Mastering Bond Enthalpy Calculations

What is Bond Enthalpy?

Bond enthalpy, also known as bond dissociation energy, is the amount of energy required to break one mole of a particular bond in a molecule in the gas phase. It is a crucial concept in thermochemistry because it helps predict the energy changes that occur during chemical reactions.

Why is Bond Enthalpy Important?

• Predicting Reaction Feasibility: By comparing the bond enthalpies of reactants and products, chemists can determine if a reaction is likely to occur.
• Calculating Enthalpy Changes: Bond enthalpy values allow for the calculation of the overall enthalpy change (ΔH) for a reaction, aiding in thermodynamic assessments.

Calculating Enthalpy Changes Using Bond Enthalpies

To calculate the enthalpy change for a reaction, you can use the following formula:

Formula:

ΔH = Σ (Bond Enthalpies of Bonds Broken) - Σ (Bond Enthalpies of Bonds Formed)

Example 1: Combustion of Ethanol

Let’s consider the combustion of ethanol (C₂H₅OH):

$$
C_2H_5OH + 3 O_2 → 2 CO_2 + 3 H_2O
$$

Step 1: Identify Bonds Broken and Formed

• Bonds Broken:

  • 1 C–C bond (C₂H₅OH)
  • 5 C–H bonds (C₂H₅OH)
  • 3 O=O bonds (O₂)

• Bonds Formed:

  • 4 C=O bonds (2 CO₂)
  • 6 O–H bonds (3 H₂O)

Step 2: Look Up Bond Enthalpy Values

• C–C: 348 kJ/mol
• C–H: 412 kJ/mol
• O=O: 498 kJ/mol
• C=O: 799 kJ/mol
• O–H: 463 kJ/mol

Step 3: Calculate the Total Bond Enthalpy

Bonds Broken:

• C–C: 1 x 348 = 348 kJ
• C–H: 5 x 412 = 2060 kJ
• O=O: 3 x 498 = 1494 kJ

Total Bonds Broken = 348 + 2060 + 1494 = 3902 kJ

Bonds Formed:

• C=O: 4 x 799 = 3196 kJ
• O–H: 6 x 463 = 2778 kJ

Total Bonds Formed = 3196 + 2778 = 5974 kJ

Step 4: Calculate ΔH

$$
ΔH = 3902 ext{ kJ} - 5974 ext{ kJ} = -2072 ext{ kJ}
$$

Conclusion: The negative sign indicates that the reaction is exothermic, meaning it releases energy.

Example 2: Formation of Ammonia

Consider the formation of ammonia (NH₃) from nitrogen and hydrogen:

$$
N_2 + 3 H_2 → 2 NH_3
$$

Step 1: Identify Bonds Broken and Formed

• Bonds Broken:

  • 1 N≡N bond (N₂)
  • 6 H–H bonds (H₂)

• Bonds Formed:

  • 6 N–H bonds (2 NH₃)

Step 2: Look Up Bond Enthalpy Values

• N≡N: 945 kJ/mol
• H–H: 436 kJ/mol
• N–H: 391 kJ/mol

Step 3: Calculate the Total Bond Enthalpy

Bonds Broken:

• N≡N: 1 x 945 = 945 kJ
• H–H: 6 x 436 = 2616 kJ

Total Bonds Broken = 945 + 2616 = 3561 kJ

Bonds Formed:

• N–H: 6 x 391 = 2346 kJ

Total Bonds Formed = 2346 kJ

Step 4: Calculate ΔH

$$
ΔH = 3561 ext{ kJ} - 2346 ext{ kJ} = 1215 ext{ kJ}
$$

Conclusion: The positive value indicates that the reaction is endothermic, meaning it absorbs energy.

Summary

Bond enthalpy calculations are vital for understanding the energy dynamics of chemical reactions. By applying the formula for ΔH and using bond enthalpy values, you can predict whether reactions will be exothermic or endothermic. These calculations not only enhance comprehension but also provide insights into reaction mechanisms and efficiencies.