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)

N≡N

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 ΔHf°.

Answer 1

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

ΔH^o_rxn = ΔH^o_products - ΔH^o_reactants

            = 2 ΔH^o_NH3 - (3 x ΔH^o_H2 + ΔH^o_N22)

            = 2 x -46.11 - (0+0)

             = -92.2 kJ /mol