1) The combustion reaction for benzoic acid C6H5CO2H(s) + 15 2 O2(g) 7 CO2(g) +

Question

1) The combustion reaction for benzoic acid C6H5CO2H(s) + 15 2 O2(g) 7 CO2(g) + 3 H2O() has H 0 = 771.2 kcal/mole. Use Hess’s Law to calculate H0 f for benzoic acid. Answer in units of kcal/mol.

2) Calculate the heat of formation of butane (C4H10) using the following balanced chemical equation and information. Write out the solution according to Hess’s law. C(s) + O2(g) CO2(g) H0 f = 396.3 kJ/mol H2(g) + 1 2 O2(g) H2O() H0 f = 284.5 kJ/mol 4 CO2(g) + 5 H2O C4H10(g) + 13 2 O2(g) H0 c = 2877.6 kJ/mol Answer in units of kJ.

3) Calculate the molar heat of vaporization of a substance given that 0.228 mol of the substance absorbs 28.5 kJ of energy when it is vaporized. Answer in units of kJ/mol.

4) Calculate H0 rxn for the reaction CH3Cl + F2 CH3F + Cl2 . Is the reaction endothermic or exothermic?

1. 242 kJ, exothermic

2. 300 kJ, endothermic

3. 242 kJ, endothermic

4. 242 kJ, endothermic

5. 379 kJ, exothermic

6. 300 kJ, exothermic

7. 379 kJ, endothermic

8. 242 kJ, exothermic

9. 379 kJ, exothermic

10. 379 kJ, endothermi

Explanation / Answer

1) The combustion reaction for benzoic acid

C6H5CO2H(s) + 15 2 O2(g) 7 CO2(g) + 3 H2O() has H 0 = 771.2 kcal/mole.

Use Hess’s Law to calculate H0 f for benzoic acid. Answer in units of kcal/mol.

HRxn = Hproducts - Hreactants

HRxn = (7CO2 + 3H2O(l) ) - (Benz.acid 15/2*O2)

H 0 = 771.2 kcal/mole. = -771.2 *4.184 = -3226.70 kJ/mol

HRxn = (7CO2 + 3H2O(l) ) - (Benz.acid 15/2*O2)

-3226.70 = 7*-393.5 + 3*-285.8 - Hbenzoic + 15/2*0

Hbenzoic = 7*-393.5 + 3*-285.8 +3226.70 = -385.2 kJ/mol

change to kcal

Hbenzoic formation = -385.2/4.184 = -92.065 kcal/mol

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