Physical Chemistry Lecture 3

Physical Chemistry Lecture 3 Notes (F/R-010)

• Professor: Dr. Hossieny Ibrahim
• Course: Physical Chemistry
• Institution: Badr University in Assiut, School of Biotechnology
• Contact: [email protected], Bio-326

Unit 1: Enthalpy of a Reaction

• Thermochemical measurements are made at constant volume or constant pressure.
• The magnitudes of changes vary between these two conditions.
• Internal energy (ΔΕ) is the heat change accompanying a reaction at constant volume.
• No external work is done at constant volume.
• Most chemical reactions in labs occur at constant pressure.
• Enthalpy (ΔΗ) is a new term introduced by chemists to study heat changes at constant pressure and temperature.

Unit 1: Enthalpy of a Reaction (Cont'd)

• Enthalpy (H) of a system is the sum of internal energy (E) and the product of pressure (P) and volume (V).
• H = E + PV
• Like internal energy, enthalpy is a state function, and its absolute value cannot be determined directly.
• However, the change in enthalpy (ΔH) accompanying a process is measurable.
• The change in enthalpy during a reaction is calculated using the following expression: ΔH = H_products - H_reactants

Unit 1: Enthalpy of a Reaction (Cont'd)

• For reactions involving solids and liquids, the change in volume (ΔV) is very small, and the term PΔV becomes negligible.
• ΔH ≈ ΔE
• For gas-phase reactions, the term PΔV is substantial and needs consideration in enthalpy calculations.
• ΔH = ΔE + PΔV

Unit 1: Exothermic and Endothermic Reactions

• A general chemical reaction (A + B → C + D) at constant pressure is considered.
• Enthalpies of reactants (H_A, H_B) and Products (H_C, H_D) are defined.
• ΔH = H_products - H_reactants = (H_C + H_D) – (H_A +H_B)
• ΔH value can be zero, negative, or positive.
• ΔH = 0: H_products = H_reactants (no heat absorbed or evolved).
• ΔH < 0 (Negative): H_products < H_reactants (heat evolved, exothermic reaction).
• ΔH > 0 (Positive): H_products > H_reactants (heat absorbed, endothermic reaction).

Unit 1: Exothermic and Endothermic Reactions (Cont'd)

• Examples of exothermic processes include mixing water and strong acids, mixing water with calcium chloride, rusting iron, and freezing.
• Examples of endothermic processes include melting solid salts, evaporation of water, sublimation, mixing water and ammonium nitrate.

Unit 1: Calculation of ΔH from ΔE

• ΔH = ΔE + PΔV
• Change in number of moles (Δn) is defined for a reaction (aA + bB → cC + dD) as Δn = (c + d) - (a + b).
• PΔV = RTΔn (where R is the ideal gas constant, and T is temperature).
• Substituting for PΔV in the ΔH equation, gives: ΔH = ΔE + ΔnRT

Unit 1: Solved Problem

• Heat of combustion of ethylene is calculated under both constant volume and constant pressure.
• ΔH is determined using the relation ΔH = ΔE + ΔnRT.

Unit 1: Solved Problem 2

• Calculating the heat of combustion of carbon monoxide from the value of heat at constant volume.
• The standard equation for the reaction is also mentioned

Unit 1: Thermochemical Equations

• Thermochemical equations represent the heat changes (evolved or absorbed) in a reaction or process.
• The equations should be balanced.
• The value of ΔH should be specified along with the physical states of reactants and products: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous states.

Unit 1: Thermochemical Equations (cont.)

• The sign of ΔH indicates whether the reaction is exothermic or endothermic – Positive ΔH is endothermic, and negative ΔH is exothermic.

Unit 1: Different Types of Heat (Enthalpy) of Reaction

• Heat of Formation (ΔHf): The change in enthalpy when one mole of a compound forms from its constituent elements.
• Heat of Combustion (ΔHc): The change in enthalpy when one mole of a substance is completely burned in excess oxygen.
• Heat of Solution (ΔH_solution): The change in enthalpy when one mole of a substance is dissolved in a specific amount of solvent.