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Unit IV Homework Set General Chemistry I

5. (from Unit II) On the “mini-exam” from earlier in semester, we considered the Earth-saving

chemistry of selenium, including some of the compounds listed below, and we introduced the selenate ion, 2

4SeO .

A. For each of the compounds shown, assign the oxidation numbers and write them in the spaces provided.

SeO2 Se _______ O _______ K2Se K _______ Se _______ H2Se H _______ Se _______ H2SeO4 Se ______ H2SeO3 Se _______ 7. (from Unit II) Appendicitis is caused by inflammation of the appendix...

B. In part A, the last compound is formed from the first compound according to the following chemical equation:

SeO2 + H2O H2SeO3

Is the above reaction a redox reaction? Explain your reasoning.

No. None of the oxidation numbers changed, so it is not a redox reaction.

2. Assign oxidation numbers for all of the atoms in this reaction. Write the oxidation numbers in parentheses above the formula.

___ ___ ___ ___ ___ ___ ___ ___ ___ \ / \ / | \ / \ / 4N2O(g) + CH4(g) 4N2(g) + 2H2O(g) + CO2(g)

Which element is oxidized? carbon Which is reduced? nitrogen (-4) to (+4) (+1) to (0) 14. (from Unit II) The proper assignments of oxidation numbers (ON) are necessary for recognizing and

balancing redox reactions (e.g. questions IV and V on page 3), but they are also useful for understanding other aspects of chemical reactivity. For example, I mentioned in class that oxidation numbers allow one to deduce the relative strength of analogous acids.

(+1) (-2) (-4) (+1) (0) (+1) (-2) (+4) (-2)

0 +1 -2 +3 -2 0

D. On October 12, 2008, an oleum spill at a chemical plant in western Pennsylvania forced about 2,500 residents to evaluate. Oleum – also known as fuming sulfuric acid, H2SO4xSO3 – is especially dangerous because the sulfur trioxide escapes as a gas and reacts with moisture in the air to form sulfuric acid according to the reaction SO3(g) + H2O(g) H2SO4(g).

Is this a redox reaction? Explain your reasoning. (+6) (-2) (+1) (-2) (+1)(+6)(-2)

SO3(g) + H2O(g) H2SO4(g) No change in oxidation numbers, so not a redox reaction. 16. (from Unit II) Titration with an aqueous solution of potassium dichromate is a useful procedure for

determining the amount of iodide in solution. The unbalanced chemical equation for this reaction is given below. Assign the oxidation number of the atoms indicated below (all atoms except H and O).

3. (from Unit III) We carried out the thermite reaction...

C. Write the oxidation numbers in the spaces provided. ___ ___ ___ ___ ___ ___ | \ / \ / | 2Al(s) + 3H2O(s) Al2O3(s) + 3H2(g) 9. (from Unit III) Yttrium is also a useful starting material for making other yttrium compounds... D. Circle the one that is larger? 1. O F 2. O2 F

3. Cl F E. Why would picking the larger between oxygen and chlorine be more difficult? The two trends (Zeff increases as you go to the right, and n decreases as you go up) are opposed for

oxygen and chlorine: Cl is right of O, but O is above Cl.

16. (Fall 2015) Write ground-state electronic configurations for the following atoms or ions and give the number of unpaired electrons:

Write ground-state electronic configurations for the following atoms or ions and give the number of unpaired electrons:

P(g) [Ne]3s23p3 3 unpaired electrons

Ti(g) [Ar]4s23d2 2 unpaired electrons

Pt(g) [Xe]6s24f145d8 2 unpaired electrons 21. (Fall 2008) ATOMIC PROPERTIES

A. Circle the atom expected to have the larger radius.

8. (Fall 2009) ATOMIC PROPERTIES A. For the following sets of atoms, write an “L” above the one that is largest, and an “S” above the

one that is smallest.

C. Would you expect the following reaction to be more favored towards the left (reactants) or towards the right (products)?

C(g) + O+(g) C+(g) + O(g)

We could use IE(O) and IE(C) to determine H, but first try to reason out the answer from the trends in ionization energies. In general, ionization energy increases as you go to the right on the periodic table due to the increase in Zeff. So we expect the ionization energy of oxygen to be greater than the ionization energy of carbon...

IE1(O) > IE1(C) => the carbon atom is easier to ionize than the oxygen atom

=> the products are favored (towards the right)

Now, using IE(O) and IE(C)

C(g) C+(g) + e- H = IE1(C) = 1086.5 kJ/mol O(g) O+(g) + e- H = IE1(O) = 1313.9 kJ/mol => O+(g) + e- O(g) H = -IE1(O) = -1313.9 kJ/mol C(g) C+(g) + e- H = IE1(C) = 1086.5 kJ/mol O+(g) + e- O(g) H = -IE1(O) = -1313.9 kJ/

C(g) + O+(g) C+(g) + O(g) H = 1086.5 – 1313.9 kJ/mol = -227.4 kJ/mol

exothermic => products favored (towards the right) 9. (Fall 2008) Consider the following gas phase reaction: F2(g) + I2(g) 2FI(g)

B. Now use bond energies to estimate H° for the above reaction. Using Hess’s Law F2(g) 2F(g) H°BE(F-F) I2(g) 2I(g) H°BE(I-I) 2F(g) + 2I(g) 2FI(g) -2H°BE(F-I) F2(g) + I2(g) 2FI(g) H°rxn = H°BE(F-F) + H°BE(I-I) -2H°BE(F-I)

We could use Hrxn = BE(reactants) BE(products), which we will cover on Monday, April 23. We, can also determine Hrxn from a thermodynamic cycle.

2F(g) + 2I(g) HBE(F-F) -2HBE(F-I) HBE(I-I) Hrxn F2(g) + I2(g) 2FI(g)

Hrxn = HBE(F-F) + HBE(I-I) -2HBE(F-I)

11. (Fall 2015) Draw dot structures as instructed below, and answer any associated questions. Be sure to show all non-zero formal charges.

A. Draw the best dot structure you can for OF2. B. Draw the best dot structure you can for HNNH, where the atoms are connected as written. C. Draw the best dot structure you can for the hypothetical molecule NCOF which contains all of the

nonmetal elements from the second row of the periodic table, except neon.

Which bond in NCOF do you expect to be the strongest? the NC bond

Why? The NC bond is a triple bond. The other two bonds are single bonds. D. Draw two dot structures for SeO2. best dot structure you can best dot structure you can that obeys the octet rule that exceeds the octet rule to minimize formal charge Would the molecular geometries corresponding to the two dot structures you drew above be the same or different? the same Explain why or why not. Both have three “things” around the central atom: two oxygen atoms and a lone pair, so the molecular geometry is the same (bent). 12. (Fall 2015) Consider the fluorine compounds shown below.

A. Which of these molecules contain polar bonds?

all except F2 have polar bonds

B. Which of these molecules are polar? NF3 and OF2

13. (Fall 2012) Write the best dot structure you can and assign all non-zero formal charges for the following molecules/ions. Draw a box around the structure you consider to be the best (i.e. the one you want graded).

OBrF (Br is the central atom) PF4

14. (Fall 2012) I really botched the question on the last exam involving the partial oxidation of

hydrocarbons as a route to make hydrogen gas. Let’s see if I can get it right this time. We’ll keep things simple by considering the partial oxidation of ethane as shown in the following reaction:

C2H6(g) + O2(g) 2CO(g) + 3H2(g)

And I do not want to leave out any important reactions and H values, so instead of using Hess’s law, let’s use the bond enthalpy data provided on the last page of the info sheet.

A. First, write dot structures for C2H6, O2, CO, and H2. You will need these dot structures for parts B

and C.

C2H6

O2

CO

H2 B. Now calculate H for the partial oxidation of ethane.

Use bond enthalpies to calculate H.

C. Was ethane really oxidized in this reaction? Assign oxidation numbers .

Your answer will depend on how you assign oxidation numbers for carbon and hydrogen in C2H6, but in either case carbon is oxidized to (+2) and oxygen is reduced to (-2).

(0) (+2) (-2) (0) ___ ___ ___ ___ ___ ___ C2H6(g) + O2(g) 2CO(g) + 3H2(g) Was C2H6 oxidized? __________ If so, did oxidation occur at C or H or both?

15. (Fall 2012) Throughout the semester we considered the combustion of fuel and the analogous reaction the food undergoes as part of the metabolic processes:

fuel/food + O2(g) → CO2(g) + H2O(l) The reverse of this reaction that occurs during photosynthesis is also pretty important: CO2(g) + H2O(l) → fuel/food + O2(g) A. Is this reaction endothermic or exothermic? Explain your reasoning.

The above reaction is the reverse of the reaction for combustion of fuel or metabolism of food. Combustion/metabolism is exothermic, so the above reaction is endothermic.

B. A recent “Science Q&A” feature in the New York Times began as follows:

Q. My family owns a 40-acre woodlot of relatively mature deciduous trees. How many pounds of carbon dioxide does such a forest absorb in a typical year? And how many pounds of oxygen are emitted?

A. "An approximate value for a 50-year-old oak forest would be 30,000 pounds of carbon dioxide sequestered per acre," said Timothy J. Fahey, professor of ecology in the department of natural resources at Cornell University. "The forest would be emitting about 22,000 pounds of oxygen."

Let’s assume that the carbon dioxide is sequestered as C6H12O6. Convert 30,000 pounds of carbon

dioxide to moles of CO2 and 22,000 pounds of oxygen to moles of O2.

Convert pounds to grams, and then grams to moles. You will get nearly the same number of moles for both carbon dioxide and oxygen.

Now balance the chemical equation

6 6 1 6 _______ CO2(g) + _______ H2O(l) → _______ C6H12O6(s) + _______ O2(g) Based on your calculation, if 30,000 pounds of carbon dioxide was sequestered would 22,000

pounds of oxygen be emitted? Explain your reasoning or support your answer with a calculation.

Yes.

Six moles of CO2 should give six moles of O2, so x moles of CO2 should give x moles of O2, in agreement with the calculation above.

16. (Fall 2015) Iodine is an important element. While its percent abundance on Earth is less than 0.05%, iodine is an essential element in biology, and iodine compounds have found numerous applications in industry and medicine.

A. Dot structures are given below for several compounds containing iodine (lone pairs are shown only for the central atom). Draw the structure and name the molecular geometry from the following list: linear, bent, T-shaped, see-saw, trigonal pyramidal, trigonal planar, trigonal bipyramidal, square planar, square pyramidal, tetrahedral, octahedral.

I

F

F

F

name: T-shape

IBI

I name: trigonal planar

name: bent

name: trigonal pyramidal

F

IF FF

F

name: square pyramidal

B. Resonance structure for the molecule ICN are given below. Assign nonzero formal charges for

these dot structures.

+ - 2+ 2-

Which dot structure is the best resonance structure for ICN?

B

AB

17. (Fall 2012) For each of the following ABn molecules, give one of the following descriptive names for the molecular geometry (atoms only): linear, bent, T-shaped, see-saw, trigonal pyramidal, trigonal planar, trigonal bipyramidal, square planar, square pyramidal, tetrahedral, octahedral. To the right of the name, sketch the molecular geometry, using wedges for bonds coming out of the paper and dashed lines for bonds going into the plane of the paper. You only need to show the lone pairs on the central atom. Use arrows to show any expected distortions from the idealized geometry. Finally, assuming the A—B bonds are polar, indicate whether the molecule will polar or nonpolar.

18. (Fall 2009) Assign the oxidation numbers for all of the carbon and oxygen atoms in the following organic reagent:

AB B

AB B

AB B

B

AB B

B

AB B

B

B

AB B

trigonal planar, nonpolar

bent, polar

linear, nonpolar

tetrahedral, nonpolar

bent, polar

trigonal pyramidal, polar

19. (Fall 2009) The previous question allowed you to demonstrate your ability to write dot structures and

use them to deduce geometries and hybridization. The next two examples, drawn from the recent literature, are a bit more interesting, but the fundamental tools used to describe the electronic structure and physical geometry are the same.

A. Hydrazine borane, NH2NH2BH3, has been proposed as a material for storing hydrogen (J. Am.

Chem. Soc. 2009, 131, 7444-7446). In combination with lithium hydroxide, hydrogen gas is released according to the reaction

NH2NH2BH3(s) + LiH(s) LiN2H3BH3(s) + H2(g) Write a dot structure for NH2NH2BH3 and assign all nonzero formal charges.

When NH2NH2BH3 reacts to form NH2NHBH3

, why is the hydrogen ion lost from the middle nitrogen instead of the outer nitrogen?

20. (Fall 2009) DOT STRUCTURES 1. Write the best dot structure you can and assign all non-zero formal charges for each of the

following molecules/ions. Draw a box around the structure you consider to be the best (i.e. the one you want graded).

2. Sketch the molecular geometry, using wedges for bonds coming out of the paper and dashed lines for bonds going into the plane of the paper. You only need to show the lone pairs on the central atom. Use arrows to show any expected distortions from the idealized geometry.

3. Give a descriptive name for the molecular geometry (atoms only): bent, T-shaped, see-saw, trigonal pyramidal, trigonal planar, trigonal bipyramidal, square planar, square pyramidal, tetrahedral, octahedral.

4. Give the hybridization about the central atom.

21. (Fall 2009) Xenon forms bonds with oxygen and fluorine, but compounds containing Xe-N bonds are rare (J. Am. Chem. Soc. 2009, 131, 7272-7286). The cation F3SNXeF was investigated and found to rearrange to form F4S=NXe. In both ions the atoms are connected as SNXe, and the multiplicity of the bonding between sulfur and nitrogen is indicated in the formulas.

22. (Fall 2009, e4) A dot structure for methyl amide is shown on the right.

-3573 kJ/mol

23. (Fall 2015) Most explosive compounds are nitrogen rich, but an exception to this rule is diacetone diperoxide (DADP) shown on

the right. The compound is fairly easy to make, and because it does not contain nitrogen, it is not easily detected. DADP and chemically

similar compounds are the reason bottles larger than 3.4 ounces (100 mL) in our carry-on items when we fly.

A. A balanced chemical equation for the combustion of DADP is given below. C6H12O4 + 7O2 6CO2 + 6H2O

Use bond enthalpies to estimate H for this reaction.

In this reaction, we break 12 C-H bonds, 4 C-C bonds, 4 C-O bonds, 2 O-O bonds, and 7 O=O bonds. We form 12 C=O (in CO2) bonds and 12 OH bonds.

Hrxn = BE(reactants) BE(products) =

12[BE(C-H)]+ 4[BE(C-C)]+ 4[BE(C-O)] + 2[BE(O-O) + 7[BE(O=O)] – {12[BE(C=O in CO2)] + 12[BE(O-H)]} =

12(414 kJ/mol) + 4(347 kJ/mol) + 4(351 kJ/mol) + 2(142 kJ/mol) + 7(498.7 kJ/mol) – [12(799 kJ/mol) + 12(460 kJ/mol)]

B. The hydrocarbon analog of DADP is 1,1,4,4-tetramethylcyclohexane

(TMCH).

1. Write a balanced chemical equation for the combustion of TMCH. The formula for TMCH is C10H20.

C10H20 + 15O2 10CO2 + 10H2O 2. One mole of DADP combusts more violently than one mole of TMCH. Do you expect the enthalpy of combustion for DAPA to be more or less exothermic than the enthalpy of combustion for TMCH?

more exothermic

3. Considering the bond enthalpies, explain why DADP is more reactive than TMCH.

The OO bonds in the ring are very weak (142 kJ/mol), requiring less enthalpy to break than the C—C and C—H bonds in TMCH.

TMCH

C C

C

C C

C

C

C

C

C

H

H

H

H

H

H

H

HH

H

H

H

H H HH

H H H H