Physical Chemistry Lecture 3

Physical Chemistry Lecture 3

• Thermochemical measurements are performed at constant volume or constant pressure.
• Changes observed under these conditions have different magnitudes.
• Internal energy (ΔE) is the heat change during a reaction at constant volume, as no external work is done.
• Most chemical reactions occur at constant pressure.
• Enthalpy (H) is a state function, representing the sum of internal energy and the product of pressure and volume (H = E + PV).
• Enthalpy change (ΔH) is measurable and calculated as ΔH = H_products - H_reactants (ΔH = H_p - H_r)
• For reactions involving solids and liquids, the change in volume (ΔV) is small, making the PΔV term negligible. ΔH ≈ ΔE.
• In gas reactions, the PΔV term is significant, and ΔH = ΔE + PΔV.

Enthalpy Changes and Reactions

• A general chemical reaction at constant pressure is A + B → C + D
• If H_A and H_B are enthalpy of reactants (A&B), and H_C and H_D are enthalpy of products (C&D), then ΔH = (H_C + H_D) − (H_A + H_B).
• ΔH can be zero, negative (exothermic), or positive (endothermic).
• Exothermic reactions release heat to the surroundings (ΔH < 0; products have less enthalpy).
• Endothermic reactions absorb heat from the surroundings (ΔH > 0; products have higher enthalpy).

Calculation of ΔH from ΔE

• ΔH = ΔE + PΔV.
• Change in moles (Δn) = (moles of products) - (moles of reactants).
• For reactions involving gases, ΔH = ΔE + ΔnRT. (where R is the gas constant and T is the temperature.)
• For reactions involving solids and liquids, ΔH = ΔE (PΔV is negligible).

Solved Examples

• Examples of heat of combustion calculations at constant volume and pressure. Calculations using the equation ΔH = ΔE + ΔnRT demonstrate different enthalpy calculation methods.

Thermochemical Equations

• Thermochemical equations describe reactions, including the heat change.
• Equations should be balanced and include the physical states of reactants and products (e.g., (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous).

ΔH and Exothermic/Endothermic

• Exothermic reactions have negative ΔH. Heat flows out of the system.
• Endothermic reactions have positive ΔH. Heat flows into the system.

Other Kinds of Heat Change

• Heat of Formation (ΔH_f): Enthalpy change when one mole of a compound forms from its elements.
• Heat of Combustion (ΔH_c): Enthalpy change when one mole of a substance completely burns in oxygen. ΔH_c is always negative.
• Heat of Solution (ΔH_sol): Enthalpy change when one mole of a substance is dissolved in a specific amount of solvent.

Additional Notes on Thermochemical Equations

• Stoichiometric coefficients represent the number of moles, not just atoms.
• When a reaction is reversed, the sign of ΔH is reversed as well.
• If you multiply the equation by a factor, the resulting ΔH is multiplied by the same factor.