Lecture on Enthalpy Changes

Introduction to Enthalpy

• Enthalpy (H) is related to the total heat content of a system.
• Useful in processes such as air conditioning and refrigeration.
• Indicates if a reaction is endothermic (absorbs heat) or exothermic (releases heat).

Definition and Equation

• Enthalpy is defined as: ( H = E + PV )
  • H: Enthalpy
  • E: Internal energy
  • P: Pressure
  • V: Volume
• At constant pressure, change in enthalpy ( \Delta H ) equals the heat exchanged with surroundings.

Heat (q) vs. Temperature (T)

• Heat: Form of energy transfer due to temperature difference (measured in Joules).
• Temperature: Average kinetic energy of particles (measured in °F, °C, or K).
• Example: In air conditioning, refrigerant absorbs heat, cools air, releases heat outside.

Exothermic vs. Endothermic Processes

• Exothermic:
  • Releases energy (heat/light).
  • Temperature of surroundings increases.
  • Example: Combustion of propane.
  • ( \Delta H < 0 ) because products are more stable and have lower energy.
• Endothermic:
  • Absorbs energy from surroundings.
  • Temperature of surroundings decreases.
  • Example: Ice melting.
  • ( \Delta H > 0 ) because products have higher energy and are less stable than reactants.

Chemical Potential Energy

• Stored energy in chemical bonds.
• More stable compounds = lower potential energy.
• Chemical reactions aim for more stable products.

Reaction Coordinate Diagrams

• Visualize potential energy changes.
• Show activation energy and energy absorbed/released.

Standard Enthalpy Change

• ( \Delta H^\circ ): Standard enthalpy change (measured in kJ/mol).
• Calculated using calorimetry: ( q = m \cdot C \cdot \Delta T )
  • m: Mass
  • C: Specific heat capacity (4.18 kJ/kg/K for water)
  • \Delta T: Change in temperature

Example: Acid-Base Reaction

• Reaction: Vinegar (acetic acid) and baking soda (sodium bicarbonate).
• Setup: Styrofoam cup, stirrer, lid, thermometer.
• Calculation:
  • Initial temperature: ( 22^\circ C )
  • Final temperature: ( 24^\circ C )
  • ( q = 0.7942 ) kJ
  • ( n = 0.0873 ) moles of acetic acid
  • ( \Delta H = -9.94 ) kJ/mol (exothermic)
  • Discrepancies due to heat loss, calorimeter absorption, and measurement inaccuracies.