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

Answer 1

1) The enthalpy of given rxn ,Hrxn-771.2 kcal/mol-(771.2*1000 J/mol)*4.184J/cal)-3226700.8J/mol-3226.7 KJ/mol

HrxnH^o_f(products) - H^o_f(reactants)

where H^o_f represents enthalpy of formation

The value of H^o_f :

H^o_f (benzoic acid)?

H^o_f (O2)0

H^o_f (CO2)-393.5 KJ/mol

H^o_f (H2O(l))-285.8 KJ/mol

Plugging in these values in the above eqn,

-3226.7 KJ/molHrxn(7*(H^o_f (CO2))+ 3*(H^o_f (H2O(l))) - (((H^o_f (benzoic acid) +15/2*H^o_f (O2))

or ,-3226.7 KJ/mol(7*(-393.5 )+3*(-285.8 )) -(H^o_f (benzoic acid) +0)

or, H^o_f (benzoic acid)-385.2KJ/mol

2) Enthalpy of formation of butaneH^o_f (C4H10)

given rxns:

C(s)+O2(g)--->CO2(g),H^orxn1-396.3KJ/mol ............(1)

H2(g)+1/2O2(g)--->H2O(l) ,H^o_rxn2-284.5 KJ/mol............(2)

4CO2(g)+5H2O(l) -->C4H10(g)+13/2O2(g),H^orxn32877.6 KJ/mol...........(3)

Using Hess's law, 4*Eqn(1) +5*eqn(2) -eqn(5) gives,

4C(s) +5H2(g) --->C4H10(g) , H^o_f (C4H10)4*H^orxn1+5*H^orxn2-H^orxn34*(-396.3)+5*(-284.5)-2877.6-5885.3KJ/mol

H^o_f (C4H10)-5885.3KJ/mol

3)heat of vaporization of 0.228 mol of the substance28.5 KJ

Molar heat of vaporization of the substance28.5 KJ/0.228mol+125.0 KJ/mol

4)

HrxnH^o_f(products) - H^o_f(reactants)

where H^o_f represents enthalpy of formation

The value of H^o_f :

H^o_f (CH3F)-246.1 KJ/mol

H^o_f (Cl2)0

H^o_f (CH3Cl)-83.7KJ/mol

H^o_f (F2)0

Hrxn(-246.1 +0)-(-83.7+0)-162.4 KJ/mol It isan exothermic reaction, heat is released as shown by the negative sign.