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) per mole using bond enthalpies and an energy diagram, we will follow these steps:
Step 1: Draw the Lewis structure of acetylene (C2H2) to determine its bond connections. Acetylene consists of two carbon atoms bonded together by a triple bond and each carbon atom is bonded to one hydrogen atom.
H H
\ /
C≡C
( H-C≡C-H )
Step 2: Determine the bond enthalpies for the bonds present in acetylene. Bond enthalpy is the energy required to break 1 mole of a particular bond. The bond enthalpies for the bonds in acetylene are as follows:
C≡C bond enthalpy = 837 kJ/mol
C-H bond enthalpy = 413 kJ/mol
Step 3: Write and balance the combustion reaction of acetylene. The balanced equation for the combustion of acetylene is:
2 C2H2 + 5 O2 -> 4 CO2 + 2 H2O
Step 4: Calculate the enthalpy change of the reaction using bond enthalpies. The enthalpy change of the reaction is equal to the sum of the energy required to break the bonds in the reactants minus the energy released when forming the bonds in the products. We can use the following equation:
ΔH = Σ(ΔH(bonds broken)) - Σ(ΔH(bonds formed))
For acetylene combustion, the bonds broken are:
4 C≡C bonds = 4 * 837 kJ/mol = 3348 kJ/mol
4 C-H bonds = 4 * 413 kJ/mol = 1652 kJ/mol
The bonds formed are:
8 C=O bonds (in CO2) = 8 * 799 kJ/mol = 6392 kJ/mol
2 O-H bonds (in H2O) = 2 * 463 kJ/mol = 926 kJ/mol
ΔH = (3348 kJ/mol + 1652 kJ/mol) - (6392 kJ/mol + 926 kJ/mol)
= -2118 kJ/mol
Therefore, the enthalpy of combustion of acetylene per mole is -2118 kJ/mol. The negative sign indicates that the combustion reaction is exothermic, meaning it releases energy.