Thermochemistry

1. Thermochemistry Molar Enthalpies

2. Learning Goals • use proper scientific terminology to describe molar enthalpies • calculate molar enthalpies • Calculate molar enthalpies using the calorimetry equation

3. Success Criteria • define the terms: molar enthalpy, calorimetry, calculating molar enthalpies • calculate molar enthalpies • calculate molar enthalpies using the calorimetry equation • Prepare for the calorimetry lab

4. Molar Enthalpies • Molar enthalpy is the enthalpy change associated with a physical, chemical, or nuclear change involving one mole of a substance • Molar enthalpy is represented by the symbol ∆Hx • The “x” is a letter or a combination of letters to indicated the type of change that is occurring

6. Enthalpy changes for exothermic reactions are given a negative sign • Enthalpy changes for endothermic reactions are given a positive sign

7. We can represent the molar enthalpy of a physical change, such as the vaporization of water, as follows: H2O(l) + 40.8 kJ  H2O(g) The molar enthalpy of vaporization for water is ∆Hvap = 40.8 kJ/mol

8. Molar enthalpy values are obtained empirically (textbook p. 307)

9. The amount of energy involved in a change depends on the quantity of matter undergoing that change (e.g. twice the amount of ice requires twice the amount of energy to melt) To calculate an enthalpy change ∆H you must 1) Obtain the molar enthalpy value ∆Hx from a reference source 2) Then use the formula ∆H = n∆Hx • enthalpy change(∆H) • moles (n) • molar enthalpy (∆Hx)

10. Learning Checkpoint Practice p. 308 UC # 1, 2, 3

11. Calorimetry of Physical Change • Studying energy changes requires an isolated system, one in which no matter nor energy can move in or out • Remember, the law of conservation of energy: the total energy change of the chemical system is equal to the total energy change of the surroundings ∆H system = ± ׀q surroundings׀

12. Calorimeter Assumptions • When using a calorimeter, three assumptions are made: • No heat is transferred between the calorimeter and the outside environment • Any heat absorbed or released by the calorimeter materials, such as the container, is negligible • A dilute aqueous solution is assumed to have a density and specific heat capacity equal to that of pure water (1.00 g/mL and 4.18 J/g∙ºC or 4.18 kJ/kg∙ºC)

13. Using Calorimetry to Find Molar Enthalpies Always recognize the law of conservation of energy ∆H = q (substance dissolving) (calorimeter water) You will also need to combine mathematical formulas ∆H = q n∆Hx = mc∆T

14. Learning Checkpoint Practice p. 310 UC # 4, 5 p. 311 UC # 6, 7, 8, 9