Styrene, C8H8, is one of the substances used in the production of synthetic rubber.?

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, 42.62 kJ are released per gram of styrene. Find the standard enthalpy of formation of styrene at 25°C.
(Given: ΔH°f[CO2(g)] = –393.5 kJ/mol, ΔH°f[H2O(l)] = –285.8 kJ/mol, ΔH°f[H2O(g)] = –241.8 kJ/mol)

schmiso · Accepted Answer

Reaction equation for the combustion reaction:
C₈H₈(l) + 10 O₂(g) → 8 CO₂(g) + 4 H₂O(l)

ΔH°c = -42.62kJ per g Styrene
Since styrene has a molar mass of 104.15g/mol
the molar enthalpy of reaction for this combustion reaction is 
ΔH°r = -42.62kJ ∙ 104,15 = -4438.9 kJ/mol

According to Hess' law:
ΔH°r = 8∙ΔH°f[CO₂(g)] + 4∙ΔH°f[H₂O(l)] - ΔH°f[C₈H₈(l)]
(oxygen does not appear because enthalpy of formation of the elements in their standard state equals zero)

= 

ΔH°f[C₈H₈(l)] = 8∙ΔH°f[CO₂(g)] + 4∙ΔH°f[H₂O(l)] - ΔH°r
= 8∙(- 393.5 kJ/mol) + 4∙(-285.8kJ/mol) - (-4438.9kJ/mol)  
= 147,7 kJ/mol

Styrene, C8H8, is one of the substances used in the production of synthetic rubber.?

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