Question

Q10. Hydrocarbon molecules can react with halogens like Cl2 or Br2 (represented generically by X2) where C-H bonds are replaced by C-X bonds, forming HX molecules in the process. Balance the following chemical reaction and compute the estimated heat of reaction using the values above.

C3H8(g) + X2(g) → C3H2X6(g) + HX (g)
You are given the following average bond energies:
H-H 436 kJ/mol
H-X 431 kJ/mol
H-C 414 kJ/mol
C-X 339 kJ/mol
X-X 243 kJ/mol

Answer 1

To balance the chemical reaction, you use stoichiometry to equate the number of atoms on both the reactant and product sides, resulting in C3H8(g) + 6X2(g) → C3H2X6(g) + 6HX(g). Using bond energies, the total energy consumed is 4380 kJ/mol, and the total energy released is 4620 kJ/mol, leading to an exothermic heat of reaction of 240 kJ/mol.

Step-by-step explanation:

Balancing the Chemical Reaction

To balance the given chemical reaction, we will first write the unbalanced equation:

C3H8(g) + X2(g) → C3H2X6(g) + HX(g)

By inspection, we can see that 6 moles of X2 are needed to replace the 6 hydrogen atoms in propane to form C3H2X6, and 6 moles of HX are produced. Therefore, the balanced chemical equation is:

C3H8(g) + 6X2(g) → C3H2X6(g) + 6HX(g)

Computing the Estimated Heat of Reaction

To estimate the heat of the reaction (ΔH), we consider the bond energies:

Forming of: 6 C-X bonds (6 × 339 kJ/mol) and 6 H-X bonds (6 × 431 kJ/mol)

The total energy consumed is:

8 × 414 kJ/mol + 6 × 243 kJ/mol = 4380 kJ/mol

The total energy released is:

6 × 339 kJ/mol + 6 × 431 kJ/mol = 4620 kJ/mol

Therefore, the estimated ΔH for the reaction is:

ΔH = Energy released - Energy consumed = 4620 kJ/mol - 4380 kJ/mol = 240 kJ/mol

This ΔH indicates the reaction is exothermic, as energy is being released.

Answer 2

Answer: To balance the given chemical equation, we can start by counting the number of atoms on both sides of the equation. We have 3 carbon atoms and 8 hydrogen atoms on the left side, and 3 carbon atoms, 6 X atoms, and 1 hydrogen atom on the right side.

C3H8(g) + X2(g) → C3H2X6(g) + HX(g)

To balance the equation, we can add a coefficient of 3 in front of HX on the product side:

C3H8(g) + X2(g) → C3H2X6(g) + 3HX(g)

Now, we have the same number of H atoms on both sides (8 H atoms on each side), and the equation is balanced.

To estimate the heat of reaction, we can use the bond energy values to calculate the energy required to break the bonds in the reactants and the energy released by forming the bonds in the products. We can then subtract the energy required to break the bonds from the energy released by forming the bonds to obtain an estimate of the heat of reaction.

Breaking bonds in the reactants:

3 C-H bonds × 414 kJ/mol = 1242 kJ/mol

1 X-X bond × 243 kJ/mol = 243 kJ/mol

Forming bonds in the products:

6 C-X bonds × 339 kJ/mol = 2034 kJ/mol

1 C-H bond × 414 kJ/mol = 414 kJ/mol

3 H-X bonds × 431 kJ/mol = 1293 kJ/mol

Estimated heat of reaction:

Energy released - energy required

(2034 kJ/mol + 414 kJ/mol + 1293 kJ/mol) - (1242 kJ/mol + 243 kJ/mol) = 2756 kJ/mol

Therefore, the estimated heat of reaction for the given chemical equation is 2756 kJ/mol. Note that this is only an estimate and actual experimental values may differ due to factors such as reaction conditions and the presence of catalysts.