Tarleton State University

Department of Chemistry, Geosciences, and Environmental Science

 

          Chemistry 1054-010                        College Chemistry                                  Fall 2005

 

                                                                     Exam 3-A-Key

 

Name:_______________________________________________________

                              (Please write your name legibly)

 

Read all directions and questions carefully!!  This exam consists of two parts.  The first part consists of 10 multiple choice questions worth four points each for a total of 40 points.  The second part consists of five numerical problems worth either 10 or 15 points per question for a total of 60 points.  Show all your work necessary for the numerical problems as partial credit will be given for those problems.

 

Possibly Useful Constants and Equations

 

            For light:                                                         Speed of light:  c = 2.998 × 108 m/s

            Planck's Law:                                 Planck's constant:  h = 6.626 × 10-34 J·s

            DeBroglie's wavelength: 

            Bohr's model of hydrogen atom: 

 

 

Score

 

                                    Part 1 (40 points):_____________________

 

 

                                    Part 2 (60 points):_____________________

 

 

                                    Total (100 points):_____________________

 

 

Don’t forget to put your name on this test!

 

Good Luck!!


Part 1

 

Multiple Choice

 

Please indicate the answer to each question by putting your choice in the space provided.  There is only one correct answer for each question.  There will be 10 multiple choice questions worth 4 points each.

 

1.  Which one of the following frequencies of light will have the lowest energy for a photon?

       (a)  90.1 MHz                                                                   (c)  1080 kHz

       (b)  2.67 × 104 Hz                                                             (d)  5.25 × 106 1/s

 

Answer:  A  (This is the largest frequency when all the frequencies are in units of Hz)

 

2.  Which quantum number is related to the average distance of the electron from the nucleus?

       (a)  ml                                                                               (c)  n

       (b)  l                                                                                 (d)  ms

 

Answer:  C

 

3.  Which is not an allowed combination of n and l quantum numbers

       (a)  n = 7 l = 0                                                                  (c)  n = 5 l = 4

       (b)  n = 2 l = 1                                                                  (d)  n = 6 l = 3

                                                            (e)  n = 3 l = 3

 

Answer:  E  (The value of “l” cannot be greater than “n–1”)

 

4.  Which one of the following elements has the largest atomic radius?

       (a)  O                                                                                (c)  F

       (b)  Al                                                                               (d)  P

                                                            (e)  B

 

Answer:  B  (Atomic radii decrease as you proceed from left to right and they increase as you go down the periodic table)

 

5.  Which one of the following ions has the smallest ionic radius?

       (a)  P3-                                                                               (c)  Cl¯

       (b)  S2-                                                                              (d)  Ca2+

                                                            (e)  K+

 

Answer:  D  (In an isoelectronic series of ions, the smallest ion is the one with the largest positive charge)

 

6.  Which one of the following elements has the largest 1st ionization energy?

       (a)  Se                                                                               (c)  P

       (b)  S                                                                                (d)  As

                                                            (e) Te

 

Answer:  B  (1st Ionization Energies increase as you move from left to right and decrease as you move down the Periodic Table)

 

7.  Which of the following salts has the largest lattice energy?

       (a)  MgO                                                                           (c)  SrO

       (b)  BaO                                                                           (d)  CaO

                 (e)  All of these salts have the same lattice energy.

 

Answer:  A  (Lattice energies increase with decreasing size of the ion.  In these salts, Mg2+ is the smallest cation in the series Mg2+, Ca2+, Sr2+, and Ba2+)

 

8.  Which one of the following elements has the smallest electronegativity?

       (a)  S                                                                                 (c)  P

       (b)  As                                                                              (d)  Se

                                                            (e)  Ge

 

Answer:  E  (Electronegativities increase as you move from left to right, and they decrease as you move down the periodic table)

 

9.  Shown above is one possible Lewis structure for the cyanate ion, OCN¯.  What is the formal charge on the oxygen atom in this structure?

       (a)  –2                                                                               (c)  0

       (b)  –1                                                                               (d)  +1

                                                            (e)  +2

 

Answer:  D (=6–5)

 

10.  When comparing single, double, and triple bonds, which bond has the longest bond length and which bond has the smallest bond enthalpy?

       (a)  Single bonds have the longest bond length and triple bonds have the smallest bond enthalpy.

       (b)  Triple bonds have the longest bond length and single bonds have the smallest bond enthalpy.

       (c)  Single bonds have the longest bond length and smallest bond enthalpy.

       (d)  Triple bonds have the longest bond length and the smallest bond enthalpy.

 

Answer:  A

 

 


Part 2

 

Numerical Problems

 

Solve the following problems, keeping track of significant figures where applicable.  Please show all the work necessary to obtain your answer in order to receive partial credit for possibly wrong answers.  Generally, full credit will not be given for the correct answer without any of the work performed to obtain the answer being shown on the paper.  Each question is worth either 10 or 15 points.

 

11.  (10 points)  Predict the products of the following reactions and balance the reaction:

 

(a)  Na(s)  +  O2(g)  ¾¾®  ???

The reaction of sodium with oxygen produces sodium peroxide, Na2O2.  The balanced reaction is:

2 Na(s)  +  O2(g)  ¾¾®  Na2O2(s)

 

(b)  Ca(s)  +  H2O(l)  ¾¾®  ???

The reaction of calcium with water produces calcium hydroxide and hydrogen gas.  The balanced reaction is:

Ca(s)  +  2 H2O(l)  ¾¾®  Ca(OH)2(aq)  +  H2(g)

 

 

(c)  Al(s)  +  Br2(l)  ¾¾®  ???

The reaction of aluminum with bromine produces aluminum bromide, AlBr3.  The balanced reaction is:

2 Al(s)  +  2 Br2(l)  ¾¾®  2 AlBr3(s)

 

(d)  Rb(s)  +  S8(s)  ¾¾®  ???

The reaction of rubidium with sulfur produces rubidium sulfide, Rb2S.  The balanced reaction is:

16 Rb(s)  +  S8(s) ¾¾®  8 Rb2S(s)


12.  (15 points)  Give the ground state electron configurations for the following atoms or ions.  You may used the condensed notation if you wish.

 

(a)  Cr

1s22s22p63s23p64s13d5  or [Ar]4s13d5

 

 

 

(b)  As

1s22s22p63s23p64s23d104p3 or [Ar] 4s23d104p3

 

 

(c)  Zn2+

The neutral atom’s configuration is:  1s22s22p63s23p64s23d10

Removing 2 electrons from the 4s orbital, the electron configuration for Zn2+ is:

1s22s22p63s23p63d10 or [Ar]3d10

 

 

 

(d)  S¯

The neutral atom’s configuration is:  1s22s22p63s23p4

Adding one electron, the electron configuration for S¯ is:

1s22s22p63s23p5 or [Ne]3s23p5

 

 

 

(e)  Fe

1s22s22p63s23p64s23d6 or [Ar] 4s23d6


13.  (10 points)  What is the frequency and energy of a photon of wavelength equal to 125 mm?

 

First, convert the wavelength to meters:

The frequency of this photon is equal to:

The energy of this photon is given by Planck’s Law:

 

 

14.  (10 points)  Use the attached table of bond enthalpies to estimate the DH for the following reaction:

A table of bond enthalpies can be found on page 330 of your book.

Add up all the bond enthalpies in the reactants:

Add up all the bond enthalpies in the products

The DH for this reaction is the difference of the sum of the reactants’ bond enthalpies minus the sum of the products’ bond enthalpies:

DH = 3338 kJ – 3642 kJ = –304 kJ


15.  (15 points)  Write the most correct Lewis structure for the following molecules or ions:

 

(a)  SiH4                                                                                  (d)  ClO2¯

 

 

                                                                        

 

 

 

 

(b)  BCl3                                                                                  (e)  HNO3

 

 

                                                               

 

 

 

 

 

(c)  PCl3                                                                                  (f)  SCl4