Exothermic and Endothermic Reactions


Exothermic Reactions

Exothermic reactions involve a temperature rise beause they always transfer heat energy to the surroundings. The products have less energy than the reactants.

They are making bonds in the reaction (BendoMex).

More energy is released when bonds are formed than needed to break bonds = exothermic reaction.

Example Reactions:

  • Hand warmers
  • Self heating cans
  • Neutralisation reactions
  • Oxidation reactions
  • Combustion reactions
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Endothermic Reactions

Endothermic Reactions have a decrease in temperature therefore, they take in heat energy from the surroundings to the reaction. The products always have more energy than the reactants.

Endothermic reactions break bonds (BendoMex).

More energy is needed to break bonds than needed to make bonds = endothermic reaction.

Example Reactions:

  • Sports Injury packs
  • Sherbet
  • Thermal Decomposition
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Reaction Profile Diagram (Exothermic)

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Reaction Profile Diagram (Endothermic)

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Reversible Reactions

If a reaction is exothermic in one direction, it will always be the opposite in the other direction (endothermic).

There will always be the same amount of energy transferred in each direction.

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Bond Enthalpy

Bond Enthalpy is the average energy required to break 1 mole of gaseous compound.

  • If bond breaking is endothermic the change in energy is positive (+).
  • If bond making is exothermic the change in energy is negative (-).

Breaking bonds energy - Making bond energy = Change in energy

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