Natural gas is a source of methane - Scottish Highers Chemistry - Question 7 - 2022

The bond enthalpy method involves breaking and forming bonds. First, we calculate the total bond enthalpies of the reactants and products:

• Reactants: 1 C–H bond broken (4 bonds total), and 2 O=O bonds (2 bonds)
• Products: 2 C=O bonds and 4 H–O bonds Using the bond enthalpies from the data booklet, compute the enthalpy change:

$ext{ΔH} = ext{Bond energies of reactants} - ext{Bond energies of products}$

If calculated correctly, should yield approximately -816 kJ mol⁻¹.

The formula for atom economy is given by:

ext{Atom Economy} = rac{ ext{Molar mass of desired product}}{ ext{Total molar mass of all products}} imes 100

In this case, the desired product is hydrogen (3 H₂) with a mass of 6 g. The total mass of products (1 CO + 3 H₂) is:

GFM: CO = 28 g and 3 H₂ = 6 g → Total = 34 g.

Thus, the atom economy is:

ext{Atom Economy} = rac{6}{34} imes 100 = 17.65 ext{%}

To maximize the yield of nitrogen dioxide (NO₂):

• Temperature: low (to favor the exothermic forward reaction)
• Pressure: high (since there are fewer moles of gas on the right side of the reaction).

To calculate the enthalpy of the reaction, we use:

$ext{ΔH}_{reaction} = ext{ΔH}_{products} - ext{ΔH}_{reactants}$ Inserting the respective enthalpy values:

• For products: $4 imes ext{ΔH for } CO + 12 imes ext{ΔH for } H_2O + 9 imes ext{ΔH for } N_2$.
• For reactants: $4 imes ext{ΔH for } CH₄NH₃ + 5 imes ext{ΔH for } N_2O₄$. Calculating yields approximately -4632 kJ.

The full structural formula for methylhydrazine, CH₄N₂H₈, is as follows:

  H  H
   |  |
H--N--N--H
   |  |
   H  H

This depicts two nitrogen atoms bonded together, each bonded to two hydrogen atoms and one carbon atom.