Question

1.) Using enthalpies of formation, calculate the standard change in enthalpy for the thermite reaction. The...

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

Homework Answers

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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