Chem8

During a chemical reaction:

• Energy must be supplied to break bonds
• Energy is released when bonds are formed

These changes can be represented on an  energy level diagram

In an exothermic reaction the energy released from forming new bonds is greater than the energy needed to break existing bonds

In an endothermic reaction the energy needed to break existing bonds is greater than the energy released from forming new bonds

The activation energy is the energy required for a reaction to occur. A catalyst lowers the activation energy.

• You can calculate the energy transferred using energy level diagrams or the following equation when supplied with bond energies:
• The energy transferred = energy taken in during bond breaking + energy released during bond forming
• The energy released must always be negative

E.g. the combustion of methane

Bond energies:

•  C-H: 413
•  O=O: 498
•  C=O: 805
•  H-O: 464

Balanced symbol equation: CH4 + 2O2 -> 2H2O + CO2

Energy change for breaking bonds in the reactants = (413 x 4) + (498 x 2) = 2648

Energy change for making bonds in the products = (464 x 4) + (805 x 2) = -3466

Energy transferred = 2648 + 3466 = -818 kJ/mol