Bond Breaking and Forming, Bond Enthalpy, and Calculating Enthalpy Change ($ΔH$)
Bond Breaking and Bond Forming: The Energy Perspective
Breaking Bonds: An Endothermic Process
- Breaking a chemical bond requires energy.
- Why? Because bonds represent stable arrangements of atoms, and separating them disrupts this stability.
- Since energy is absorbed, bond breaking is an endothermic process, and the enthalpy change is positive ($+ΔH$).
Definition
Bond enthalpy
The energy required to break one mole of a specific bond in the gaseous state is called its bond enthalpy (or bond dissociation energy)
Breaking a hydrogen molecule (H₂) into two hydrogen atoms can be represented as:
$$\text{H}_2(g) \to 2\text{H}(g) \quad \Delta H = +436 \, \text{kJ mol}^{-1}$$
Forming Bonds: An Exothermic Process
- In contrast, forming a new chemical bond releases energy.
- This occurs because atoms achieve a more stable, lower-energy arrangement when bonded.
- As a result, bond formation is an exothermic process, and the enthalpy change is negative ($−ΔH$).
When two hydrogen atoms combine to form a hydrogen molecule:
$$2\text{H}(g) \to \text{H}_2(g) \quad \Delta H = -436 \, \text{kJ mol}^{-1}$$
The Balance of Bond Breaking and Forming
- In any chemical reaction, energy is absorbed to break bonds in the reactants and released when new bonds form in the products.
- The overall energy change of the reaction depends on the balance between these two processes:
- If more energy is released in bond formation than is absorbed in bond breaking, the reaction is exothermic ($ΔH< 0$).
- If more energy is absorbed in bond breaking than is released in bond formation, the reaction is endothermic ($ΔH >0$).
To determine whether a reaction is exothermic or endothermic, compare the total energy of the bonds broken with the total energy of the bonds formed.
Bond Enthalpy: A Measure of Bond Strength
Average Bond Enthalpy
- In reality, the energy required to break a particular bond can vary depending on the molecular environment.
- To account for these variations, bond enthalpies are often reported as average bond enthalpies.
