Bond Energies Note** - READ TEXTBOOK PAGES 300-306 – SECTION 8.8
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- Energy must be added/absorbed to BREAK bonds (endothermic) in order to overcome the coulombic attraction between each nuclei and the shared electrons. Energy is released when bonds are FORMED (exothermic) because the resultant coulombic attraction between the bonded atoms lowers potential energy causing a release. This is a giant misconception among students! Once again, it “takes” energy to break bonds and energy is released when a bond forms.
- ΔH = sum (Σ) of the energies required to break old bonds (positive signs since energy is added to the system) plus the sum of the energies released in the formation of new bonds (negative signs since energy is lost from the system).
ΔH = Σ Bond Energies broken – Σ Bond Energies formed
Example: Using the table of average bond energies below, the ΔH for the reaction is ______kJ. (answer: -217 kJ/mol)
Bond:C≡CC–CH–IC–IC–H
Δ (kJ/mol):839348299 240413
∆H0rxn =(Bonds Broken) – (Bonds Formed)
(1* C≡C + 2*H–I ) – (2*C–H + 2*C–I + 1*C-C)
∆H0rxn = [1(839) + 2(299)] – [2(413) + 2(240) + 1(348)]
∆H0rxn = -217 kJ (answer)
**Note – 2 of the C-H bonds in reactants did not break and
are present as products, therefore do not form.
- Using the table of bond dissociation energies, the ΔH for the following gas-phase reaction is ______kJ. (ans.=-44kj/mol)
- The reaction below is used to produce methanol. Calculate the C-H bond energy given the following data: (ans.= 417 kJ/mol)
C≡O (g) + 2H-H (g) → CH3OH (l)ΔHrxn = -128 kJ
3.A ______covalent bond is the longest.
a.single
b.double
c.triple
d.they are all the same length
e.strong
4.Bond enthalpy is ______.
a.always positive
b.always negative
c.sometimes positive, sometimes negative
d.always zero
e.unpredictable
5.Of the bonds C-N, C=N, and C=N, the C-N bond is ______.
a.strongest/shortest
b.strongest/longest
c.weakest/shortest
d.weakest/longest
e.intermediate in both strength and length
6.Using the table of average bond energies below, the H for the reaction:
is _____ kJ.
Bond:C≡CC–C H–IC–IC–H
Bond energy (kJ/mol):839348299 240413
A) +160
B) -160
C) -63
D) -217
E) +63
7.Using the table of bond dissociation energies, the H for the following reaction is _____ kJ.
2HCl(g) + F2(g) 2HF(g) + Cl2(g)
Bond Bond Energy(kJ/mol)
H-Cl 431
F-F 155
H-F 567
Cl-Cl 242
a.-359
b.-223
c.359
d.223
e.208
- a. Draw a potential energy diagram for an exothermic reaction below and label the following: reactants, products, Eact, and H.
b. Draw a dotted line on the same graph to show the effects of adding a catalyst.
- a. Draw a potential energy diagram for an endothermic reaction below and label the following: reactants, products, Eact, and H.
b. Draw a dotted line on the same graph to show the effects of adding a catalyst.