6.8 Describing a Reaction: Bond Dissociation Energies

Bond dissociation energies are key to understanding reaction energetics. They tell us how much energy it takes to break specific bonds, helping predict if reactions will release or absorb energy overall.

High-energy compounds in our bodies, like ATP, have weak bonds that break easily to power cellular processes. This connects to how the strength of chemical bonds influences energy storage and release in living systems.

Bond Dissociation Energies and Reaction Energetics

Calculation of bond dissociation energy

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• Bond dissociation energy (BDE) quantifies energy required to break a specific bond in a molecule
  • Expressed in units of kJ/mol or kcal/mol
  • Each bond type has a characteristic BDE value
    • C-H bond BDE approximately 413 kJ/mol
• Calculate energy required to break a bond using the specific bond's BDE value
  • Breaking a C-H bond requires 413 kJ/mol of energy input
• When breaking multiple bonds, sum the BDE values for each bond to determine total energy required
  • Breaking two C-H bonds requires 2 × 413 kJ/mol = 826 kJ/mol of energy

Bond strengths and reaction thermodynamics

• Exothermic reactions release energy as they proceed
  • Product bonds are stronger than reactant bonds
  • Difference in bond strengths results in net energy release
• Endothermic reactions absorb energy as they proceed
  • Reactant bonds are stronger than product bonds
  • Difference in bond strengths results in net energy absorption
• Overall reaction energy change depends on balance between energy required to break bonds and energy released when new bonds form
  • If more energy is released than absorbed, reaction is exothermic (negative $\Delta H$)
  • If more energy is absorbed than released, reaction is endothermic (positive $\Delta H$)
• Bond strength influences the enthalpy change of a reaction

High-energy compounds in biochemistry

• High-energy compounds have relatively weak bonds easily broken to release energy
  • ATP (adenosine triphosphate) and glucose
• Energy released from breaking weak bonds drives other cellular processes
  • ATP hydrolysis ($ATP + H_2O \rightarrow ADP + P_i$) releases energy for biochemical reactions
• Compounds with stronger bonds are more stable and less likely to release energy under physiological conditions
  • Amino acids and proteins have strong peptide bonds, making them stable and less reactive
• Relative bond strengths of biomolecules determine reactivity and energy storage capacity in living systems
  • Carbohydrates (glucose) have weaker bonds than proteins, allowing for easier energy release
  • Lipids (fatty acids) have strong C-C and C-H bonds, serving as efficient energy storage molecules

Thermochemistry and Reaction Kinetics

• Thermochemistry studies heat changes in chemical reactions
• Bond dissociation energies are crucial in determining reaction enthalpy
• Activation energy represents the minimum energy required for a reaction to occur
• Bond strength influences the activation energy needed to initiate a reaction