Calculate the enthalpy of combustion of acetylene(per mol acetylene)using bond enthalpies and energy diagram

To calculate the enthalpy of combustion of acetylene (C2H2) using bond enthalpies and an energy diagram, you will need the following information:

1. Bond enthalpy values for the bonds present in acetylene:
C≡C bond enthalpy: 835 kJ/mol
C-H bond enthalpy: 414 kJ/mol

2. The balanced chemical equation for the combustion of acetylene:
C2H2 + 5/2 O2 -> 2 CO2 + H2O

Step 1: Calculate the total bond enthalpies of the reactants.
C≡C + 2 × C-H
= 1 × 835 kJ/mol + 2 × 414 kJ/mol
= 1668 kJ/mol + 828 kJ/mol
= 2496 kJ/mol

Step 2: Calculate the total bond enthalpies of the products.
2 × C=O + 2 × O-H
= 2 × 805 kJ/mol + 2 × 464 kJ/mol
= 1610 kJ/mol + 928 kJ/mol
= 2538 kJ/mol

Step 3: Calculate the change in enthalpy (ΔH) for the reaction.
ΔH = Total bond enthalpies of reactants - Total bond enthalpies of products
= 2496 kJ/mol - 2538 kJ/mol
= -42 kJ/mol

Therefore, the enthalpy of combustion of acetylene per mole of acetylene is -42 kJ/mol. Since the value is negative, it indicates that the combustion reaction is exothermic, releasing energy.

To calculate the enthalpy of combustion of acetylene using bond enthalpies, we need to follow these steps:

Step 1: Write the balanced chemical equation for the combustion of acetylene.
The chemical equation for the combustion of acetylene (C₂H₂) is:

C₂H₂ + 5/2 O₂ → 2CO₂ + H₂O

Step 2: Determine the bonds broken and formed in the reaction.
In the combustion reaction, we need to break the bonds in acetylene and oxygen and form the bonds in carbon dioxide and water. The bond energies for these bonds can be found in reference sources.

The bonds broken in acetylene (C₂H₂) are:
- C≡C: 837 kJ/mol
- H─H: 436 kJ/mol

The bonds formed in carbon dioxide (CO₂) are:
- C=O: 799 kJ/mol (twice, since two carbon dioxide molecules are formed)

The bond formed in water (H₂O) is:
- O─H: 463 kJ/mol (twice, since two water molecules are formed)

Step 3: Calculate the total energy change.
The total energy change (enthalpy of combustion, ΔH) is determined by the difference between the energy required to break the bonds and the energy released by forming the new bonds.

ΔH = (bonds broken) - (bonds formed)

ΔH = [2(C≡C) + 5/2(H─H)] - [4(C=O) + 2(O─H)]

ΔH = [2(837 kJ/mol) + 5/2(436 kJ/mol)] - [4(799 kJ/mol) + 2(463 kJ/mol)]

ΔH = [1674 kJ/mol + 1090 kJ/mol] - [3196 kJ/mol + 926 kJ/mol]

ΔH = 2764 kJ/mol - 4122 kJ/mol

ΔH = -1358 kJ/mol

Therefore, the enthalpy of combustion of acetylene is -1358 kJ/mol. The negative sign indicates that the combustion of acetylene is an exothermic reaction, releasing energy.