Chem. 121 Mid-Term, Sec 12
Chem. 121 Mid-term, Sec 12
Name
Student I.D.
Please show your work!
1. The Haber Process for the production of ammonia is represented by:
3 H2(g) + N2(g) → 2 NH3(g)
If a mixture of 30 g of hydrogen with 10 g of nitrogen produced 8.4 g of ammonia, what was the percent yield? (4 marks)
2. If 4.0 ×1024 molecules of N2, 4.0 g of H2, 6.0 moles of O2 are placed in an 8.2 L container at 127°C, what would be (i) the mole fraction of each gas, (ii) the total pressure in the flask, and (iii) the partial pressure of each gas? (6 marks)
3. The reaction of aluminum with hydrochloric acid produces hydrogen gas. The balanced chemical equation for the reaction is given below:
2Al (s) + 6HCl (aq) → 2AlCl3 (aq) + 3H2 (g)
If 35.5 mL of H2(g) is collected over water at 26 ◦C and a barometric pressure of 755 mmHg, how many moles of HCl must have been consumed? (The vapor pressure of water at 26 ◦C is 25.2 mmHg.) Show your calculations; include units in every step. (5 marks)
R= 0.08206 atm.L. mol-1. K-1 , T (K) = t (◦C) + 273.15
4. Which of the following reactions gives a positive value for the electron affinity?
Explain why? (3 marks)
A) O- (g) + e- → O2-(g)
B) Cl(g) + e- → Cl- (g)
C) S(g) + e- → S-(g)
D) Br(g) + e- → Br-(g)
5. Sketch (draw) all of the orbitals with a principal quantum number equal to 2. (3 marks)
6. The existence of discrete (quantized) energy levels in an atom may be inferred from:
(2 marks)
A) diffraction of electrons by crystals
B) X-ray diffraction by crystals
C) atomic line spectra
D) experiments on the photoelectric effect
7. Calculate the wavelength in meters of light absorbed by an electron in an atom of hydrogen as it moves from n = 3 to n = 6. (4 marks)
8. An orbital: (2 marks)
I) describes a region of high electron density.
II) describes a region in an atom where an electron is likely to be found.
III) is a wave function resulting from specific values assigned to quantum numbers in wave equations.
IV) describes a spherical region around a nucleus where an electron can be found.
A) II only
B) I and II
C) II and III
D) I, II, and III
E) I, II, III, and IV
9. In a hydrogen atom, orbitals with the same energy: (2 marks)
I) extend the same distance from the nucleus.
II) have the same principle quantum number, n.
III) are referred to as iso-electronic.
IV) are referred to as degenerate.
V) are represented by 1s and 2s orbitals.
A) I, III, IV
B) I, II, IV
C) I, IV, V
D) I, II, III
E) I, II, III, and IV
10. The ground state electron configuration of Sc is ______. (2 marks)
A) 1s22s22p62d103s1
B) 1s21p62s22p63d5
C) 1s22s22p63s23p63d3
D) 1s22s22p63s23p64s23d1
E) 1s22s22p63s23p64s24p1
11. Show the orbital diagram for the ground state electron configuration of the following:
Cu , Mn, and Cr3+? (6 marks)
12. Choose the paramagnetic atom or ion: Ca, Ne, Sc3+, Cl-, Na; show the orbital diagrams.
(3 marks)
13. The following are electron configurations for a nitrogen atom, indicate whether the configurations represent the ground state or a state that is inconsistent with Hund's Rule. Justify your answers. (3 marks)
1s 2s 2px 2py 2pz
a) ↑↓ ↑↓ ↑ ↓ ↑
b) ↑↓ ↑↓ ↑ ↑ ↑
c) ↑↓ ↑↓ ↑ ↑ ↓
d) ↑↓ ↑↓ ↓ ↓ ↓
14. Concerning the concept of subshells and orbital, (3 marks)
a) How many subshells are in the n=3 level? What are the names of these subshells?
b) How many orbitals have the values n=4, ℓ = 3?
c) What is the total number of orbitals in the n=4 level?
15. Explain why the series of elements from Sc to Zn shows the smallest difference in atomic radii? (3 marks)
16. Write an orbital diagram for the ground state electron configuration of Fe 2+ and Fe 3+?
Using the orbital diagrams, explain why Fe 2+ is easily oxidized to Fe 3+? (3 marks)
17. Using the orbital diagram showing the valence electron configuration of magnesium, Mg, aluminum, Al, phosphorous, P, and sulfur, S, explain the following exceptions:
a) Why the first ionization energy of Al is smaller than that of Mg, and b) the first ionization energy of S is smaller than that of P? (6 marks)
18. Chlorine has a large negative electron affinity. Therefore, chlorine is most likely to:
(2 marks)
A) act as an oxidizing agent
B) have a small first ionization energy
C) act as a reducing agent
D) release electrons readily to other atoms
19. Which of the following has the largest radius? (2 marks)
Cl-, Ar, P3-, Sc3+, K+