Bond Enthalpies- Example

Bond Enthalpies- Example

Problem

Calculate the enthalpy change when hydrogen is added to ethyne to produce ethane using bond enthalpies.

Solution

To find the enthalpy change, we need to consider the bonds broken and formed during the reaction.

Step 1: Molecular Structures

• Write out the structural formulas for all molecules involved in the reaction to identify the bonds.

Reactants:

• Ethyne (C$_2$H$_2$): C≡C bond
• Hydrogen (H$_2$): H-H bonds (x2)

Product:

• Ethane (C$_2$H$_6$): C-C bonds (x2) and H-H bonds (x6)

Step 2: Bond Breaking

• List the bonds broken in the reactants and their respective bond enthalpies from the data book.

Reactants Bond Breaking:

• C≡C bond: 837 kJ/mol (x1)
• H-H bonds (from H2): 436 kJ/mol (x2)

Step 3: Bond Making

• List the bonds formed in the products and their respective bond enthalpies.

Product Bond Making:

• C-C bonds (from C$_2$H$_6$): 348 kJ/mol (x2)
• H-H bonds (from H$_2$): 436 kJ/mol (x6)

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Bond Enthalpies - Example

Step 4: Calculating Enthalpy Change

• Calculate the total energy change by subtracting the energy required to break bonds from the energy released when bonds are formed.

Enthalpy Change (ΔH):

• ΔH = (Energy for bond breaking) - (Energy for bond making)
• ΔH = [(837 kJ/mol) + (436 kJ/mol x 2)] - [(348 kJ/mol x 2) + (436 kJ/mol x 6)]
• ΔH = [837 + 872] - [696 + 2616]
• ΔH = 1709 - 3312
• ΔH = -1603 kJ/mol

Conclusion:

The enthalpy change for the reaction in which hydrogen is added to ethyne to produce ethane is -1603 kJ/mol. This negative value indicates that the reaction is exothermic, meaning it releases energy.