1)Using average bond enthalpies, predict which of the following reactions will b
ID: 896040 • Letter: 1
Question
1)Using average bond enthalpies, predict which of the following reactions will be most exothermic:
a) C(g) + 2 F2(g)
b) CO(g) +3 F2(g) CF4(g) + OF2(g)
c) CO2(g) + 4 F2(g) CF4(g) + 2 OF2(g)
2) Ammonia is produced directly from nitrogen and hydrogen. The chemical reaction is N2(g) + 3 H2(g) 2 NH3(g)
NN
a) Use Table 8.4 p 326 in textbook (or you can find the table in the chapter 5 slides on the UR course), to estimate the enthalpy change using the bond enthalpy for the reaction. Is it exothermic or endothermic?
b) Calculate the standard enthalpy change for this reaction, using standard enthalpies of formation Hf°.
Explanation / Answer
1) a) answer : C(g) + 2 F2(g) most exothermic delta H = -925 kJ i got
for b ) -789 kJ and for c) -875 kJ i calculated
2 )
(a)
from bond enthalphy calculating enthalpy of reaction.
N2 + 3 H2 --------------------> 2 NH3
NH3 has 3 N-H bonds . so bond energy = 3 x 391 = 1173 kJ
2 molecules of NH3 = 2 x 1173 = 2346 kJ
N2 energy = 945 kJ
H2 energy = 432 kJ
for 3 moles H2 = 3 x 432 = 1296 kJ
total reactant energy = N2 + 3 H2 = 945 + 1296 = 2241 kJ
energy of the reaction = reactant energy - product energy
= 2241 - 2346
= -105 kJ
enthalpy of reaction = -105 kJ
so it is exothermic reaction
b) the standard enthalpy change for this reaction
Horxn = Hoproducts - Horeactants
= 2 HoNH3 - (3 x HoH2 + HoN22)
= 2 x -46.11 - (0+0)
= -92.2 kJ /mol
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