Chemistry 3810
Answers to Problem Set #4
Topic: Hydrogen Chemistry. Based on material in Ch.8 of Shriver-Atkins, 3 rd Edition
1
Assign oxidation numbers to the elements in (a) H2S (b) KH, (c) [ReH9]2–, (d) H2SO4 (e) H2PO(OH).
2
Write balanced chemical equations for three major industrial preparations of hydrogen gas. Propose two different
reactions that would be convenient for the preparation of hydrogen in the laboratory.
3
Preferably without consulting reference material, construct the periodic table, identify the elements, and (a) indicate
positions of salt-like, metallic, and molecular hydrides, (b) add arrows to indicate trends in ∆Gf° for the hydrogen
compounds of the p-block elements, (c) delineate the areas where the molecular hydrides are electron-deficient, electronprecise, and electron-rich.
4
Name and classify the following hydrogen compounds: (i) BaH2, (ii) SiH4, (iii) NH3, (iv) AsH3, (v) PdH0.9, (vi) HI. Identify the
compounds (i) to (vi) that provide the most pronounced example of the following chemical characteristics and give a
balanced equation that illustrates each of the characteristics: (a) hydridic character, (b) Bronsted acidity, (c) variable
composition, (d) Lewis basicity. Divide the compounds (i) to (vi) into those that are solids, liquids, or gases at room
temperature and pressure. Which of the solids are likely to be good electrical conductors?
5
What is the expected infrared stretching wavenumber of gaseous 3HCl given that the corresponding value for 1HCl is 2991
cm-1?
We need the equations for reduced mass vs. the wavenumber given on page 3 of Lecture 9 (spectroscopy notes). These
equations can be combined in a comparative fashion when the bond strength is identical . We also need to calculate the
reduced mass using the equation provided, using the dominant 35Cl isotope (mass = 34.968852) and the isotopes 1H (mass =
1.00882) and 3H (mass = 3.01605).
ν1
=
ν2
µ2
µ1
and
µ=
m1m2
now solve the two reduced masses, and input terms. Let ν 2 be the frequency of 1HCl.
m1 + m2
1.00782 × 34.968852
3.01605 × 34.968852
= 0.97959 µTCl =
= 2.77657
1.00782 + 34.968852
3.01605 + 34.968852
0.97959
ν1
=
and thus:
; ν 1 = 1777 cm− 1 is the predicted stretching frequency for 3TCl
−1
2991cm
2.77657
µHCl =
6
Sketch the qualitative splitting pattern and relative intensities within each set for the 1H- and
We would expect to measure the same coupling constants in both
sets of spectra. The proton resonance spectrum should be a simple
doublet with a (large) splitting into a doublet due to the one-bond
coupling to the 100% abundant 31P nucleus.
The corresponding phosphorus resonance spectrum (frequency about
2/5th MHz that of the proton, so they do not overlap) is expected to
be a 1:3:3:1 quartet from coupling to three equivalent 1H nuclei,
which are over 98% abundant.
7
31
P-NMR spectra of PH3.
1J{31 P-1 H}
1 H-spectrum
1 J{31P-1H}
31P-spectrum
Use Lewis structures and the VSEPR model to predict the shapes of H2Se, P2H4, and H30+ and to assign point groups.
Assume a skew structure for P2H4. Model each compound in HyperChem using the model builder followed by AM1
minimization.
8
Identify the compound in the following list that is most likely to undergo radical reactions with alkyl halides, and describe
the reason for your choice: H2O, NH3, (CH3)3SiH, (CH3)3SnH.
9
Arrange H2O, H2S, and H2Se in order of (a) increasing acidity, (b) increasing basicity toward a hard acid such as the
proton.
10 Describe the three different common methods for the synthesis of binary hydrogen compounds and illustrate each one with
a balanced chemical equation.
11 Borane exists as the molecule B2H6 and trimethylborane exists as a monomer B(CH3)3. In addition, the molecular formulas
of the compounds of intermediate compositions are observed to be B2H5(CH3), B2H4(CH3)2, B2H3(CH3)3, and B2H2(CH3)4.
Based on these facts, describe the probable structures and bonding in this series of mixed alkyl hydrides.
So long as there is a single B–H bond, dimerization via bridging H atoms is possible (3c,2e bonds). Unlike the larger Al atom,
B is incapable of bridging methyl groups, presumably because they are too large. Hence BMe 3 is a monomer.
12 Give balanced chemical equations for practical laboratory methods of synthesizing (a) H2Se, (b) SiD4, and (c) Ge(CH3)2H2
from Ge(CH3)2Cl2, and for the commercial synthesis of (d) SiH4 from elemental silicon and HCl.
13 Does B2H6, survive in air? If not, write the equation for the reaction. Describe a step-by-step procedure for transferring
B2H6 quantitatively from a gas bulb at 200 Torr into a reaction vessel containing diethyl ether.
14 What is the trend in hydridic character of [BH4]–, [AlH4]–, and [GaH4]–? Which is the strongest reducing agent? Give the
equations for the reaction of [GaH4]– with excess 1 M HCl(aq).
15 Given NaBH4, a hydrocarbon of your choice, and appropriate ancillary reagents and solvents, give formulas and
conditions for the synthesis of (a) B(C2H5)3, (b) Et 3NBH3.
16 Write balanced chemical equations for the formation of pure silicon from crude silicon via silane.
17 Describe the important physical differences and a chemical difference between each of the hydrogen compounds of the pblock elements in Period 2 and their counterparts in Period 3.
18 What type of compound is formed between water and krypton at low temperatures and elevated krypton pressure?
Describe the structure in general terms.
19 Sketch the approximate potential energy surfaces for the hydrogen bond between H2O and the Cl – ion and contrast this
with the potential energy surface for the hydrogen bond in [FHF] –.
20 (a) Sketch a qualitative molecular orbital energy level diagram for the HeH+ molecule-ion and indicate the correlation of
the molecular orbital levels with the atomic energy levels. The ionization energy of H is 13.6 eV and the first ionization
energy of He is 24.6 eV. (b) Estimate the relative contribution of H 1s and He ls orbitals to the bonding orbital and predict
the location of the partial positive charge of the polar molecule. (c) Why do you suppose that HeH+ is unstable on contact
with common solvents and surfaces? You may use HyperChem to help you with this exercise, but note the following: the
structure must be created using Model Build. Then perform a SemiEmpirical calculation, using the Extended Hückel
method only. How many eV of stabilization occurs on forming HeH+ from the constituents?
(a) See diagram at right.
eV
-11
(b) The bonding orbital is extremely
dominated by the He 1s atomic orbital,
and hence we expect there to be a large
positive charge on the H atom..
Extended Hückel calculates +0.85 on H
and +0.15 on the He. There is only 0.
-13
(c)
There is only 0.4 eV of
stabilization on the formation of the
adduct; thus we are not surprised that
the “molecule” is unstable on reaction
with surfaces and other molecules.
-21
HeH +
H+
He
2σ
-15
-17
-19
-23
-25
↑↓
1σ
↑↓
21 Use this theory to study the following Lewis AB reactions. Use the AM1 method throughout, and all fragments and adducts
in this case are neutral with a spin multiplicity of 1.
a) Create AM1 minimized models of the acids (A) BH3 and (B) BCl 3. Turn on the log file and perform Single Point
calculations on each in order to confirm the point group symmetry and to obtain ∆Hf for each base. Turn the log file
off again. Use the Calculate Orbitals function to display the LUMO and identify which orbital type this is. Use the
Copy function to obtain a graph of the MO energy levels (labels turned on).
b) Create AM1 minimized models of the bases (C) NH3 and (D) PH3 and do exactly the same for these as done for A and B.
c) Create AM minimized models of the adducts A–C, A–D, B–C and B–D. Obtain their ∆Hf also, and their orbital
energies. Use this data to create orbital interaction diagrams for each acid with each base. Identify the MO’s in the
adducts that correspond to the fragment orbital HOMO-LUMO interactions. Note that these may not be the HOMO or
LUMO of the adduct! Calculate the ∆Hrxnfor the formation of each adduct from its constituents. Finally, correlate the
enthalpy changes with the compatibility of the orbital energies of the interacting fragments, and use this to identify the
best acid-base combination amongst A, B, C and D.
AM1 Calculated orbital energies of two Lewis acids and two Lewis bases and the Four Possible Adducts
BH3
∆Hf = +26.2 kcal/mol
BCl 3
∆Hf = –97.1 kcal/mol
NH3
∆Hf = –7.3 kcal/mol
PH3
∆Hf = +10.1 kcal/mol
H3B–NH3
∆Hf = –19.4 kcal/mol
H3B–PH3
So ∆Hrxn is –38.2
Cl3B–NH3
∆Hf = –140.3 kcal/mol
∆Hf = +7.2 kcal/mol
So ∆Hrxn is –29.2
Cl3B–PH3
So ∆Hrxn is –35.9
∆Hf = –70.8 kcal/mol
So ∆Hrxn is +16.2
METHOD: ∆Hrxn =[∆Hf adduct] – [∆Hf acid + ∆Hf base]
We find for each base that BH3 is a better Lewis acid than BCl 3 according to this gas phase reaction scheme. There does not
seem to be an obvious correlation between the value of the orbital energies of the acid and the base and the ∆Hrxn. However, it
is obvious from the structure of the weakest adduct that little re-organisation of the shape of the molecule has occurred on
adduct formation.
Each PH3 adduct is weaker than the corresponding NH3 adduct. This certainly fits our expectations given that PH3 is a soft base,
ammonia a hard base, and the acids are both hard acids.
22 Derive a bonding model for the linear [F–H–F]– ion. Use HyperChem to assist you. You may use the model builder in this
case, despite the unusual nature of the compound. Then minimize it in AM1 being sure to set the correct charge and
multiplicity (how many electrons are there in this ion?) Now create an interaction diagram as follows: using the SAO’s
provided in the Symmetry Tools document on the web page for a linear system with p orbitals (do NOT forget about the p
orbitals that have σ-symmetry!) to create SAO’s for the two terminal fluorine atoms as one fragment. The central H atom
will be the second fragment (what symmetry does the hydrogen AO have in the D∝h point group?) Calculate the bond
order, provide topological sketches of the MO’s, and discuss the bonding in this unusual species in as much detail as
possible.
The [F–H–F]– ion does minimize as a linear molecule: The bond distances are 1.084 Å. The energy level is now presented, with
topologically correct sketches of the MO’s. Note the close resemblance of many of these to the SAO’s for a linear molecule –
and not surprising, since only a single hydrogen 1s orbital has been inserted in between!
[F-H-F]-
z
+15
•
•
•
3σg
↑↓ 2σ u
•
•
•
2σu
πg
•
•
•
πg
•
•
•
πu
•
•
•
2σg
•
•
•
1σu
•
•
•
1σg
3σ g
y D∞ h
x
eV
F---F
H
-3
-4.5
-5
-10
↑↓
↑↓
↑↓ ↑↓
πu
↑↓
2σ g
break
in scale
-34
-39
↑↓
↑↓
1σ u
σu
σg
1σ g
G
The nature of the MO’s is most interesting. Firstly, the 1σg orbital is extremely low in energy, yet is stabilized 5 eV compared
to the completely non-bonding 1σu orbital. This means that it is an important bonding MO. The contribution of H to this MO is
only as a result of second-order mixing! The second significant bonding orbital is clearly the 2σg orbital, a central H 1s orbital
in phase with fluorine 2pz functions. These two between them correspond to the bonds in the [F–H–F]– ion, and although
formally this should be a 3c,2e bond, it certainly looks from the MO diagram as if the bond order is closer to 2 than to one.
Consistently there is a very high enthalpy of formation for this ion.
The remaining F p orbitals are not capable of mixing with the H orbital by symmetry. What is going on with these? To a first
approximation, π g is completely non-bonding, π u is weakly bonding across the H nucleus, and σu is quite significantly antibonding, again by long-distance interaction across the H nucleus. These two favourable and one unfavourable interactions
almost cancel each other out, but not quite. The 2σu destabilization is slightly greater, and thus this to some extent cancels out
what 2σg gains, so that the bond order is definitely not a full two units. Probably about 1.5 for the molecule, or 0.75 for each F–
H bond. Still, it’s a pretty nice molecule, this “hydrogen bonded ion”!