CHEM 1A03 TUTORIAL #4 Oct 1 Oct. 5, 2012 Solutions Chapter 10. The following problems (1-8) are based on material from Chapter 10 and class notes.

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1 HEM 1A03 TUTRIAL #4 ct 1 ct. 5, 2012 olutions hapter 10 The following problems (1-8) are based on material from hapter 10 and class notes. 1. Arrange the following in terms of increasing electronegativity: (a) Mg, N,, i, (b), e, n, Te (a) Mg < i < N < EN increases left to right across a period and bottom to (b) n < Te < e < top of a group. 2. or the following anions: 3 2-, 3 2-, N 3 -, 3 -, and 2 - : a) Draw Lewis structures and resonance forms b) What are the average bond orders for the central-atom-to-oxygen bonds in these species? c) What is the average formal charge on the oxygen atoms in each of these species? d) Indicate whether the species is an exception to the octet rule (i.e. it has more than an octet of electrons around one of the atoms). e) Give the VEPR AB n E m classes f) Predict the shapes of the anion. Note that the figures used to illustrate the shapes in point f) do not indicate the bond orders and the formal charges a) 26 valence electrons b) or 3 2-, the average bond order is 4/3. c) The average formal charge on oxygen in 3 2- is -2/3. d) The charge-minimized resonance structures do not follow the octet rule. e) AB 3 E f) triangular pyramidal Page 1 of 9

2 HEM 1A03 TUTRIAL #4 ct 1 ct. 5, 2012 olutions hapter a) 24 valence electrons b) or 2-3, the average bond order is 4/3. c) The average formal charge on oxygen in 2-3 is -2/3. d) The resonance structures do follow the octet rule. e) AB 3 f) triangular planar or trigonal planar N 3 - a) 24 valence electrons N N N b) or N - 3, the average bond order is 4/3. c) The average formal charge on oxygen in N - 3 is -2/3. d) The resonance structures do follow the octet rule. e) AB 3 f) triangular planar or trigonal planar N 3 - a) 26 valence electrons b) or 3 -, the average bond order is 5/3. c) The average formal charge on oxygen in 3 - is -1/3. d) The charge-minimized resonance structures do not follow the octet rule. e) AB 3 E f) triangular pyramidal or trigonal pyramidal Page 2 of 9

3 HEM 1A03 TUTRIAL #4 ct 1 ct. 5, 2012 olutions hapter a) 20 valence electrons b) or 2 -, the average bond order is 3/2. c) The average formal charge on oxygen in 2 -, is -1/2. d) The charge-minimized resonance structures do not follow the octet rule. e) AB 2 E 2 f) bent (or angular or v-shape) 3. xalic acid, (H) 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,a(h) 2, the salt a 2 4 is formed. a) Draw adequate Lewis structures to represent the bonding in the oxalate ion, There are 34 valence electrons. b) What is the average carbon-oxygen bond order in oxalate ion? Average bond order = ( )/4 = 1.5 c) What is the average formal charge on each oxygen atom in oxalate ion? Average formal charge on oxygen = ( (-1) + 0)/4 = -1/2 d) Give the shape around each atom in the oxalate ion. Each atom is AX 3, trigonal planar (triangular planar). Page 3 of 9

4 HEM 1A03 TUTRIAL #4 ct 1 ct. 5, 2012 olutions hapter Phosphoric acid, H 3 P 4, contains three oxygen-hydrogen single bonds. When one mole of phosphoric acid reacts with one mole of sodium hydroxide, the salt sodium dihydrogen phosphate, NaH 2 P 4, results. a) Draw adequate Lewis structures for H 3 P 4 and H 2 P 4 -. H 3 P 4 has 32 valence electrons. H 2 P 4 has 32 valence electrons. H P H H P H P H H H b) alculate bond orders for all the phosphorus-oxygen bonds of H 3 P 4 and H 2 P 4 - In H 3 P 4, there is one P- bond of bond order 2, and three P- bonds of bond order 1. In H 2 P 4 -, there are two P- bonds of bond order 1.5, and two P- bonds of bond order 1. c) alculate the formal charges on each oxygen atom in H 3 P 4 and H 2 P 4 -. In H 3 P 4, all oxygen atoms have a formal charge of zero. In H 2 P 4 -, two oxygen atoms (bonded to H) have a formal charge of zero, while the other two have a formal charge of -1/2. 5. A gaseous molecule "X" is made by burning sulfur 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" = 2, "Y" is 3, and "Z" is H 2 4. b) Write a balanced equation for each reaction. 8 (s) (g) --> 8 2 (g) or (s) + 2 (g) --> 2 (g) 2 2 (g) + 2 (g) --> 2 3 (g) 3 (g) + H 2 (l) --> H 2 4 (aq) c) Draw Lewis diagrams of the molecules "X" and "Y". Determine their AB n E m VEPR classes and name the molecular shapes. Indicate the bond angles, and if they are ideal or less than ideal. Are the "X" and "Y" molecules polar or non-polar? Page 4 of 9

5 HEM 1A03 TUTRIAL #4 ct 1 ct. 5, 2012 olutions hapter 10 2 has 18 valence electrons. Its Lewis structure is (2 double bonds) INRRET: RRET: Petrucci does not show the charge-minimized structure, as seen on the right-hand side. We will draw this structure. 3 has 24 valence electrons. Its Lewis structure is.... or 2, (b) its VEPR class is AX 2 E. The shape is bent (or angular, or v-shape). The -- bond angle will be less than the ideal angle of 120, because of the influence of the lone electron pair on sulfur. The molecule is polar (the - bonds are polarized towards oxygen, and the resulting molecular dipole is represented with a cross-base arrow that bisects the -- angle), pointing to the right, in this case. Repulsion Repulsion or 3, its VEPR class is AX 3. The shape is trigonal planar (or triangular planar). The -- bond angles will be the ideal angle of 120. The molecule is non-polar (the - bonds are polarized towards oxygen, but there is no resulting molecular dipole - the bond dipoles cancel out because of the symmetry of the molecule). d) When 1 mole of "Z" is titrated with 2 moles of NaH, a soluble salt "W" forms. Write an appropriate Lewis electron dot structure for the anion of this salt. The anion of the salt "W" is There are six resonance structures. ne of them is Page 5 of 9

6 HEM 1A03 TUTRIAL #4 ct 1 ct. 5, 2012 olutions hapter or the molecules, I 3, I 5, P 3, and 2 Br 2 : a) determine the AB n E m class; name the molecular shape. b) decide which have dipole moments, and for those that do, decide the direction of the dipole moment. has 16 valence electrons. Its Lewis structure (with bond dipole moments represented in black cross-base arrows and molecular dipole moment shown in a red cross-base arrow) is Total It is an AB 2 molecule (linear shape), with a net dipole moment, represented with a cross-base arrow pointing to. I 3 has 28 valence electrons. Its Lewis structure is I It is an AB 3 E 2 molecule (T-shaped), with a net dipole moment pointing towards the equatorial (the two axial bond moments cancel). I I 5 has 42 valence electrons. Its Lewis structure is Page 6 of 9

7 HEM 1A03 TUTRIAL #4 ct 1 ct. 5, 2012 olutions hapter 10 I It is an AX 5 E molecule (square-based pyramid shape), with a net dipole moment pointing to the fluorine that is situated 180 o from the nonbonding pair. I P 3 has 26 valence electrons. Its Lewis structure is P It is an AB 3 E molecule (trigonal pyramid or triangular pyramid shape), with a net dipole moment pointing from P to the center of the triangle formed by the three fluorine atoms. P Total 2 Br 2 has 32 valence electrons. Its Lewis structure is Br Br It is an AB 4 molecule (tetrahedral shape), with a net dipole moment pointing from and bisecting the -- angle. Total Br Br Page 7 of 9

8 HEM 1A03 TUTRIAL #4 ct 1 ct. 5, 2012 olutions hapter 10 hallenge and Interest Question: 7. Dichlorodiphenyltrichloroethane (DDT) is one of the most well investigated class of organochlorine chemical that has been widely used as an insecticide. (a) Determine the molecular shape around the central atom (H), and the molecular dipole moment of DDT. H * Plane of symmetry Despite the polarity of - bond due to its moderate electronegativity difference, DDT is overall a non-polar molecule due to its symmetric structure and plane of symmetry where all bond dipoles cancel out AX 4 Tetrahedral shape (b) Relate its molecular properties above with the tendency for DDT to bio-accumulate in the fatty tissue of living organisms. ince DDT is a nonpolar molecule, it has poor solubility properties in polar solvents, such as water, yet is extremely soluble in organic solvents and fats lipophilic. Thus, any DDT released into the aqueous environment readily accumulates into fatty tissue of organisms bio-accumulate. ombined with its persistence and non-reactivity, DDT tends to biomagnify up the food chain, where it is highly concentrated despite its original low concentration level in the environment. (c) Justify the advantages and disadvantages of the usage of DDT in terms of its longterm impact on health and environment based on the quote below: Although DDT is generally not toxic to human beings and was banned mainly for ecological reasons, subsequent research has shown that exposure to DDT at amounts that would be needed in malaria control might cause preterm birth and early weaning, abrogating the benefit of reducing infant mortality from malaria...ddt might be useful in controlling malaria, but the evidence of its adverse effects on human health needs appropriate research on whether it achieves a favourable balance of risk versus benefit. W.J. Rogan The Lancet, 2005, 366, Advantages: 1. Insecticidal properties that help reduce populations of insect carriers of infectious diseases, such as malaria and typhus Health & Infectious Disease Management 2. imilarly, it can be applied in agricultural fields to reduce crop damage due to insect infestations Economy, ood Production, Hunger 3. DDT can be manufactured from inexpensive reagents and it is cheap to produce 2, H 3 H 2 H and hlorobenzene Affordable access to developing countries Page 8 of 9

9 HEM 1A03 TUTRIAL #4 ct 1 ct. 5, 2012 olutions hapter Due to its chemical persistence without degrading in environment, DDT does not need to be regularly re-applied, thus reducing overall costs hemical persistence is good Disadvantages: 1. Excessive and indiscriminate usage can lead to insect-resistance, where higher amounts of DDT are needed while becoming less effective Insect-resistance 2. The same process also exposes other beneficial insects and wildlife in nature Nonselective toxicity to other insects/animals 3. Long-term exposure has also been linked to chronic disorders and negative health outcomes in humans, notably the most sensitive sub-populations hronic diseases 4. Due to its chemical persistence without degrading in environment, DDT is easily transported across the planet far from its original source (Artic) hemical persistence is bad Ultimately, government regulation of DDT as formulated in the tockholm onvention for persistent organic pollutants needs to balance long-term benefit V risk, as well as promoting safer alternatives. Page 9 of 9