Styrene, C 8 H 8 , is one of the substances used in the production of synthetic
ID: 520199 • Letter: S
Question
Styrene, C8H8, is one of the substances used in the production of synthetic rubber. When styrene burns in oxygen to form carbon dioxide and liquid water under standard-state conditions at 25°C, 42.15 kJ are released per gram of styrene. Find the standard enthalpy of formation of styrene at 25°C.
a) –4390 kJ/mol
b) –1044 kJ/mol
c) –8681 kJ/mol
d) +99 kJ/mol
e) +637 kJ/mol
Styrene, C8H8, is one of the substances used in the production of synthetic rubber. When styrene burns in oxygen to form carbon dioxide and liquid water under standard-state conditions at 25°C, 42.15 kJ are released per gram of styrene. Find the standard enthalpy of formation of styrene at 25°C.
Explanation / Answer
The overall Reaction equation for the combustion reaction:
CH(l) + 10 O(g) 8 CO(g) + 4 HO(l)
H°c = -42.15 kJ per g Styrene
Since styrene has a molar mass of 104.15g/mol
the molar enthalpy of reaction for this combustion reaction is
H°r = -42.15 kJ 104.15 = -4389.9 kJ/mol
and according to Hess' law:
H°r = 8H°f[CO(g)] + 4H°f[HO(l)] - H°f[CH(l)]
(oxygen does not appear because enthalpy of formation of the elements in their standard state equals zero)
=>
H°f[CH(l)] = 8H°f[CO(g)] + 4H°f[HO(l)] - H°r
= 8(- 393.5 kJ/mol) + 4(-285.8kJ/mol) - (-4389.9kJ/mol)
= 98.7 kJ/mol
So correct choice is d.