2012 SPRING Semester Midterm Examination
For General Chemistry I
Date: March 28 (Wed),
Time Limit: 7:00 ~ 9:00 p.m.
Write down your information neatly in the space provided below; print your Student ID in the upper
right corner of every page.
Professor
Name
Class
Problem
points
Student I.D.
Problem
1
2
/10
Name
TOTAL pts
points
/20
/8
6
7
3
/12
8
/25
4
/8
9
/20
5
/12
10
/20
/15
/150
** This paper consists of 4 sheets with 10 problems. Please check all page numbers before taking the exam.
Write down your work and answers in the Answer sheet.
Please write down the unit of your answer when applicable. You will get 30% deduction for a missing unit.
NOTICE: SCHEDULES on RETURN and CLAIM of the MARKED EXAM PAPER.
(채점답안지 분배 및 이의신청 일정)
1. Period, and Procedure
1) Return and Claim Period: April 2 (Mon), Quiz Session; 7: 00 ~ 7:30
2) Procedure: During the quiz hour, you can take your mid-term paper scored. If you have any claims on it,
you can submit a claim paper with your opinion. After writing your opinions on any paper you can get
easily, attach it with a stapler to your mid-term paper scored (Please, write your name, professor, and
class.). Submit them to your TA. The papers with the claims will be re-examined by TA.
The claim is permitted only on the period. Keep that in mind!
(A solution file with answers for the examination will be uploaded on 3/30 at the web.)
2. Final Confirmation
1) Period: April 9 (Mon)-10 (Tue)
2) Procedure: During this period, you can check final score of the examination on the website again.
** For further information, please visit a General Chemistry website at www.gencheminkaist.pe.kr.
<The Answers>
Problem
points
1
2
5+5 /10
Problem
points
4+4 /8
6
7
5+5+5 /15
3
3+3+3+3 /12
8
10+15 /25
4
4+4 /8
9
10+10 /20
5
8+4 /12
10
10+5+5 /20
TOTAL pts
14+6 /20
/150
1. (a) Calculate the maximum wavelength of electromagnetic radiation needed to eject
electrons from the surface of tungsten, which has a work function of 7.29 x 10-19 J.
(b) If the maximum speed of electrons emitted from the tungsten surface is 2.00 x 10 6 m s-1,
calculate the wavelength of the incident radiation.
Planck’s constant h = 6.626 x 10-34 J·s; speed of light c = 2.998 x 108 m·s-1
1. (a) This involves the photoelectric equation, Emax = ½meve2 = h - , where  is the
work function (7.29 x 10-19 J) representing the minimum energy (and hence maximum
wavelength) of photons required for photoelectric effect.
Emin = 7.29 x 10-19 J = hc/max
max = (6.626 x 10-34 J·s) (2.988 x 108 m·s-1) / (7.29 x 10-19 J) = 2.72 x 10-7 m or 272 nm
(b) Maximum energy of electrons ejected by impact of photons of wavelength  is
given by,
Emax = ½meve2 = ½ (9.109 x 10-31 kg) (2.00 x 106 m·s-1)2 = 18.2 x 10-19 J = hc/ - 
Hence, hc/ = 25.5 x 10-19 J
 = (6.626 x 10-34 J·s) (2.988 x 108 m·s-1) / 25.5 x 10-19 J = 0.779 x 10-7 m or 77.9 nm
2.
(a) Which of the quantum numbers govern the energy of an orbital?
(b) Which of the quantum numbers govern the shape and spatial orientation of an orbital?
2. (a) principal quantum number and orbital angular momentum quantum number
(b) orbital angular momentum quantum number and magnetic quantum number
3. Which of the following statements are true for many-electron atoms? If false, explain why.
(a) The effective nuclear charge Zeff is independent of the number of electrons present in an
atom.
(b) Electrons in an s-orbital are more effective than those in other orbitals at shielding other
electrons from the nuclear charge because an electron in an s-orbital can penetrate to the
nucleus of the atom.
(c) Electrons having l = 2 are better at shielding than electrons having l = 1.
(d) Zeff for an electron in a p-orbital is lower than for an electron in as s-orbital in the same
shell.
3. (a) false. Zeff is considerably affected by the total number of electrons present in
the atom because the electrons in the lower energy orbitals will “shield” the
electrons in the higher energy orbitals from the nucleus. This effect arises because
the e-e repulsions tend to offset the attraction of the electron to the nucleus.
(b) true.
(c) false. The electrons are increasingly less able to penetrate to the nucleus as l
increases.
(d) true.
4. (a) Which has the larger second ionization energy, B or C, and why?
(b) Which is the largest and which is the smallest among K+, Cl-, and Ca2+, and why?
4. (a) The first ionization of each removes a 2p electron, but the second removes a 2s
electron from B, but only another 2p electron from C. The change in subshell with B
means it has the higher second ionization energy.
(b) All three have the same number of electrons, but Cl– has the fewest number of
protons in its nucleus to hold these electrons in place (making it the largest) while
Ca2+ has the largest number of protons (making it the smallest). K+ is in between.
5. (a) We learned that the Coulomb potential energy of the interaction of two individual ions
z 1z 2e 2
is
, where z 1e and z 2e are the charges of the two ions and r12 is the distance
4 0 r12
between two ions. Consider the following one-dimensional nanorod made of only four ions.
The distance between two adjacent ions is d. The charge of each ion is indicated (2e or 2e). Calculate the total Coulomb potential energy of this system.
(b) In which of the nanorods Mg2+Se2-Mg2+Se2- and Ca2+Se2-Ca2+Se2- are the interactions
between the ions stronger? Why?
e2
 17 e 2
(4  4 / 2  4 / 3  4  4 / 2  4 ) 
5. (a)
4 0d
60d
(b) Mg2+Se2-Mg2+Se2-, the same charge, smaller distance, smaller positive ions
6. (a) Write the Lewis structures of SO2 and N2O. Indicate the most stable structure for each
molecule and explain the reason.
(b) Write the possible Lewis structures of HNO and HON molecules. Which one would be
energetically favored? Why?
6. (a) SO2
10 electrons on S
stable resonance structure
N2O
10 electrons on middle N
stable
most stable
least stable
 Negative charge
on more negative atom
(b)
HNO is energetically favored because of the neutral formal charge.
7. Place the following molecules or ions in order of decreasing bond length, and explain the
reason.
(a) the CO bond in CO, CO2, CO32(b) the SO bond in SO2, SO3, SO32(c) the CN bond in HCN, CH2NH, CH3NH2
7. (a) CO32- > CO2 > CO
CO32- : resonance between two single bonds and one double bond
CO2 : two double bonds; CO : triple bond
(b) SO32- > SO2 ~ SO3
SO32- : resonance between two single bonds and one double bond
SO2 : double bonds; SO3 : double bonds
(c) CH3NH2 > CH2NH > HCN
CH3NH2 : single bond; CH2NH : double bond; HCN : triple bond
8. (a) Use the VSEPR theory to sketch and name the molecular geometries of the
molecules, SCl2, SCl4, and SCl6, and state whether each is polar or nonpolar.
(b) Bromine can form compounds or ions with any number of fluorine atoms from one to five
(BrF, BrF2-, BrF3, BrF4-, BrF5). Describe their geometries based on the VSEPR theory.
8. (a)
(b) BrF(linear), BrF2-(linear), BrF3 (T-shaped), BrF4-(square planar), BrF5 (square
pyramidal)
9. Use valence (VB) theory to predict the hybridization in formaldehyde and allene. Sketch
the hybrid atomic orbitals depicting their overlap and  and -bonding.
(a) formaldehyde H2C=O
(b) allene H2C=C=CH2
9. (a) C atom is sp2-hybridized, as is the O atom. These trigonal planar sp2 orbitals bonded with 1s orbital of H atom and 2p orbital of O atom. Unhybridized 2p orbital of
C atom and O atom form a -bond which is orthogonal to the -bond.
(b) The central C atom forms one -bond on each side. These -bonds are orthogonal
to -bond. The central C atom must be sp-hybridized. The terminal C atoms are sp2
hybridized. These sp2 orbitals are -bonded with 1s orbitals of H atoms and the sp
hybrid orbital of the central C atom.
10. (a) Draw the molecular orbital energy-level diagram for N2 and label the energy levels
according to the type of orbitals from which they are made, whether they are  or -orbitals,
and whether they are bonding or antibonding.
(b) The orbital structure of the heteronuclear diatomic ion NO + is similar to that of N2. How
will the fact that the electronegativity of N differs from that of O affect the molecular orbital
energy-level diagram of NO+ compared with that of N2? Use this information to draw the
energy-level diagram for NO+.
(c) In the molecular orbitals, will the electrons have a higher probability of being at N or at
O? Why?
10.