Physical Chemistry Lecture 3

Physical Chemistry Lecture 3 Notes

• Thermochemical Measurements: Measurements are performed at constant volume or constant pressure. The change in magnitudes differs between these two conditions.
• Internal Energy (ΔE): The change in internal energy (ΔE) represents the heat change during a reaction at constant volume. No external work is done.
• Enthalpy (H): Enthalpy is a state function, defined as the sum of internal energy and the product of pressure and volume (H = E + PV). Changes in enthalpy (ΔH) can be precisely measured. ΔH = H_products - H_reactants; ΔH = ΔE + PΔV
• ΔH for Solids and Liquids: For reactions involving solids and liquids, the change in volume (ΔV) is very small, making PΔV negligible. Thus, ΔH ≈ ΔE.
• ΔH for Gases: For reactions involving gases, the change in volume (ΔV) is appreciable, so PΔV is significant. Thus, ΔH = ΔE + PΔV.

Enthalpy of Reaction

• Enthalpy Changes (ΔH): The enthalpy change (ΔH) for a reaction at constant pressure is the total heat absorbed or released by the system.

• Exothermic Reactions: ΔH is negative; heat is released from the system to the surroundings.

• Endothermic Reactions: ΔH is positive; heat is absorbed by the system from the surroundings.

• Zero ΔH: Hproducts = Hreactants and no heat is exchanged.

• ΔH Sign and Temperature: If ΔH is negative, the reaction is exothermic, and the temperature surrounding the system is elevated and the system temperature decreased. Conversely, a positive ΔH means the reaction is endothermic, and the temperature of the surroundings decreases, while the system temperature increases.

Calculations of ΔH

• ΔH = ΔE + PΔV: This equation relates the change in enthalpy to the change in internal energy and the work done by or on the system due to pressure-volume changes.

• Calculations for ΔH using ΔE and Pressure-Volume Change Calculation: The total change in volume is equal to the product of Change in Number of Moles and Volume. PΔV = RTΔn and then inserted in ΔH = ΔE + PΔV equation.

Solved Problems (Ethylene Combustion)

• Heat of Combustion at Constant Volume: Given the data and calculations, the heat of combustion at constant volume is given. ΔE = -332.19kcal
• Heat of Combustion at Constant Pressure: Calculate the heat of combustion for ethylene combustion using standard conditions of temperature and pressure. ΔH = ΔE + Δn RT.

Thermochemical Equations

• Definition: An equation showing the heat exchange during a reaction.
• Components of a Thermochemical Equation
• It must be balanced.
• The values of ΔH or ΔE must correspond to the quantities in the equation.
• Physical states of reactants and products must be specified (s, l, g, aq).
• Change in Sign: If a reaction is reversed, the sign of ΔH changes.
• Scaling: if an equation is multiplied or divided by a constant factor, the ΔH must be altered by that same factor as well. Example, if the reaction is doubled, ΔH will also be doubled.

Different Types of Heat (Enthalpy) of Reaction

• Heat of Formation (ΔH_f): The change in enthalpy when one mole of a compound is formed from its constituent elements.
• Heat of Combustion (ΔH_c): The change in enthalpy when one mole of a substance is completely burned in oxygen.
• Heat of Solution (ΔH_solution): The change in enthalpy when one mole of a substance is dissolved in a specific quantity of solvent. Heat of solution is always negative when a substance is mixed in solvent

Dive into the intricacies of thermochemical measurements in Physical Chemistry. This lecture discusses concepts of internal energy, enthalpy, and their significance in chemical reactions under different conditions. Gain insights into how enthalpy changes are calculated and their practical implications in various states of matter.