Chem 107 - Hughbanks Exam 2, March 7, 2014

Name (Print) Key UIN # Section 504 Exam 2, Key for Version A On the last page of this exam, you’ve been given a periodic table and some physical constants. You’ll probably want to tear that page off to use during the exam – you don’t need to turn it in with the rest of the exam. The exam contains 11 problems, with 4 numbered pages. You have 50 minutes to complete the exam. Please show ALL your work as clearly as possible – this will help us award you partial credit if appropriate. Even correct answers without supporting work may not receive credit. You may use an approved calculator for the exam, one without extensive programmable capabilities or the ability to store alphanumeric information. Print your name above, provide your UIN number, and sign the honor code statement below.

On my honor as an Aggie, I will neither give nor receive unauthorized assistance on this exam.

SIGNATURE:

Name (Print) Version A

1

For questions 1 through 4, consider the following gas samples (assume all the gases behave ideally): A. 20 g of Ne (g) at 0 °C in a 22.4 L container B. 300 torr of N2 (g) at 27 °C in a 100.0 L container C. 0.50 mol of Ar (g) at 27 °C in a 5.0 L container

For each question below, please indicate your choice and give a brief justification of your answer (i.e., an explanation of your reasoning). You shouldn’t need detailed numerical calculations, although you may perform some if you feel that will help. In the event that more than one of the gases satisfies one of the statements, you should say so. (For example, if you think they all are at the same pressure, then for part (b) you would write “All have the same pressure because....”)

(1) (5 pts) Which gas sample contains the fewest molecules?

Ans. 1_____c_____

(2) (5 pts) Which gas sample is at the lowest pressure?

Ans. 2_____b______

(3) (5 pts) In which of the samples is the average kinetic energy of the molecules highest?

K.E. of ideal gases depends only on temperature, b and c have equal temps.

Ans. 3____b & c___

(4) (5 pts) In which of the samples is the average molecular speed slowest?

Ans. 4______c_____

(For grading)

Scores 1 /5 2 /5 3 /5 4 /5 5 /7 6 /7 7 /7 8 /18 9 /10 10 /16 11 /15 Tot. /100

Name (Print) Version A

2

(5) (7 pts) Lithium metal has an electron binding energy of 3.94 × 10–19 J. What is the longest wavelength of light (in nm) than can eject electrons from lithium? (Put your answer with the correct units in the space provided.)

E = hc/λ ⇒ λ = hc/E = (6.626 × 10–34 J•s)(2.9979 × 108 m/s)/(3.94 × 10–19 J) = 5.04 × 10–7 m

Ans. 5 504 nm

(6) (7 pts) Write out the full electronic configuration of a selenium atom (Se, no inert gas formalism).

Ans. 6 1s22s22p63s23p63d104s24p4

(7) (7 pts) In a selenium atom, which electron is least effective in screening a 3p electron from the nucleus,

(a) a 1s electron (b) a 3s electron (c) another 3p electron (d) a 4p electron (e) a – d are all equally effective

Ans. 7 d (8) (18 points) In (a) – (c), BE SURE TO READ EACH STATEMENT VERY CAREFULLY!

(a) (6 pts) From the following choices, circle the molecule with the strongest C–O bond(s) and put a box around the molecule with the longest C–O bond(s).

H3COCH3 F2 HC(O)OH CO2 CO H2CO

(b) (6 pts) Put the atoms, molecules, or ions in order, starting with those with the fewest unpaired electrons to most unpaired electrons: Na, Ni2+, Zn2+, N2, N

Fewest unpaired N2 = Zn2+ < Na < Ni2+ < N Most unpaired

(c) (6 pts) Atoms of which one of the following elements has the highest ionization energy? Be B C N O F Ne

Name (Print) Version A

3

Questions 9-11 – show all work (42 pts) (9) (10 points) N2O5 is an unstable gas that decomposes to form NO2 and O2 gases (only). What would

be the total pressure of gases present if a 10.0 L container at 27 ˚C begins with 0.800 atm of N2O5 and the gas completely decomposes? Assume the final temperature is also 27 ˚C.

Balanced equation: 2 N2O5 → 4 NO2 + O2

Since nothing else changes, the pressure change will be solely due to the change in the number of moles of gas. 2 moles of reactant gas yields 5 moles of product gases, therefore the pressure will increase by a factor of 5/2; the final pressure will be 2.5(0.800 atm) = 2.00 atm

(10) (16 points) Excited mercury atoms emit light at many wavelengths, two of which are at 435.8 and 546.1 nm. Both of these transitions are to the same final state.

(a) (5 pts) Give the energy difference between the initial excited state and the lower final state for the transition at 435.8 nm.

∆E = hc/λ = (6.626 × 10–34 J•s)(2.9979 × 108 m/s)/(4.358 × 10–7 m) = 4.56 × 10–19 J

(b) (5 pts) Give the energy difference between the initial excited state and the lower final state for the transition at 546.1 nm.

∆E = hc/λ = (6.626 × 10–34 J•s)(2.9979 × 108 m/s)/(5.461 × 10–7 m) = 3.64 × 10–19 J

(c) (6 pts) If a transition between the two higher energy excited states could be observed, what would be the frequency of the light?

The energy difference between the two excited states is ∆(∆E) = (4.56 × 10–19 J) – (3.64 × 10–19 J) = 9.21 × 10–20 J

Therefore, the frequency is ν = ∆(∆E)/h = (9.21 × 10–20 J)/(6.626 × 10–34 J•s) = 1.39 × 1014 s–1

Name (Print) Version A

4

(11) (15 points) Below are shown four ions and one neutral molecule containing nitrogen-oxygen bonds. The diagrams given show only the way the atoms are connected together, they are NOT complete Lewis diagrams. There are five distinct observed N–O bond distances (in pm), in order of increasing length: 106.2, 112.8, 118.8, 122.2, 125.7 pm for the bonds that are labeled below. (Some bonds have the same length as others, so there are more labeled bonds than there are bond distances given above.) Pay attention to the charges on the ions – only N2O is a neutral molecule.

Fill in the table below. Use Lewis diagrams, including any necessary resonance structures, to find

the N-O bond orders and decide which observed bond distances belong to which molecules. If you can’t assign an exact value to a bond order, give the best estimate you can by using inequalities (for example, if you think a bond order is greater than 2 but less than 2.5, write 2 < B.O. < 2.5).

Bond Bond order Bond distance* Bond distance*

a 3 106.2 pm 106.2 pm b 1 < B.O. < 1.5 122.2 pm 125.7 pm c 2 112.8 pm 112.8 pm d 2 112.8 pm 112.8 pm e 1½ 118.8 pm 118.8 pm f 1½ 118.8 pm 118.8 pm g 1⅓ 125.7 pm 122.2 pm h 1⅓ 125.7 pm 122.2 pm i 1⅓ 125.7 pm 122.2 pm

*Bond distance choices are (in pm): 106.2, 112.8, 118.8, 122.2, 125.7

Answers in either of the two “bond distance columns” will be given full credit.

ONOO

N

OON

O

+

+

+–

–

[ ]

N ON

–

>50%

<50%–

ea

b

b

equivalent

f

i

g

h

ONN

c d+ONO

twoequivalent

0