5 - Chemistry Energetics
Exothermic and Endothermic Reactions
Definitions
Exothermic Reaction:
Transfers thermal energy to the surroundings.
Results in an increase in temperature of the surroundings.
Reaction examples include:
Combustion reactions (e.g., burning fuel in the presence of oxygen).
Neutralization reactions (e.g., acid-base reactions).
Most oxidation reactions.
Endothermic Reaction:
Absorbs thermal energy from the surroundings.
Leads to a decrease in temperature of the surroundings.
Bond Energy Considerations
Breaking Bonds:
Is typically endothermic.
Requires more energy input to break bonds than the energy released from breaking them, overriding the reaction's overall energy output.
Example: Breaking a marker requires added energy, while the sound and thermal energy released is minimal compared to that energy input.
Making Bonds:
Is exothermic as more energy is released than absorbed.
A reaction is classified as exothermic if the energy required to break bonds is less than the energy released when new bonds are formed.
Activation Energy and Enthalpy Change
Activation Energy (Ea):
The minimum energy required for colliding particles to react.
Enthalpy Change (ΔH):
Represents the difference in energy between reactants and products.
Formula: ΔH = (Energy of Reactants) - (Energy of Products).
If ΔH is negative (ΔH < 0), the products have less energy than the reactants, indicating an exothermic reaction.
If ΔH is positive (ΔH > 0), the products contain more energy than the reactants, indicating an endothermic reaction.
Summary of Energy Changes
In summary, ΔH is:
Negative for exothermic reactions (indicating energy release).
Positive for endothermic reactions (indicating energy absorption).
Visual Representation of Energy Changes
Graphical representation typically depicts:
For Exothermic Reactions:
Energy level decreases as reactants transform into products.
Energy change is negative.
For Endothermic Reactions:
Energy level increases as reactants are converted to products.
Energy change is positive.