Energetics in Chemistry

Key Concepts

• Distinguish between exothermic and endothermic reactions.

  • Exothermic reactions: Release heat, causing the surroundings to get hotter (e.g., combustion, neutralization).
  • Endothermic reactions: Absorb heat, causing the surroundings to get colder (e.g., photosynthesis, thermal decomposition).

• Energy Profile Diagrams:

  • Illustrate the enthalpy of reactants and products, the change in enthalpy, and the activation energy.
  • In exothermic reactions, the enthalpy of products is less than the enthalpy of reactants (∆H is negative).
  • In endothermic reactions, the enthalpy of products is greater than the enthalpy of reactants (∆H is positive).

Enthalpy (H)

• Represents the energy content of a substance.
• Cannot be measured directly, only changes in enthalpy (∆H) can be measured.
• Change in enthalpy (∆H) = Enthalpy of products - Enthalpy of reactants.

Conservation of Energy

• Energy is neither created nor destroyed.
• Total energy remains the same before and after a reaction.
• Exothermic reaction: Energy released to surroundings, products have lower energy.
• Endothermic reaction: Energy absorbed from surroundings, products have higher energy.

Catalysts

• Speed up reactions by providing alternative pathways with lower activation energy.
• Do not affect the overall energy change (∆H) of the reaction.

Calculations

• Specific Heat Capacity (c): Quantity of heat required to raise temperature of 1g of substance by 1°C (water = 4.2 J/g°C).
• Heat of reaction (Q): Measured by calorimetry, calculated using formula:

Q = m * c * ∆T

• m = Mass of solution/reactant.
  • c = Specific heat capacity.
  • ∆T = Change in temperature.

Calculating Heat of Solution

• Heat change when one mole of solute dissolves in a solvent.
• Consider energy absorbed to break solute and solvent bonds versus energy released in solvation.

Example Problems

• Potassium nitrate dissolution:

  • Initial temperature drop indicates endothermic reaction.
  • Calculate moles using mass and molar mass.
  • Use ∆T, m, and c to find heat absorbed, then scale to find heat per mole.

• Neutralization Reaction:

  • Acid + base -> salt + water, typically exothermic.
  • Calculate heat using calorimetry; use average initial temperature of reactants.

Exam Tips

• Always consider the sign of ∆H in context of temperature change (positive for endothermic, negative for exothermic).
• For neutralization, typical ∆H is ~ -56 kJ/mol for strong acid and base.
• Ensure correct labeling and use of signs in energy profiles and calculations.

These notes provide a detailed guide to understand energetics in chemistry, focusing on the key points necessary for exams, including types of reactions, how to calculate enthalpy changes, and interpreting energy profiles.