Lecture 11: Chemical bonding 4
Read:
HW:
BLB 8.7–8.8
BLB 8:61,63,64,71
Sup 8:7–10
Know:
• bond length & enthalpy
Exam 1: Monday, Feb 9 @ 6:30!!! start preparing now!! only
non-text programmable calculators are allowed—no PDAs,
blackberries, cell phones, etc. will be permitted. Bring: pencils,
student ID and a calculator—Absolutely NO text-programmable
calculators or wireless devices (will be checked)
Form a study group, use the CRC, take advantage of SI (info on web), use the
online resources, and work those problems—practice, practice, practice
Sheets’s office hours: Mondays 12:30-2; Tuesdays
10:30-12 in 324 Chem (or 326 Chem).
Bonus deadline for Skill check test 6 is
SUNDAY, 2/8
Sheets
Page 1
Lecture 11
Covalent bond lengths & energies
• bond length: distance between nuclei
check out
BLB Tables 8.4 & 8.5
bond
C–C
C=C
C≡C
bond energy
kJ/mol
348
614
839
• more electrons shared,
bond
C–H
C–Cl
C–Br
bond energy
kJ/mol
413
328
276
bond length d
Å
1.54
1.34
1.20
bond length
bond length d
Å
1.10
1.76
1.96
• shorter bond length,
the bond
Sheets
Lecture 11
Page 2
Sheets
Page 3
Lecture 11
More on covalent bond lengths
& energies
• covalent bonds are strong (100–1000 kJ/mol)
• overall stability of a molecule related to the
bonds it contains
• strength of bond = energy (E) needed to
break the bond
• bond (dissociation) energy (D): enthalpy of
bond-breaking reaction in the gas phase
• always D > 0
(ΔH > 0); value is always +
(endothermic)—requires
to break bond
• value depends on identity of atoms & # of
shared e– pairs
example
H
H C H
H
C
4H
ΔH = 1660 kJ
D(C–H) = 1660 kJ/mol / 4 = 415 kJ/mol
Sheets
Page 4
Lecture 11
Bond energy
• for diatomics, D is ΔH of one reaction
H–H(g) → 2H(g)
D(H–H) = ΔHrxn = 436 kJ/mol
• for polyatomics, D is an averaged quantity
H–O–H(g) → HO(g) + H(g)
H–O(g)
→ H(g) + O(g)
+494 kJ/mol
+424 kJ/mol
D(O–H) = 463 kJ/mol *
* value obtained from averaging over many
different molecules
• exact values are known for some bonds, but
the average values are not exact for any one
case (unlike ΔHf°←more on this when we cover
Chap 5 [~Lectures 38 & 39])
Sheets
Page 5
Lecture 11
Before next class:
Review: Lectures 1–11: Chap 1, 2, 6, 7 & 8.
Please bring any last minute questions you
may have. This is for touch-up or clarification
questions, NOT a lecture repeat!
Sheets
Page 6
Lecture 11