Worksheet: Hess Cycles Using Bond Energies

In this worksheet, we will practice calculating reaction enthalpies by comparing the standard bond enthalpies of the reactants and the products.

Q1:

What is the approximate enthalpy change for the reaction shown?

H ( ) + B r ( ) 2 H B r ( ) 2 2 g g g

Use the bond enthalpies from the table.

Bond B r B r H B r H H
Bond Enthalpy (kJ/mol) 190 370 436

Q2:

What is the approximate enthalpy change for the reaction shown? CH()+3O()2CO()+2HO()24222gggg Use the bond enthalpies from the table.

Bond C H C C C O H O O O
Bond Enthalpy (kJ/mol) 415 611 741 464 498

Q3:

What is the approximate enthalpy change for the reaction shown? CH()+I()CHI()+HI()423gggg Use the bond enthalpies from the table.

Bond C H C I H I I I
Bond Enthalpy (kJ/mol) 415 240 295 150

Q4:

When a molecule can form two different structures, the structure with the stronger bonds is usually the more stable form. Using the bond energy values listed in the table, identify the correct structure of the hydroxylamine molecule.

Bond H N H O N O
Bond Enthalpy (kJ/mol) 390 464 200
  • A
    N O H H H
  • B
    N O H H H

Q5:

Given that the energy of a CHH3 bond is 104 kcal/mol and that of an HOH bond is 119 kcal/mol, what is the reaction enthalpy for the following reaction? HOCHHOCH++423

Q6:

The energies of selected bonds are shown in the table.

Bond C H C C l H C l C l C l
Bond Energy (kcal/mol) 104 85 103 58

The chlorination of methane is described by the equation shown. CH+ClCHCl+HCl423 Using the values in the table, calculate the energy change in this reaction per mole of methane reacted.

Q7:

Hydrogen fluoride gas is formed by the reaction of hydrogen with fluorine, as shown. H()+F()2HF()22ggg

The standard enthalpy change for this reaction is 535 kJ/mol. The molar bond energies of H2 and F2 are 436 kJ/mol and 159 kJ/mol respectively. Calculate the energy of one HF bond.

  • A 1 . 3 8 × 1 0 J
  • B 6 . 7 4 × 1 0 J
  • C 8 . 0 6 × 1 0 J
  • D 9 . 3 8 × 1 0 J
  • E 1 . 4 3 × 1 0 J

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