CHM 1045
Dr. Michael Blaber

Name_________________________

SS #__________________________

Exam #2 100 points total

  1. A sample of aluminum requires 112 Joules to raise its temperature from 300 K to 350 K. How many moles are in this sample of aluminum? (The specific heat of aluminum is 0.9 J g-1 K-1. (5 points)

    2. Grams = heat energy transferred/(DT * S.H.)

    = 112J/(50K * 0.9 J g-1 K-1)

    = 2.49 g

    = 2.49 g * (1 mole/26.98g) = 0.092 moles

     

  2. 0.3 liters of an aqueous solution of hydrochloric acid (HCl) and 0.1 liters of an aqueous solution of sodium hydroxide (NaOH) are allowed to react in a constant pressure calorimeter. The temperature is observed to increase from 317 to 348 K. Calculate the enthalpy change for this reaction. (10 points)

    3. DT of solution = 348-317 = 31 K

    specific heat = 4.18 J g-1 K-1

    mass = 300 ml + 100 ml = 400 ml * 1g ml-1 = 400 g

    DHsoln = 4.18 J g-1 K-1 * 400 g * 31 K = 51,832 J or 51.8 kJ

    DHrxn = -DHsoln = -51.8 kJ

     

  3. Given the following data:

    4. CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g) DH = -802 kJ

    C(s) + O2(g) -> CO2(g) DH = -394 kJ

    2H2(g) + O2(g) -> 2H2O(l) DH = -572 kJ

    Calculate DH for the formation of 10 grams of methane from carbon and hydrogen gas (10 points)

    C(s) + O2(g) -> CO2(g) DH = -394 kJ

    2H2(g) + O2(g) -> 2H2O(l) DH = -572 kJ

    CO2(g) + 2H2O(g) -> CH4(g) + 2O2(g) DH = 802 kJ

    C(s) + 2H2(g) -> CH4(g) DH = -394-572+802 = -164 kJ

    (-164 kJ / mole) * (1 mole /16 g ) * 10g = -103 kJ

     

  4. The frequency of a certain electromagnetic radiation is 3.28 x 1014 s-1 .

    5. a. What is the wavelength of this radiation in nm?

    b. What is the speed of this radiation?

    c. Is this radiation visible to the human eye? If so, what is the perceived color? If not, in what region of the E.M. spectrum is this located? (5 points total)

    Freq * wavelength = speed of light

    Wavelength = (3 x 108 m/s) / (3.28 x 1014 s-1 ) = 9.15 x 10-7 m = 915 nm

    Speed is c or 3x 108 m/s, Not visible, it is infrared

     

  5. An electron transitions from the n=2 to the n=4 principle quantum number in the hydrogen atom. What is the wavelength of the associated photon? Is energy emitted or absorbed for the associated electron transition?(5 points)

    6. Freq = (RH /h)*(1/ni2 – 1/nf2) = (2.18 x 10-18 J/6.63 x 10-34 J s) * (1/4 – 1/16) = 6.17x1014 s-1

    Wavelength = c/freq = (3x108 m/s)/( 6.17x1014 s-1 ) = 4.87 x 10-7, or 487 nm

    Energy is absorbed (freq is positive)

     

  6. List the various subshells, and the number of orbitals in each subshell, for the shell with a principle quantum number of 4. How many total orbitals are there? (10 points)

    7. n=4 , l = 0,1,2,3 (s, p, d, f)

    4s 1 orbital

    4p 3 orbitals

    4d 5 orbitals

    4f 7 orbitals

    16 orbitals total

     

  7. A certain orbital has a principle quantum number of 3, and an azimuthal quantum number of 0. Write the designation for this orbital, and draw a radial electron distribution diagram for an electron in this orbital (5 points)

    8. 3s

     

  8. What is the effective nuclear charge on the valence electrons of the Nitrogen atom? (5 points)

    9. Nitrogen is [1s2] 2s2 2p3, with Z = 7

    Zeff = 7-2 = 5+

     

     

  9. What elements are represented by the following electron configurations? (5 points)

    10. 1s2 2s2 2p6 3s2 3p4 S

    [Ar] 4s2 3d4 Cr

    [Kr] 5s1 Rb

    1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2 Ge

    1s2 He

  10. Write orbital diagrams for the following elements (10 points)

    11. Mn

     

    S

     

  11. How does the effective nuclear charge change as you move from left to right across the periodic table? How does the effective nuclear charge change as you move down a group in the periodic table? (5 points)

    12. Zeff increases as you move from left to right

    Zeff unchanged as you move down

  12. For the following elements write down their electron configurations and indicate which orbitals constitute the "valence" electrons (10 points)

    13. Sc

    1s2 2s2 2p6 3s2 3p6 4s2 3d1

    In

    1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p1

  13. Which element, or ion, below has the highest ionization energy? (5 points)

    14. Ca2+ Na Rb Sr2+

    Ca2+

     

  14. Write the balanced chemical equation for the reaction of Strontium with water. Be sure to include information about the physical state of reactants and products (5 points)

    15. Sr(s) + 2H2O(l) -> Sr(OH)2(aq) + H2(g)

     

  15. Write the balanced chemical equation for the reaction of Rubidium with Bromine. Be sure to include information about the physical state of reactants and products (5 points)

2Rb(s) + Br2(g) -> 2RbBr(s)

 

Exam II Appendix

Specific Heat (J g-1 K-1)

Al(s) 0.90

H2O(l) 4.18

Density (g ml-1)

H2O(l) 1.0

Constants

Rydberg 2.18 x 10-18 J

Planck’s 6.63 x 10-34 J s