1. The enthalpy of combustion (H° c ) of cinnamic acid (C 9 H 8 O 2 ) is -4359.3

Question

1. The enthalpy of combustion (H°c) of cinnamic acid (C9H8O2) is -4359.30 kJ/mol. Using the appropriate information given below, calculate the enthalpy of formation (H°f), in kJ/mol, for cinnamic acid.

Report your answer to two decimal places.



H°f (CO2 (g)) = -393.51 kJ/mol

H°f (H2O (l)) = -285.83 kJ/mol
2. Determine the mass (in g) of cinnamic acid produced, if H° was determined to be -459.11 kJ during an experiment in which cinnamic acid was formed.

Report your answer to three significant figures

Explanation / Answer

C9H8O2

Combustion equation:

C9H8O2 + O2 = CO2 + H2O

balance

C9H8O2 + 10O2 = 9 CO2 + 4 H2O

so..

Hcombustion = Hproducts - Hreactants

Hcombustion = (9*H-CO2 + 4*H2O) - (H-C9H8O2 + 10*H-O2)

nte that

10*H-O2 = 0 since O2 is elemental and CO2, H2O are given as well as Hcombustion

Substitute

-4359.30 = 9*(-393.51 ) + 4*(-285.83 ) - H-cinnamic acid

Hcinamic acid = 9*(-393.51 ) + 4*(-285.83 ) +4359.30 = -325.61 kJ/mol

Q2.

mass of acid produced if

Q = -459.11 kJ

so

Q = n*Hrxn

n = Q/HRxn = (-459.11/-4359.30 ) = 0.105 mol of acid

MW of acid = 148.1586 g/mol

so

mass = mol*MW = 148.1586 *0.105 = 15.556 g of cinnamic acid = 3 sig fig 15.6 g of acid

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