Physical Chemistry Lecture 3

Study 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.

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).

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

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 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

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.