Chemical Bond Energy Worksheet

Learning Objectives

By the end of this worksheet, you will be able to: * Calculate total bond energy in molecules * Interpret energy diagrams of chemical reactions * Predict energy changes in reactions based on bond strengths * Compare molecular stability using bond energy concepts

Part 1: Total Bond Energy Calculations

Key Concept

The total bond energy of a molecule is the sum of all individual bond energies within that molecule.

$$ E_{total} = \sum E_{bonds} $$

Example

For water (H₂O), we have two O-H bonds: * O-H bond energy = 463 kJ/mol * Total bond energy = 2 × 463 kJ/mol = 926 kJ/mol

Practice Problems

1. Calculate the total bond energy for methane (CH₄) using the following bond energy:

   • C-H bond = 413 kJ/mol

2. Calculate the total bond energy for carbon dioxide (CO₂) using:

   • C=O bond = 799 kJ/mol

3. Propane (C₃H₈) contains C-C and C-H bonds. Calculate its total bond energy using:

   • C-C bond = 348 kJ/mol
   • C-H bond = 413 kJ/mol

Part 2: Energy Diagrams for Chemical Reactions

Key Concept

Energy diagrams show the energy changes during a chemical reaction, including: * Energy of reactants * Energy of products * Activation energy (if applicable) * Overall energy change (ΔH)

Study the diagram below:

Energy |
       |                  ↑
       |                 / \  Activation
       |                /   \  Energy
       |               /     \
       |              /       \
       | Reactants →  •        \
       |                        \
       |                         \ → Products
       |                          •
       |                   Energy released (ΔH < 0)
       |
       |-----------------------------------→ Reaction Progress

Practice Problems

4. Describe what each of these features represents in a reaction: a) The height difference between reactants and products b) The peak of the curve c) An upward slope from reactants to the peak

5. Draw a simple energy diagram for an endothermic reaction where the products have higher energy than the reactants.

Part 3: Predicting Energy Changes from Bond Strengths

Key Concept

When predicting whether a reaction releases or absorbs energy: * Energy is required to break bonds (endothermic) * Energy is released when bonds form (exothermic) * Net energy change: ΔH = Energy needed to break bonds - Energy released when bonds form

$$ \Delta H = \sum E_{bonds\ broken} - \sum E_{bonds\ formed} $$

• If ΔH < 0: Exothermic reaction (releases energy)
• If ΔH > 0: Endothermic reaction (absorbs energy)

Example

For the reaction: H₂ + Cl₂ → 2HCl * Bonds broken: H-H (436 kJ/mol) and Cl-Cl (242 kJ/mol) * Bonds formed: 2 H-Cl (431 kJ/mol each) * ΔH = (436 + 242) - (2 × 431) = 678 - 862 = -184 kJ/mol * This is exothermic (releases energy)

Practice Problems

6. For the reaction CH₄ + 2O₂ → CO₂ + 2H₂O, determine whether it's exothermic or endothermic using these bond energies:

   • C-H bond = 413 kJ/mol
   • O=O bond = 498 kJ/mol
   • C=O bond = 799 kJ/mol
   • O-H bond = 463 kJ/mol

7. Predict whether this reaction will release or absorb energy: N₂ + 3H₂ → 2NH₃

   • N≡N bond = 941 kJ/mol
   • H-H bond = 436 kJ/mol
   • N-H bond = 391 kJ/mol

Part 4: Comparing Molecular Stability

Key Concept

• Molecules with stronger/higher total bond energy are generally more stable
• Lower energy states are more stable than higher energy states
• Molecules tend to react to form more stable products

Practice Problems

8. Compare the stability of ethane (C₂H₆) and ethene (C₂H₄) using these bond energies:

   • C-C bond = 348 kJ/mol
   • C=C bond = 614 kJ/mol
   • C-H bond = 413 kJ/mol

9. Rank these molecules in order of increasing stability based on total bond energy:

   • H₂ (H-H bond = 436 kJ/mol)
   • O₂ (O=O bond = 498 kJ/mol)
   • N₂ (N≡N bond = 941 kJ/mol)

10. Explain why diamond (a network of C-C bonds) is more stable than graphite (sheets of carbon with alternating single and double bonds) at standard conditions.

Reflection Questions

11. Why does breaking a chemical bond always require energy?

12. When a reaction is highly exothermic, what can you infer about the relative bond strengths of the reactants versus the products?

13. How might the concept of bond energy help explain why some compounds are more reactive than others?

Challenge Problem

14. Ozone (O₃) in the upper atmosphere absorbs harmful UV radiation and breaks down into O₂ and O. Given that the O-O single bond energy is 142 kJ/mol and the O=O double bond energy is 498 kJ/mol, calculate the energy change for this reaction: O₃ → O₂ + O
    • Hint: Ozone has a resonance structure with one double bond and one single bond between oxygen atoms.