Question

1.) Using enthalpies of formation, calculate the standard change in enthalpy for the thermite reaction. The enthalpy of formation of Fe3O4 is −1117 kJ/mol.

8 Al(s) + 3 Fe3O4(s) → 4 Al2O3(s) + 9 Fe(s)

2. a) Nitroglycerin is a powerful explosive, giving four different gases when detonated.

2 C3H5(NO3)3(l) → 3 N2(g) + 1/2 O2 (g) + 6 CO2(g) + 5 H2O(g)

Given that the enthalpy of formation of nitroglycerin, ΔHf°, is −364 kJ/mol, calculate the energy (heat at constant pressure) released by this reaction.

b) Calculate the energy liberated when 10.0 g of nitroglycerin are detonated.

3.) Diborane (B2H6) is a highly reactive boron hydride, which was once considered as a possible rocket fuel for the U.S. space program. Calculate ΔH for the synthesis of diborane from its elements, according to the equation

2 B(s) + 3 H2(g) → B2H6(g)

using the following data:

2 B(s) + 3/2 O2(g) → B2O3(s) ΔH −1273 kJ

B2H6(g) + 3 O2(g) → B2O3(s) + 3 H2O(g) ΔH −2035 kJ

H2(g) + 1/2 O2(g) → H2O(g) ΔH −241.8 kJ

Answer 1

1)

For

8 Al(s) + 3 Fe3O4(s) → 4 Al2O3(s) + 9 Fe(s)

Hrxn = Hproduycts - Hreactants

Hrxn = (4*Al2O3 + 9*Fe)- (8*Al + 3*Fe3O4)

since elements have H = 0

Hrxn = (4*Al2O3) - ( 3*Fe3O4)

Hrxn = 4*(−1675.7) - 3*(−1118.4) = -3347.6 kJ/mol

2)

for

2 C3H5(NO3)3(l) → 3 N2(g) + 1/2 O2 (g) + 6 CO2(g) + 5 H2O(g)

Hrxn = Hproducts - Hreactants

Hrxn = (3 N2+ 1/2 O2+ 6 CO2 + 5 H2O) - 2*(-364 )

Hrxn = (6 CO2 + 5 H2O) - (-364*2 )

Hrxn = (6*−393.51 + 5*−241.826) - (-364*2 ) = - 2842.19 kJ/mol

for

m = 10 g of nitroglycerin

MW = 227.0865

mol = mass/MW = 10/227.0865 = 0.04403

then

Q = 2842.19 *0.04403 = 125.1416257 kJ are released

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