Bond Energies

Bond Energies

  • Energy is essential for reactions.

    • In order for a chemical reaction to occur, energy is needed. The minimum amount of energy, the activation energy, is supplied to break the reactant bonds.

  • Bonds are broken in reactants.

    • In order for particles to react, existing bonds must be broken to release the atoms, so they are free to make new bonds.

  • Bond breaking is endothermic.

    • When bonds are broken, this process is endothermic. Energy is taken in from the surroundings in order to break old bonds.

  • Bonds are formed in products.

    • When a reaction has taken place, bonds are formed between atoms to create products. Energy is  released when new bonds are made.

  • Bond formation is exothermic.

    • When new bonds are formed, this process is exothermic. Energy is released to the surroundings through the formation of new bonds.

Calculating Overall Energy Change in a Reaction

  • Bonds have specific energies.

    • Every chemical bond has it’s own ‘energy’.
    • This is the amount of energy required to break the bond, which is the same as the amount of energy released to form the bond.

  • Reactions have energy changes.

    • Since bonds are broken and formed during a reaction, we can calculate the overall energy change of a reaction.
    • To do this, we need to know the total bond energies of the reactants and also the total bond energies of the products.
    • We can then use a simple equation to calculate the overall energy change of the reaction.

Exothermic Reactions

  • Exothermic reactions have a negative energy value.
    • If the overall energy change of a reaction is negative when a reaction is complete, then the reaction is exothermic.
    • This is due to a low amount of energy being used to break bonds in the reactants, and a high amount of energy being released via formation of bonds in the products.

Endothermic Reactions

  • Endothermic reactions have a positive energy value.
    • If the overall energy change is positive when a reaction is complete, then the reaction is endothermic.
    • This is due to a high amount of energy being used to break bonds in the reactants, and a low amount of energy being released via formation of bonds in the products.