygen in CO32- is -2/3.

The average formal charge on oxygen in NO3- is -2/3 (note that N has a formal charge of +1 in this ion).

The average formal charge on oxygen in ClO3- is -1/3.

The average formal charge on oxygen in ClO2-, is -1/2.

d) Choose all of the species of part a that violate the octet rule.

The species that violate the octet rule are SO32-,ClO3-, and ClO2-

e) Give the AXnEm classes and predict the shapes of all the anions.

SO32- - AX3E - triangular pyramidal

CO32- - AX3 - triangular planar

NO3- - AX3 - triangular planar

ClO3- - AX3E - triangular pyramidal

ClO2- - AX2E2 - bent

2. Oxalic acid, (COOH)2, has two ionizable hydrogens and contains a carbon-carbon single bond. When one mole of oxalic acid reacts with one mole of calcium hydroxide,Ca(OH)2, the salt CaC2O4 is formed.

a) Draw adequate Lewis structures to represent the bonding in the oxalate ion, C2O42-.

There are 34 valence electrons.

b) What is the carbon-oxygen bond order in oxalate ion?

Bond order = (1 + 2 + 1 + 2)/4 = 1.5

c) What is the formal charge on each oxygen atom in oxalate ion?

Formal charge on oxygen = (-1 + 0 + (-1) + 0)/4 = -1/2

3. N2O has a linear, unsymmetrical structure that may be thought of as a hybrid of two resonance forms. Draw the two resonance forms of N2O.

There are 16 valence electrons. (Also remember that N and O obey the octet rule.)

4. A solid non-metallic element (A) from Group 5A was burned in excess oxygen to give a white solid (B) (Rxn 1). The white solid (B) dissolves in water to give a solution of a substance (C) (Rxn 2); (C) turns blue litmus red. To this solution of (C) was added an excess of aqueous KOH to produce an aqueous solution of (D) (Rxn 3). Write balanced equations for Rxns 1, 2, and 3; identify A, B, C, D.

Reaction 1: P4(s) + 5O2(g) --> P4O10(s). Thus A is P4, and B is P4O10

Reaction 2:P4O10(s) + 6H2O(l) --> 4H3PO4(aq). Thus C is H3PO4.

Reaction 3:H3PO4(aq) + 3KOH(aq) --> K3PO4(aq) + 3H2O(l). Thus D is K3PO4.

5. Phosphoric acid, H3PO4, contains three oxygen-hydrogen single bonds. When one mole of phosphoric acid reacts with one mole of sodium hydroxide, the salt sodium dihydrogen phosphate, NaH2PO4, results.

a) Draw adequate Lewis structures for H3PO4 and H2PO4-.

H3PO4 has 32 valence electrons.

H2PO4- has 32 valence electrons.

b) Calculate bond orders for all the phosphorus-oxygen bonds of H3PO4 and H2PO4-.

In H3PO4, there is one P-O bond of bond order 2, and three P-O bonds of bond order 1.

In H2PO4-, there are two P-O bonds of bond order 1.5, and two P-O bonds of bond order 1.

c) Calculate the formal charges on each oxygen atom in H3PO4 and H2PO4-.

In H3PO4, all oxygen atoms have a formal charge of zero.

In H2PO4-, two oxygen atoms (bonded to H) have a formal charge of zero, while the other tow have a formal charge of -1/2.

6. For the molecules OCS, ICl3, IF5, SO2, PF3, and CCl2Br2:

a) determine the AXnEm class;

b) decide which have dipole moments, and for those that do, decide the direction of the dipole moment.

OCS has 16 valence electrons. Its Lewis structure (with bond moments shown) is

It is an AX2 molecule, with a net dipole moment pointing to O.

ICl3 has 28 valence electrons. Its Lewis structure (with bond moments shown) is

It is an AX3E2 molecule, with a net dipole moment pointing towards the equatorial Cl (the two axial bond moments cancel).

IF5 has 42 valence electrons. Its Lewis structure is

It is an AX5E molecule, with a net dipole moment pointing to the fluorine that is situated 180o from the nonbonding pair.

SO2 has 18 valence electrons. Its Lewis structure is

It is an AX2E molecule, with a net dipole moment pointing from S to a point midway between the O atoms.

PF3 has 26 valence electrons. Its Lewis structure is

It is an AX3E molecule, with a net dipole moment pointing from P to the center of the triangle formed by the three fluorines.

CCl2Br2 has 32 valence electrons. Its Lewis structure is

It is an AX4 molecule, with a net dipole moment pointing from C and bisecting the Cl-C-Cl angle.

7. A gaseous molecule "X" is made by burning a yellow elemental solid in air. "X" is converted to "Y" by reaction with oxygen in the presence of a catalyst. "Y" reacts vigorously with water to produce "Z" which turns litmus red.

a) Identify "X", "Y", and "Z".

"X" = SO2, "Y" is SO3, and "Z" is H2SO4.

b) Write a balanced equation for each reaction.

S(s) + O2(g) --> SO2(g)

2SO2(g) + O2(g) --> 2SO3(g)

SO3(g) + H2O(l) --> H2SO4(aq)

c) Draw Lewis diagrams and indicate the shapes of the molecules "X" and "Y".

SO2 has 18 valence electrons. Its Lewis structure is

The molecule is AX2E, and is bent.

SO3 has 24 valence electrons. Its Lewis structure is

The molecule is AX3, and is triangluar planar.

d) State whether "X" and "Y" possess dipole moments and, if so, in which direction they point.

SO2 has a dipole moment pointing from S to midway between the oxygen atoms.

SO3 has no dipole moment.

e) When 1 mole of "Z" is titrated with 2 moles of NaOH, a soluble salt "W" forms. Write an appropriate Lewis electron dot structure for the anion of this salt.

"W" is SO42-. There are six resonance structures. One of them is