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Energy Changes in Chemical Reactions
Breaking and forming bonds
When methane, CH4 burns in oxygen gas, O2, bonds must first be broken in both molecules before new bonds forming the products can be made.
Energy is measured in kilojoules or kJ.
When bonds break, energy must be absorbed from their surroundings. Taking in energy reduces the temperature of the surroundings - this is called an endothermic reaction. This value is always given a positive sign, for example, +345kJ.
When bonds are made, energy is released to the surroundings. Energy that is released to the surroundings is called an exothermic reaction. This value is always given a negative value, for example, -345kJ.
Remember, when a reaction takes place bonds break (endothermic) then bonds are made (exothermic).
Overall, the reaction will be exothermic if more energy is released into the surroundings than was absorbed.
An endothermic reaction will occur overall if, more energy is absorbed from the surroundings than is released.
Bond energy: This is the energy required to break one mole of bonds. The bond energy is also the energy given out when a mole of bonds is formed.
Activation energy: This is the minimum amount of energy required to break bonds to start the reaction off.