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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.