energy change of reactions

• in a chemical reaction, new substances are produced:

  • the bonds in the reactants are broken
  • new bonds are made to form the products.
• breaking a chemical bond requires energy - it's an endothermic process.
• when a new chemical bond is formed, energy is given out - it's an exothermic process.
• if more energy is required to break bonds than is released when bonds are formed, the reaction must be endothermic. If more energy is released when bonds are formed than is needed to break bonds, the reaction must be exothermic.