KVPY 2019 SB SX Exam Previous Papers

Question : 105 of 120

Marks: +1, -0

Given that, the molar combustion enthalpy of benzene, cyclohexane and hydrogen are x, y and z respectively, the molar enthalpy of hydrogenation of benzene to cyclohexane is

x – y + z
x – y + 3z
y – x + z
y – x + 3z

Validate

Solution:

Hydrogenation of benzene is given by

{C}_{6} {H}_{6}+3 {H}_{2} ⟶ {C}_{6} {H}_{12} \;\;\;\;\;⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(a)

Enthalpies are given for,

{C}_{6} {H}_{6}+\;{15}\/{2} {O}_{2} ⟶ 6 {CO}_{2}+3 {H}_{2} {O} ; ∆ H=x \;\;\;\;\;⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(b)

{C}_{6} {H}_{12}+9 {O}_{2} ⟶ 6 {CO}_{2}+6 {H}_{2} {O} ;\;\;\;\;\;⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(c)

∆ H=y \\ {H}_{2}+\;{1}\/{2} {O}_{2} ⟶ {H}_{2} ; ∆ H=z \;\;\;\;\;⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(d)

Reaction (a)

=( {b})-( {c})+3( {d})

∴ ∆ H

of a

=x-y+3 z

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