GCSE AQA Chemistry - Triple Higher - Energy Changes

You can load the pictutes by clicking on Load as PDF - they are important diagrams describing the energy changes.

  • Created by: jeban02
  • Created on: 30-05-20 12:05

Exothermic Reactions

Exothermic reactions are when energy is transferred from the system (reactants) to the surroundings. Energy is given out in this reaction - this is seen by measuring a temperature increase. It loses energy to the surroundings and makes them hotter.


You can tell whether a reaction is exothermic when the products are lower than the reactants. In an exothermic reaction, there enrgy required to break the bonds in the reactants is less than the energy RELEASED when new bonds are formed. The change in H is the energy change for the reaction and is measured in kJ / mol. The value for exothermic reactions is always negative. 

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Endothermic Reactions

Endothermic reactions are reactions in which energy is taken in during a reaction, and we notice a decrease in the temperature. They use up the energy and therefore the surroundings become cooler. 


In an endothermic reaction, the energy required to break the bonds in the reactants is more than the energy RELEASED when new bonds form. The energy change in an endothermic reaction is always a positive value, as the reactant are higher in energy than the products. 

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Exothermic or Endothermic?

Endothermic Reactions

  • Thermal decomposition is an endothermic reaction, for example, CaO and CuCO3 - heat is given in. Thermal decomposition is when a compound is split into two different compounds, for example CuCO3 · CuO + CO2
  • Photosynethesis is an endothermic reaction, as it takes in light energy from the sun. 
  • Dissolving can be endothermic or exothermic, but an example of an endothermic dissolving reaction would be an ice pack, which takes in energy and decreases in temperature. 

Exothermic Reactions

  • Respiration is an exothermic reaction as it releases energy. 
  • Combustion or burning fuels is always exothermic, because it is heating something, therefore the surroundings increase in temperature. 
  • Dissolving can be endothermic or exothermic, but an example of an exothermic dissolving reaction is hand warmers, as the temperature increases. 
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