ThermochemistryHard
Question
Enthalpy of combustion of CH4C2H6 and C3H8 are - 210, - 368.4 and - 526.2 k cal mol-1 respectively. Enthalpy of conbustion of hexane can be predicated as
Options
A.-840k cal mol-1
B.-684k cal mol-1
C.-1000k cal mol-1
D.none of these
Solution
ᐃHc(C2H6) - ᐃHc(CH4)
= -368.2-(-210.8) = - 157.4
ᐃHc(C3H8) - ᐃHc(C2H6)
= - 526.2 - (-368.4) = - 157.8
Thus, average ᐃHc (-CH2) =
= - 157.6kcal mol-1
Then,
ᐃHc(C6H14) = - ᐃHc(C3H8) + 3ᐃHc(-CH2)
= - 526.2 + 3 (- 157.6) = - 99 k cal
= -368.2-(-210.8) = - 157.4
ᐃHc(C3H8) - ᐃHc(C2H6)
= - 526.2 - (-368.4) = - 157.8
Thus, average ᐃHc (-CH2) =
= - 157.6kcal mol-1
Then,
ᐃHc(C6H14) = - ᐃHc(C3H8) + 3ᐃHc(-CH2)
= - 526.2 + 3 (- 157.6) = - 99 k cal
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