bond enthalpy

• What do chemical reactions involve?

  Breaking and forming of bonds
• What results from breaking and forming bonds in chemical reactions?

  Enthalpy changes
• What is the overall enthalpy change (ΔH) of a reaction?

  Combined effect of bond breaking and forming
• What type of process is breaking bonds in reactants?

  Endothermic process requiring energy
• What happens when stronger bonds are broken?

  More energy is required
• What type of process is forming bonds in products?

  Exothermic process releasing energy
• What does a positive ΔH indicate about a reaction?

  Bond breaking requires more energy
• What does a negative ΔH indicate about a reaction?

  Bond breaking requires less energy
• What is mean bond enthalpy?

  Energy to break one mole of a bond
• Why do mean bond enthalpy values vary?

  Based on the bond's molecular environment
• What is true about bond enthalpy values?

  They are always positive
• What do mean bond enthalpy values represent?

  Average energy needed to break a bond
• How is the enthalpy change of a complete reaction (ΔHr) determined?

  Using average bond enthalpies of bonds broken and formed
• What is the formula for calculating ΔHr?

  ΔHr = Σ(bonds broken) - Σ(bonds formed)
• Why are Hess cycles more accurate for calculating ΔHr?

  They consider actual enthalpy changes
• What is the balanced equation for the combustion of methane?

  CH4 + 2O2 ➔ CO2 + 2H2O
• What is the mean bond enthalpy for C-H bonds?

  +414 kJ mol-1
• How many C-H bonds are broken in methane combustion?

  4 C-H bonds
• How many O=O bonds are broken in methane combustion?

  2 O=O bonds
• What is the total energy absorbed in breaking bonds during methane combustion?

  2,652 kJ mol-1
• What is the total energy released in forming bonds during methane combustion?

  3,454 kJ mol-1
• What is ΔH for the combustion of methane?

  802 kJ mol-1
• What is the balanced equation for the synthesis of ammonia?

  N2 + 3H2 ➔ 2NH3
• What is the mean bond enthalpy for N≡N bonds?

  +945 kJ mol-1
• How many N≡N bonds are broken in ammonia synthesis?

  1 N≡N bond
• How many H-H bonds are broken in ammonia synthesis?

  3 H-H bonds
• What is the total energy absorbed in breaking bonds during ammonia synthesis?

  2,253 kJ mol-1
• What is the total energy released in forming bonds during ammonia synthesis?

  2,346 kJ mol-1
• What is ΔH for the synthesis of ammonia?

  93 kJ mol-1
• What are the steps to calculate ΔH for a reaction?

  1. Identify and count bonds broken and formed
  2. Calculate total energy absorbed in breaking bonds
  3. Calculate total energy released in forming bonds
  4. Use ΔH = Σ(bonds broken) - Σ(bonds formed)
• What are the differences between using mean bond enthalpies and Hess cycles for calculating ΔHr?

  • Mean bond enthalpies provide average values
  • Hess cycles consider specific enthalpy changes
  • Hess cycles yield more accurate ΔHr values