Enthalpy Change in Bond Breaking and Bond Making | Chemistry for Grade 11 (IGCSE) PDF Download

Bond Breaking & Bond Forming

• Whether a reaction is endothermic or exothermic depends on the energy required to break existing bonds and the energy released during new bond formation.
• Bond breaking involves absorbing energy from the surroundings to break chemical bonds, making it an endothermic process.
• For example, breaking the bonds in water (H₂O) to form hydrogen (H₂) and oxygen (O₂) requires energy input.
• On the other hand, bond making releases energy to the surroundings during bond formation, classifying it as an exothermic process.
• When hydrogen (H₂) and oxygen (O₂) combine to form water (H₂O), energy is released.

Exothermic Reactions

• If a reaction releases more energy than it absorbs, it is termed exothermic.
• Exothermic reactions involve more energy being released when new bonds are formed compared to the energy required to break the bonds in the reactants.
• The change in energy is negative since the products possess less energy than the reactants.
• Consequently, an exothermic reaction is characterized by a negative ΔH value.
• Making new chemical bonds releases energy, which is emitted as heat into the surroundings:
  

Endothermic Reactions

• If a reaction absorbs more energy to break bonds than it releases to form new bonds, it is termed endothermic.
• Endothermic reactions involve the products containing more energy than the reactants.
• The symbol ΔH (delta H) represents the change in heat energy, where H signifies enthalpy, measuring the total heat of a chemical reaction.
• Therefore, an endothermic reaction is identified by a positive ΔH value, evident in energy level diagrams and calculations.
• Breaking chemical bonds requires energy which is taken in from the surroundings in the form of heat:
  

Bond Energy Calculations

Energy of reaction calculations

• Every chemical bond possesses a distinct bond energy.
• This energy represents the amount needed to break the bond or released upon its formation.
• Calculating heat release or absorption in a reaction involves utilizing these bond energies.
• Knowledge of the bonds in both reactants and products is essential for this calculation.

Method

• If an equation is not provided, create a balanced one.
• Optionally, illustrate the displayed formula to discern bond type and quantity.
• Sum the bond energies across all bonds in the reactants, representing the "energy in" component.
• Sum the bond energies across all bonds in the products, representing the "energy out" component.
• Calculate the enthalpy change using the formula: ΔH = Energy absorbed - Energy released.