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Chemical Energetics
Overview of Chemical Reactions
In any chemical reaction, reactants are on the left-hand side, and products are on the right-hand side.
Example: A + B ➜ C, where A and B are reactants, and C is the product.
Energy Changes in Reactions
In the bond-breaking process, energy is required.
In the bond-making process, energy is released.
Bond breaking and making occur simultaneously in chemical reactions.
Types of Chemical Reactions
Endothermic Reactions
Definition: Reactions where the energy absorbed in breaking the bonds is greater than the energy released in forming the bonds.
Involves absorption of energy from surroundings.
Example: Adding potassium nitrate to water cools the beaker; indicates energy absorption.
Key Features:
Temperature of the reaction mixture drops as energy is absorbed.
Energy required to break bonds > Energy released when products are formed.
Exothermic Reactions
Definition: Reactions where the energy absorbed in breaking bonds is less than the energy released in forming bonds.
Involves release of energy to surroundings.
Example: Adding sodium hydroxide to water warms the beaker; indicates energy release.
Key Features:
Products have lower energy than reactants.
Energy required to break bonds < Energy released when products are formed.
Enthalpy (H) and Enthalpy Change (ΔH)
Enthalpy is the heat or energy content of a reactant or product, represented by the symbol H.
Enthalpy change (ΔH) measures the difference in energy between products and reactants:
Formula: ΔH = Energy content of products - Energy content of reactants.
For Endothermic Reactions:
ΔH is positive (products have higher energy).
For Exothermic Reactions:
ΔH is negative (products have lower energy).
Energy Profile Diagrams
Y-axis: Energy (measured in kilojoules per mole)
X-axis: Reaction pathway.
The curve shows the relationship between the energy of reactants and products.
Activation Energy (Ea): The energy difference between reactants and the highest energy point on the diagram.
ΔH: The difference in energy levels between reactants and products.
Endothermic Energy Profile
Products lie at a higher energy level than reactants.
ΔH is positive: Energy content of products > Energy content of reactants.
Exothermic Energy Profile
Products lie at a lower energy level than reactants.
ΔH is negative: Energy content of products < Energy content of reactants.
Effect of Catalysts
Catalyst: A substance that speeds up a chemical reaction by providing an alternative pathway with lower activation energy.
Impact on Energy Profile Diagram:
Without catalyst: Higher activation energy.
With catalyst: Lower activation energy while maintaining the same initial energy level of reactants and final energy level of products.
Summary
In all reactions, bonds are broken (energy required) and formed (energy released).
More bonds formed than broken = Exothermic reaction.
More bonds broken than formed = Endothermic reaction.
Endothermic: Positive ΔH; Exothermic: Negative ΔH.