TUTORIAL #6 – Chemical Bonding I (with answers)
1.
Which has the higher lattice energy: NaCl or CsI?
2.
Calculate the lattice energy (in kJ/mol) for sodium hydride, NaH using the following
information: Ea for H = –72.8 kJ/mol
IE1 for Na = +495.8 kJ/mol
Heat of sublimation for Na = +107.3 kJ/mol
Bond dissociation energy for H2 = +435.9 kJ/mol
∆𝐻𝑓0 (NaH) = –60 kJ/mol
3.
Calculate the overall energy change (in kJ/mol) for the formation of CsF from its element
using the following data:
Ea for F = –328 kJ/mol
IE1 for Cs = +375.7 kJ/mol
IE2 for Cs = +2422 kJ/mol
Heat of sublimation for Cs = +76.1 kJ/mol
Bond dissociation energy for F2 = +158 kJ/mol
Lattice energy for CsF = +740 kJ/mol
4.
*Calculate the overall energy changes (in kJ/mol) for the formation of MgF and MgF2 from
their elements. The following data are needed:
Ea for F = –328 kJ/mol
IE1 for Mg = +737.7 kJ/mol
IE2 for Mg = +1450.7 kJ/mol
Heat of sublimation for Mg = +147.7 kJ/mol
Bond dissociation energy for F2 = +158 kJ/mol
Lattice energy for MgF2 = +2952 kJ/mol
Lattice energy for MgF = +930 kJ/mol
Based on your calculations, which compound is more like to form in the reaction of
magnesium with fluorine: MgF or MgF2? Why?
5.
*Assume that 20.0 g of Sr is allowed to react with 25.0 g of Cl2 to give SrCl2.
(a) What is the net energy change (in kJ/mol) for the formation of SrCl2 from its elements?
Heat of sublimation for Sr = +164.44 kJ/mol
IE1 for Sr = +549.5 kJ/mol
IE2 for Mg = +1064.2 kJ/mol
Bond dissociation energy for Cl2 = +243 kJ/mol
Ea for F = –348.6 kJ/mol
Lattice energy for SrCl2 = +2156 kJ/mol
(b) Which reactant is limiting and how many grams of SrCl2 are formed?
(c) How much energy (in kJ) is released during the reaction?
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6.
Which of the following substances are ionic, polar covalent and which are covalent?
(a) HF
(d) BBr3
(b) HI
(e) NaOH
(c) PdCl2
7.
Use the electronegativity values (refer to textbook) to predict which bond in each of the
following is more polar:
(a) C—H or C—Cl
(b) Si—Li or Si—Cl
(c) N—Cl or N—Mg
8.
What is ‘octet rule’? Why does it apply primarily to representative element and not to
transition metals?
9.
Draw the Lewis structures for the following molecules or ions:
(a) CBr4
(i) S2Cl2
(b) NCl3
(j) CH3Cl
–
(c) BF4
(k) SiO44–
(d) O22–
(l) ClF4+
(m) HOCl
(e) NO+
(n) HNO2 (Hint: there is no oxygen(f) ClF
(g) H2Se
oxygen bond)
(h) PO43–
10.
Draw all resonance structures for the following molecules or ions:
(a) HN3
(b) SO3
(c) SCN–
11.
Assign formal charges to the atoms in the following resonance forms of ClO2–:
12.
Calculate the enthalpy for forming 2 mol of ammonia from molecular nitrogen and molecular
hydrogen. Is this reaction exothermic or endothermic?
Given: N≡N = 946 kJ/mol ; H–H = 436 kJ/mol ; N–H = 391 kJ/mol
13.
*Using bond energies, determine ∆Ho for the reaction of molecular hydrogen with each of the
gaseous molecular halogens: fluorine, chlorine, bromine and iodine. Which is the most
exothermic reaction?
Given:
H–H = 436 kJ/mol ;
F–F = 158 kJ/mol ; Cl–Cl = 242 kJ/mol ;
Br–Br = 193 kJ/mol ; I–I = 151 kJ/mol ;
H–F = 566 kJ/mol ;
H–Cl = 431 kJ/mol ; H–Br = 366 kJ/mol ; H–I = 299 kJ/mol.
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ANSWERS :
1.
Na+ is smaller than Cs+ and Cl– is smaller than I–, therefore the distance between ions is
smaller in NaCl than in CsI. As a result, NaCl has the larger lattice energy.
2.
808 kJ/mol
3.
–537 kJ/mol
4.
∆𝐻𝑓0 (MgF) = –294 kJ/mol , ∆𝐻𝑓0 (MgF2) = –1114 kJ/mol
5.
(a) –832 kJ/mol ; (b) limiting reactant = Sr, 36.1 g SrCl2 is produced , (c) 190 kJ
6.
(a) polar covalent
(d) polar covalent
7.
(a) C—Cl , (b) Si—Cl , (c) N—Mg
8.
Octet rule states that the representative elements tend to undergo reactions that will leave
them with eight valence electrons. The transition metals are characterized by partially filled d
orbitals that can be used to expand their valence shell beyond the normal octet of electrons.
9.
Drawing
10.
Drawing
11.
Formal charges for resonance structures of ClO2–:
12.
–92 kJ, exothermic
13.
∆Ho(HF) = –538 kJ/mol ; ∆Ho(HCl) = –184 kJ/mol ; ∆Ho(HBr) = –97 kJ/mol ;
∆Ho(HI) = –11 kJ/mol ; the reaction of H2 with F2 is the most exothermic.
(b) polar covalent
(c) polar covalent
(e) Na+ —OH– is ionic but OH– is polar covalent
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