Energy Changes

Exothermic reactions

• Exothermic reaction:
• A reaction that gives out heat
• Reaction vessel will feel hot
• Energy of the products is less than that of reactant
• Examples: Neutralisation reactions, combustion reactions, precipitation reactions, displacement reactions
• Uses: Hand warmers, self-heating cans
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Endothermic reactions

• Endothermic reaction:
• A reaction that takes in heat
• Reaction vessel will feel cold
• Energy of the products is greater than that of the reactants.
• Examples: Photosynthesis, thermal decompositions
• Uses: Sports injury packs
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Activation Energy

• Activation Energy: The minimum energy that molecules must possess for an interaction between them to result in a successful collision and the formation of products.
• Activation energy depends on the reaction in question.
• The use of a catalyst lowers the activation energy for a reaction.
• Successful collision: A collision where molecules have the correct orientation and sufficient kinetic energy to break bonds. Successful collisions lead to the formation of products. However, the vast majority of collisions are unsuccessful.
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• Legenda: Exothermic reaction with the use of a catalyst

Bond Energy

• Breaking of a bond is endothermic i.e. requires energy to be put in
• Formation of a bond is exothermic i.e. releases energy
• Difference between the total energy needed to break all the bonds and the total energy released when forming all the bonds determines whether a reaction is exothermic or endothermic, i.e. whether the overall energy input is positive or negative (A negative energy input would imply that energy is released from the system)
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Calculation energy change

• Example: Burning methane in oxygen involves the breaking of four C-H bonds and two O=O bonds and the formation of two C=O bonds and four O-H bonds
• The amount of energy released/ taken in by a bond being formed/broken is given by its bond energy
• \(CH_4 + O_2 \rightarrow CO_2 + H_2O\)
• The fact that this reaction is exothermic is why methane is so useful as a fuel.
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