Physical Chemistry Lecture 3

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

• Thermochemical Measurements: Measurements are made at 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 by chemists to study heat changes for reactions occurring at constant pressure and temperature. Most lab reactions occur at constant pressure.
• Definition of Enthalpy: Enthalpy (H) is defined as the sum of internal energy (E) and the product of pressure (P) and volume (V). H = E + PV.
  • Enthalpy is a state function, meaning its value depends only on the initial and final states, not the path taken.
  • The absolute value of enthalpy cannot be measured; only changes in enthalpy (ΔH) can be determined.
• ΔH Calculation: Change in enthalpy (ΔH) accompanying a process is accurately measured using the expression: ΔH = H_products – H_reactants = H_p – H_r.

Enthalpy of Reaction (Solids & Liquids)

• ΔH and ΔE Relationship (Solids & Liquids): For reactions involving solids and liquids, the change in volume (ΔV) is very small. Therefore, the term PΔV is negligible, and ΔH ≈ ΔE.

Enthalpy of Reaction (Gases)

• ΔH and ΔE Relationship (Gases): For reactions involving gases, the change in volume (ΔV) is appreciable. The term PΔV is significant, and ΔH = ΔE + PΔV; meaning the change in enthalpy is the sum of the change in internal energy and the work done during expansion or contraction.

Exothermic and Endothermic Reactions

• Exothermic Reaction: A chemical reaction that releases heat to the surroundings; ΔH < 0
  • Products have less enthalpy than reactants.
  • System loses heat, surroundings gain heat, and temperature increase in surroundings decreases in system
• Endothermic Reaction: A chemical reaction that absorbs heat from the surroundings; ΔH > 0 or ∆H is positive.
  • Products have more enthalpy than reactants.
  • Surroundings lose heat, system gain heat, and temperature decrease in surroundings and temperature increase in system.

Calculation of ΔH from ΔE

• General Reaction: aA + bB ⇌ cC + dD
• Δn = change in the number of moles (moles of products - moles of reactants).
• Δn = (c + d) – (a + b).
• ΔH = ΔE + ΔnRT (where R is the ideal gas constant and T is the temperature)

Solved Problem (Ethylene Combustion)

• Heat of Combustion: The heat released during the combustion of 1 mole of ethylene at constant volume is -332.19 kcal at 17°C . Calculating this value at constant pressure.
• Chemical Equation: C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(l).
• Constant Volume Conditions: ΔE = -332.19 kcal.
• Constant Pressure Conditions: ΔH = ΔE + ΔnRT. Given temperature, R, and calculation of change in number of moles
• Result: Calculation gives the heat of combustion at constant pressure. This is done by carefully considering the number of moles reactants vs product moles.

Solved Problem 2 (Carbon Monoxide Combustion):

• Heat of Combustion: The heat of combustion of carbon monoxide at a constant volume is given at a certain temperature. Calculate the heat of combustion at constant pressure.
• Chemical Equation: CO(g) + ½ O₂(g) → CO₂(g).
• Constant Volume Condition: ΔE = -283.3 kJ.
• Constant Pressure Condition: ΔH = -284.5 kJ.

Thermochemical Equations

• Definition: Equations that describe chemical reactions and include quantitative information about heat change. This includes state of reactants and products.
• Balancing: The equation must be balanced, reflecting the correct stoichiometric coefficients of reactants and products.
• Quantities of Heat (ΔH/ΔE): The magnitude and sign (positive or negative) indicate whether the reaction is endothermic or exothermic.
• Physical States: The physical states (solid (s), liquid (l), gas (g), aqueous (aq)) of all reactants and products must be specified.

Different Types of Heat (Enthalpy)

• Heat of Formation (ΔH_f): The change in enthalpy when one mole of a compound is formed from its elements in their standard states.
• Heat of Combustion (ΔH_c): The change in enthalpy when one mole of a substance is completely burned in excess of air (oxygen).
• Heat of Solution (ΔH_solution): The change in enthalpy when a specified amount of solute is dissolved in a given amount of solvent.

Explore the concepts of thermochemical measurements, internal energy, and enthalpy in this quiz. Understand how heat changes during chemical reactions at constant volume and pressure, and learn the definitions and calculations of enthalpy. Test your knowledge of these essential physical chemistry principles.