Lesson # 2: Enthalpy Calculations Thermochemistry Lesson # 2: Enthalpy Calculations

Calorimetry Calculation Rules Any thermal energy transferred from the calorimeter into the outside environment is negligible. Any thermal energy absorbed by the calorimeter is negligible. All dilute aqueous solutions have the same density (1.00 g/mL) and specific heat capacity (4.18 J/g°C) as water. In a calorimeter, the solute is considered the system, and the solvent is the surroundings. qsystem = -qsurroundings

Example 1 A student places 80.0 mL of liquid water at 21.00°C into a coffee-cup calorimeter. She places a sample of gold at 100.00°C in the calorimeter. The final temperature of the water is 21.33°C. Calculate the quantity of thermal energy, q, absorbed by the water in the calorimeter.

Example 2 Calculate the specific heat capacity of the sample of gold in the question above, if its mass is 6.77 g. Assume that the final temperature of the gold sample the same as the final temperature of the water in the calorimeter.

Example 3 A 50.0 mL sample of an aqueous solution of HCl was mixed with 50.0 mL of an aqueous solution of NaOH at 25.0°C in a calorimeter. After the solutions were mixed by stirring, the temperature was 31.9°C. Determine the quantity of thermal energy transferred by the reaction to the water and state whether the reaction was endothermic or exothermic.

Enthalpy & Reactions Remember that heat (q) is a type of energy, and enthalpy is the change in heat energy (ΔH). ΔHsystem = │qsurroundings│ For a chemical reaction, ΔH = Hproducts – Hreactants When ΔH > 0, the reaction is endothermic. When ΔH < 0, the reaction is exothermic.

Molar Enthalpy Typically enthalpy is more commonly known as molar enthalpy – the energy change for one mole of a substance. Looking up ΔH values will be based on one mole. ΔH = nΔHx (where x = type of change – combustion, vaporization, etc.)

Example 4 If a 1.00 g sample of ethanol is spread across the skin and evaporated, what is the expected enthalpy change? The molar enthalpy of vaporization of ethanol is 38.6 kJ/mol.

Example 5 A student places 125 g of water at 24.2°C into a calorimeter and then adds 10.5 g of solid potassium bromide, also at 24.2°C. He stirs the liquid until it dissolves and determines the temperature has changed to 21.1°C. Calculate the molar enthalpy change for this dissolution reaction, ΔHsol.

Example 6 A 50.00 mL volume of 0.300 mol/L CuSO4 (aq) is mixed with 50.00 mL of NaOH (aq) solution that contains an excess of NaOH. The initial temperature of both solutions is 21.40°C. After mixing the two solutions in a simple calorimeter, the highest temperature reached is 24.60°C. Determine the enthalpy change for the reaction: CuSO4 (aq) + 2 NaOH (aq) → Cu(OH)2 (s) + 2 Na2SO4 (aq)

Representing Molar Enthalpy Changes Example: H2O (l) + 285.8 kJ → H2 (g) + ½ O2 (g) This reaction would be – heat needs to be taken from the surroundings for this reaction to occur You would see a in temperature of the surroundings ΔH = Example: Mg (s) + ½ O2 (g) → MgO (s) + 601.6 kJ This reaction would be – heat is released when the reaction occurs from the system to the surroundings You would see a in temperature of the surroundings

Potential Energy Diagrams

Example 7 The combustion of methane is an exothermic reaction. When 1 mol of methane burns, 802.3 kJ of energy is released. a) Write the thermochemical equation both by representing the energy change as a ΔH value, and by representing the energy change as an energy term in the equation. b) Draw the potential energy diagram for this reaction.