9.3 Bond enthalpies

Question 1

average bond enthalpy

Answer 1

is the energy required to break one mole of a specified type of bond in a gaseous molecule

Question 2

energy is required to break bonds,
therefore bond enthalpies are always exothermic/endotthermic

Answer 2

endothermic
have a positive enthalpy value

Question 3

energy is released when bonds form.
bond making is exothermic/endothermic

Answer 3

exothermic

Question 4

bond breaking is exothermic/endothermic

Answer 4

endothermic

Question 5

calculating enthalpy changes from average bond enthalpies.
for a reaction involving GASEOUS molecules of covalent substances:

Answer 5

ΔrH = Σ(bond enthalpies in reactants) - Σ(bond enthalpies in products)

reactants - products

Question 6

using average bond enthalpies, calculate ΔrH for the reaction of propane with oxygen.
C3H8(g) + 5O2(g) –> 3CO2(g) + 4H2O(g)

C-H = 413 kJ mol-1
C-C = 347
O=O = 498
C=O = 805
O-H = 464

Answer 6

propane = 2(347) + 8(413)
= 3998
oxygen = 5(498)
= 2490
carbon dioxide = 3(2x805)
= 4830
water = 4(2x464)
=3712

reactants - products:
(3998 + 2490) - (4830 + 3712)
= 6488 - 8542
= -2054 kJ mol-1