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BOND ENERGY
(p. 688-689)
•
Bond Dissociation Energy:
• the quantity of energy required to break one mole of covalent bonds in
a gaseous species, usually expressed in kJ/mol.
•
Average Bond Energy:
• average of bond-dissociation energies for a number of different
species containing a particular covalent bond.
•
∆H for reactions can also be found using bond energies.
•
The ∆H calculated by this method will be different than that found using
Hess’s Law or that found experimentally.
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Bond energies found in tables are average values for a particular bond type.
Some Average Bond Energies
Bond
H-H
H-C
H-N
H-O
H-S
H-F
H-Cl
H-Br
H-I
Bond Energy
(kJ/mol)
436
414
389
464
368
565
431
364
297
Bond Bond Energy Bond Bond Energy
(kJ/mol)
(kJ/mol)
C-C
347
N-N
163
C=C
611
N=N
418
837
946
C≡C
N≡N
C-N
305
N-O
222
C=N
615
N=O
590
891
O-O
142
C≡N
C-O
360
O=O
498
C=O
736
F-F
159
C-Cl
339
Cl-Cl
243
Br-Br
193
I-I
151
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The bond energy for a H-H bond, or an F-F bond can be determined to a high
degree of precision because H2 and F2 molecules contain single bonds of a
particular type.
•
The exact bond energy for a C-H bond cannot be determined exactly because
there is no such thing as a C-H molecule.
2
•
The C-H bond always has other atoms bonded to it which affect the
bond energy. (i.e. the C-H bond energy in CH4, CH2Cl2 and CH3F are
all slightly different since the C-H bond is in different environments.)
•
∆H values calculated using average bond energies are therefore slightly
different than the values calculated by some other method.
•
Recall that bond formation is an exothermic process while bond breakage is
an endothermic process.
•
To find ∆Hrxn from average bond energies we use the formula:
∆H = ∆(Bond energies of all reactant bonds broken) –
∆(Bond energies of all product bonds formed)
∆Hrxn = ∆H(bond breakage) – ∆H(bond formation)
∆Hrxn = ΣBE(reactants) - ΣBE(products)
Steps for calculating H using bond energies:
1. Determine the number (in moles) and type of bonds broken and formed
from the balanced equation. This may require you to draw the molecules
to identify how many of each bond type are present.
2. Multiply the number of bonds by the average bond energy given in the
table to determine the energy change.
3. Substitute in values into the formula to calculate the H of the overall
reaction.
Examples
1.
Calculate the energy of the reaction for the burning of methane in oxygen
to form carbon dioxide gas and water gas, using heats of formation. The
balanced equation is given below.
C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)
2.
Find the ∆H for the formation of H2O from its elements using (a) average
bond energy values and (b) ∆Hof values.
2H2(g) + O2(g) Æ 2H2O(g)
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Examples
1. Calculate the energy of the reaction for the burning of methane in oxygen to
form carbon dioxide gas and water gas, using heats of formation. The balanced
equation is given below.
C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)
Steps 1 and 2
1) Determine the number and types of bonds broken and formed.
2) Determine the energy change.
Bonds Broken (reactants)
Type
#
Bond Energy
C-C
2
347 kJ/mol
+ 694 kJ
C-H
8
414 kJ/mol
+3,312 kJ
O=O
5
498 kJ/mol
+2,490 kJ
Total
Energy
+6,496 kJ
Bonds formed (products)
O-H
8
464
3,712 kJ
C=O
6
736
4,416 kJ
Total
8,128 kJ
Step 3)
H = Energy of Bonds Broken - Energy of Bonds formed
H = 6,496 kJ - 8,542 kJ
H = - 2,046 kJ
Example:
Find the ∆H for the formation of H2O from its elements using (a) average bond
energy values and (b) ∆Hof values.
2H2(g) + O2(g) Æ 2H2O(g)
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Thermochemistry Worksheet #9
Use the table of average bond energies given above to complete the following:
1. Use bond energies to estimate the enthalpy change for the combustion of
butane. (Ans: -2110 kJ)
C4H10(g) + 13/2 O2(g) Æ 4 CO2(g) + 5 H2O(g)
2. Use bond energies to estimate the enthalpy change for:
N2(g) + 3 H2(g) Æ 2 NH3(g)
Compare the value you obtained using average bond energies to that which you
would obtain using standard heats of formation. (Ans: -80 kJ vs. –92.22 kJ)
3. One reaction that methane undergoes with chlorine is:
CH4(g) + 3 Cl2(g) Æ CHCl3(g) + 3 HCl(g)
Using average bond energies calculate the enthalpy change for this
reaction.(Ans: -339kJ)
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Some Final Points!
Catalyst
• a substance that provides an alternative mechanism of lower activation
energy for a chemical reaction.
• The reaction is speeded up and the catalyst is regenerated.
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Catalysts do not effect the enthalpy (∆H) of a reaction!
Physical, Chemical and Nuclear Changes
•
These differ in the amount of energy involved:
Physical Change:
• one or more physical properties of a sample of matter change, but the
composition remains unchanged.
• The energy associated with a phase change is usually in the 10’s of kJ/mol.
Chemical Reaction:
• a process in which one set of substances (reactants) is transformed into a
new set of substances (products).
• The energy associated with a chemical change is usually in the 100’s or
1000’s of kJ/mol
Nuclear Reactions:
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These are of two general types.
a) Nuclear Fission:
• a radioactive decay process in which a heavy nucleus breaks up into
two lighter nuclei and several neutrons, accompanied by the release of
energy.
b) Nuclear Fusion:
• process by which small atomic nuclei are fused into larger ones, with
some of their mass being converted to energy.
•
The energy associated with nuclear reactions is usually in the millions or
billions of kJ/mol!