Physical Chemistry Lecture 3

Physical Chemistry Lecture 3 Notes

• Thermochemical Measurements: Measurements of heat changes are made at either constant volume or constant pressure. The changes observed under these conditions differ.

• Internal Energy (ΔE): The heat change accompanying a chemical reaction at constant volume. No external work is performed.

• Enthalpy (ΔH): A new term introduced to study heat changes at constant pressure and temperature. It's related to internal energy, pressure and volume:

  • H = E + PV, where:
    • E is the internal energy
    • P is the pressure
    • V is the volume of the system
  • Enthalpy is also a state function, meaning its value depends only on the current state of the system, not how it got there. You cannot measure absolute enthalpy; only changes in enthalpy can be measured.
  • ΔH = H_products - H_reactants = H_p - H_r

• Relationship between ΔH and ΔE for Solids and Liquids:

  • For reactions involving solids and liquids, the change in volume (ΔV) is very small.
  • The term PAV (pressure times change in volume) is considered negligible.
  • Thus, ΔH ≈ ΔE

• Relationship between ΔH and ΔE for Gases:

  • For reactions involving gases, the change in volume (ΔV) is appreciable.
  • The term PAV is not negligible.
  • Thus, ΔH = ΔE + PΔV.

Exothermic and Endothermic Reactions

• ΔH = 0: No heat is evolved or absorbed (H_products = H_reactants)

• ΔH < 0 (Negative): Exothermic reaction; heat is released to surroundings (H_products < H_reactants)

• ΔH > 0 (Positive): Endothermic reaction; heat is absorbed from surroundings (H_products > H_reactants).

Calculation of ΔH from ΔE

• ΔH = ΔE + PΔV (Equation 1)
• Considering a general reaction aA + bB → cC + dD:
  • ∆n = (c + d) – (a + b) (change in number of moles)
  • PΔV = RT∆n (where R is the ideal gas constant, T is the temperature)
• Substituting PΔV in Equation 1:
  • ΔΗ = ΔΕ + RT∆n

Solved Problems

• Problem 1: Given the heat of combustion at constant volume, calculate the heat of combustion at constant pressure. Relevant data such as ΔE, Δn, temperature (T) and gas constant (R) are provided.

• Problem 2: Given the heat of combustion at constant volume, calculate the heat of combustion at constant pressure for a different reaction.

Thermochemical Equations

• Thermochemical Equation: An equation that shows the heat change associated with a reaction or process.

  • Must be balanced.
  • Show the value of ΔE or ΔH.
  • Specify the physical states of reactants and products (s, l, g, aq)

• Rules:

  • Changing the direction of the reaction alters the sign of ΔH
  • Multiplying/dividing reactions by a factor multiplies ΔH by the same factor

Different Types of Heat (Enthalpy) of Reaction

• Heat of Formation (Hf): Change in enthalpy when one mole of a compound forms from its elements.

• Heat of Combustion (Hc): Change in enthalpy when one mole of a substance burns completely in excess oxygen. Heat of combustion is always negative.

• Heat of Solution (H_solution): Change in enthalpy when one mole of a substance dissolves in a specific amount of solvent.