Answer to Question #100293 in General Chemistry for Beverlie

Question

5. Given the standard enthalpies of formation for the following substances, determine the change in enthalpy for the combustion of 1.0 mol of propane, C3H8.
Substance Δhf in kJ/mol
C3H8(g) -103.8
CO2(g) -393.5
H2O(l) -285.8
6. Calculate the energy released as heat when 45.8 g of Fe(s) is combined with 27.9g of O2(g) if the ΔH°rxn is -1648 kJ/mol. The balanced equation for the reaction is
4 Fe(s) + 3O2 = 2Fe2O3(s)

Answer 1

5.

Combustion of propane is given by :

$$C_3H_8+5O_2\to 3CO_2+4H_2O$$

enthalpy of combustion of propane =

$$3\times\Delta H_f(CO_2) +4\times \Delta H_f(H_2O)-\Delta H_f(C_3H_8)$$

$$3\times(-393.5)+ 4\times(-285.8)-(-103.8)\\=2219.2\ kJ/ mol$$

6.

$$4 Fe(s) + 3O_2 \to 2Fe_2O_3(s)$$

Iron is the limiting reagent in this problem

Now,

224 gm of iron releases 1648 kJ energy

then, 45.8 gm of Iron will release = $\frac{45.8}{224}\times1648 =337 \ kJ \ energy$