Bond enthalpies

Understanding Bond Enthalpies

• Bond enthalpy refers to the amount of energy required to break apart one mole of a particular type of bond in each molecule of a gaseous substance.
• Energy is needed to break bonds, hence this process is an endothermic one, where energy is absorbed.
• When a bond forms, conversely, energy is released, leading to an exothermic process.

Average Bond Enthalpies

• Bond enthalpy is often given as an average value, derived from measuring the energy needed to break the bond in various molecules.
• Energy required to break a specific bond can vary depending on the molecule in which it is present.
• The bond enthalpy value represents the energy required to break the bond on average across a selection of compounds.

Calculating Energy Changes Using Bond Enthalpies

• You can estimate the enthalpy change in a chemical reaction using average bond enthalpies.
• Remember the formula: ∆H = Energy required to break bonds - Energy released when new bonds are formed. This can be usually represented as ∆H = Bond breaking - Bond making.
• The units used for bond enthalpies are Kilojoules per mole (KJ mol-1).
• The larger the bond enthalpy, the stronger the bond. Stronger bonds are harder to break, and thus require greater amounts of energy.

Limitations of Using Average Bond Enthalpies

• Calculating enthalpy changes this way leads to an approximation rather than a precise value.
• Bond enthalpy values are averaged across a range of compounds, not specific to a particular reaction or molecule.
• These values tend to be suitable for gas-phase reactions, but they should be handled with caution when using for reactions in solution or solid-state reactions.