Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 6.11 Bond enthalpy and reaction insights-Study Notes - New Syllabus

6.11 Bond Enthalpy and Reactivity

Bond enthalpy data provides insight into which bonds are most likely to break during a reaction and therefore helps predict reactivity and reaction rate.

Key Idea

• Bond enthalpy = energy required to break a bond.
• Lower bond enthalpy → bond breaks more easily.
• Higher bond enthalpy → bond is stronger and harder to break.

Which Bond Breaks First?

• The bond with the lowest bond enthalpy is most likely to break first.
• This is because it requires less energy.

Example

• C–Cl bond (~338 kJ mol\(^{-1}\)) is weaker than C–H (~413 kJ mol\(^{-1}\)).
• Therefore, C–Cl bond breaks more easily.

Effect on Reaction Rate

• Reactions involving weaker bonds occur more easily.
• Lower energy required → lower activation energy.
• Faster reactions at room temperature.

Difficulty of Reaction

• Strong bonds (high bond enthalpy) require more energy to break.
• Reactions are slower or may require heating/catalyst.

Link to Activation Energy

• Bond breaking contributes to activation energy.
• Stronger bonds → higher activation energy → slower reaction.

Important Limitation

• Bond enthalpy alone does not fully determine rate.
• Other factors:
• Activation energy pathway
• Reaction mechanism
• Orientation of molecules

Therefore, bond enthalpy data is useful for predicting reactivity but must be considered alongside other factors.