Question

Acetylene gas (ethyne; HC≡CH) burns with oxygen in an oxyacetylene torch to produce carbon dioxide, water vapor, and the heat needed to weld metals. The heat of combustion for acetylene is −1259 kJ/mol. Calculate the C≡C bond energy._________ kJ/mol.

Answer 1

Answer : The bond energy of $C\equiv C$ is 800 kJ/mol.

Explanation :

The given chemical reaction is:

$2C_2H_2+5O_2\rightarrow 4CO_2+2H_2O$

For 1 mole, the reaction will be:

$C_2H_2+2.5O_2\rightarrow 2CO_2+H_2O$

As we know that:

The enthalpy change of reaction = E(bonds broken) - E(bonds formed)

$\Delta H=[(B.E_{C\equiv C})+(2\times B.E_{C-H})+(2.5\times B.E_{O=O})]-[(2\times B.E_{C=O})+(2\times B.E_{H-O})]$

Given:

$\Delta H$ = heat of combustion = -1259 kJ/mol

$B.E_{C-H}$ = 413 kJ/mol

$B.E_{O=O}$ = 498 kJ/mol

$B.E_{O-H}$ = 467 kJ/mol

$B.E_{C=O}$ = 799 kJ/mol

Now put all the given values in the above expression, we get:

$-1259kJ/mol=[(B.E_{C\equiv C})+(2\times 413kJ/mol)+(2.5\times 498kJ/mol)]-[(4\times 799kJ/mol)+(2\times 467kJ/mol)]$

$B.E_{C\equiv C}=800kJ/mol$

Therefore, the bond energy of $C\equiv C$ is 800 kJ/mol.