PSI 9. The enthalpy of combustion (at 25 ),AH comb. for benzoic acid, C7H602(s)

Question

PSI 9. The enthalpy of combustion (at 25 ),AH comb. for benzoic acid, C7H602(s) is-3227 kJ mol-1 and Substance" ll'f kJ mol-I | C(s) CO2(g) H2(g) H200) 02(g) GH602( So J mol . i K . i 5.69 213.6 130.6 700 205 167.6 0 3935 -285.8 0 ? a) Write the balanced chemical equation that describes the combustion of one mol of benzoic acid. b) Write the balanced chemical equation that describes the standard formation of benzoic acid. e) Using the information given above and the equations in parts a) and b). calculate H f (kJ mol-1) for benzoic acid. 8 0

Explanation / Answer

(a) benzoic acid combustion can be written as:

2C7H6O2 + 17O2 -------> 14CO2 + 6H2O ------------(1)

(b) standard formation equation can be written as:

7C(s) + 3H2(g) + O2(g) --------> C7H6O2 ---------------(2)

(c)

Enthalpy of combustion is given -3227 kJ/mol

2C7H6O2 + 17O2 -------> 14CO2 + 6H2O

enthalpy of combustion = enthalpy of formation of products - enthalpy of formation of reactants

-3227 kJ/mol x 2 mol = 14mol x -393.5 kJ/mol + 6mol x -285.8 kJ/mol - 2 mol x Hf

Hf = +384.9 kJ/mol

heat of combustion of benzoic acid = +384.9 kJ/mol

(d) from equation (2) we get Sf

7C(s) + 3H2(g) + O2(g) --------> C7H6O2

Sf = 167.6 J/mol.K x 1 mol - {1 mol x 205 J/mol.K + 3 mol X 130.6 J/mol.K + 7 mol x 5.69 J/mol.K}

        = -469.03 J/K

use equation (1) to find entropy of reaction

2C7H6O2 + 17O2 -------> 14CO2 + 6H2O

Sr =14 mol x 213.6 J/mol.K + 6 mol x 70.0 J/mol.K -(17 mol x 205J/mol.K + 2 x 167.6 J/mol.K)

      = -409.8 J/K

chegg ask us to do only 4 parts of a question

but I can give you some hint for sure

to find Gf = Hf - TSf

we already have Hf = +384.9KJ/mol

Sf = -469.03 J/mol.K

T = 25 + 273 = 298 K

similarly using the information from table and equation (1) you can find Hr

and find Gr

(f)

If G < 0   it is thermodynamically favoured or else not

If you calculate i think the Gf should come positive and hence thermodynamically not favoured and Gr should be favoured as Gr should come negative *** please verify yourself as it is just a guess

thanks

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