Complete revision notes for Topic 6 of the CIE IGCSE Chemistry course: Chemical energetics. Explanations with diagrams for every specification point. These notes are written for candidates taking the Extended paper.
Describe the meaning of exothermic and endothermic reactions
Exothermic:
A reaction in which energy is given out to surroundings (temperature of environment
increases)
Examples:
o Combustion of fuels
o Reaction of acids and metals
o Neutralisation reactions
Endothermic:
A reaction in which energy is taken in from surroundings (temperature of
environment decreases)
The energy change is positive
Examples:
o Thermal decomposition of carbonates
o Electrolysis
o First stages of photosynthesis
Exothermic reactions always give off heat and they feel hot, whereas endothermic reactions
take heat in and they feel cold.
Describe bond breaking as an endothermic process and bond forming as an exothermic
process
Whether a reaction is endothermic or exothermic depends on the difference between the
energy needed to break bonds and the energy released when the new bonds are formed.
, Enthalpy
Very similar to energy, but not the same
Enthalpy is a property of a thermodynamic system, defined as the sum of the
system's internal energy and the product of its pressure and volume
Endothermic: bond breaking
In an endothermic reaction, more energy is required to break the bonds than that
gained from making the new bonds
The change in energy is positive since the products have more energy than the
reactants
The symbol ΔH (delta H) is used to show the change in heat energy. H is the symbol
for enthalpy, which is a measure of the total heat of reaction of a chemical reaction
Therefore, an endothermic reaction has a positive ΔH value
Exothermic: bond making
If more energy is released than is absorbed, then the reaction is exothermic
More energy is released when new bonds are formed than energy required to break
the bonds in the reactants
The change in energy is negative since the products have less energy than the
reactants
Therefore, an exothermic reaction has a negative ΔH value
Draw and label energy level diagrams for exothermic and endothermic reactions using data
provided
Energy level diagrams:
Graphical representations of the heat changes in chemical reactions
The enthalpy of the reactants and products is displayed on the y-axis
The reaction pathway is shown on the x-axis
Arrows on the diagrams indicate whether the reaction is exothermic
(downwards pointing) or endothermic (upwards pointing)
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