Decomposition of liquid benzene to gaseous acetylene: C6H6 (l) -> 3 C2H2 (g). Delta H=630.0 kJ. What is the total energy change (Delta E) of the system if 34.5 g of benzene decomposes at STP?

Answer 1

∆E_sys = q + w

q = heat

w = work

C₆H₆(l) ==> 3C₂H₂(g) .. ∆H = 630.0 kJ

34.5 g C₆H₆ x 1 mol / 78.1 g = 0.442 mol

0.442 mol x 630 kJ/mol = +278 kJ = heat = q

mols C₂H₂ formed = 0.442 mol C₆H₆ x 3 mol C₂H₂ / mol C₆H₆ = 1.33 mols C₂H₃(g)

At STP 1 mol of any ideal gas = 22.4 L

Volume C₂H₂ = 1.33 mol x 22.4 L / mol = 29.8 L

w = -P∆V = - (1atm)(29.8L) = -29.8 Latm

-29.8 Latm x 0.101325 kJ/Latm = -3.02 kJ

w = -3.02 kJ

∆E_sys = 278 kJ - 3.02 kJ = 275 kJ