1. Thermochemical equations 16.3

2. 16.3 Thermochemical equations  Thermochemical equation = a balanced chemical equation that includes the physical states of all reactants and products and the enthalpy change (∆H).  Remember that enthalpy is the heat content of a system at constant pressure.

3. EXOTHERMIC 4 Fe (s) + 3O 2 (g)  2Fe 2 O 3 (s) + 1625 kJ 4 Fe (s) + 3O 2 (g)  2Fe 2 O 3 (s) ∆H= -1625 kJ The negative value for ∆H means that the system (reaction) is losing energy, giving off heat, is exothermic.

4. ENDOTHERMIC 27 kJ + NH 4 NO 3 (s)  NH 4 + (aq) + NO 3 - (aq) NH 4 NO 3 (s)  NH 4 + (aq) + NO 3 - (aq) ∆H = 27 kJ The positive value for ∆H means that the system (reaction) is gaining energy, taking in heat, is endothermic.

5. Enthalpy of Combusion  ∆H comb is the enthalpy change for the complete burning of one mole of substance.  Combustion of glucose: C 6 H 12 O 6 (s) + 6O 2  6CO 2 + 6H 2 O ∆H comb = - 2808 kJ The negative value means heat is given off (exothermic.) For 1 mole of glucose, 2808 kJ is given off.

6. Table 16-5 on page 501  ∆H comb for octane = -5471 kJ/mol  Translation: 5471 kJ is given off from the combustion of one mole of octane.  Which gives off more energy: combustion of propane or methane?  Propane (-2219 kJ/mol)  (Methane is only -891 kJ/mol)

7. CHANGES OF STATE  Solid to liquid; liquid to gas; solid to gas  All take in energy (endothermic)  ∆H is +  Gas to liquid; liquid to solid; gas to solid  All lose energy (exothermic)  ∆H is -

8. Changes of State  ∆H vap (enthalpy of vaporization) = heat required to vaporize one mole of a liquid. (liquid to gas)  ∆H fus (enthalpy of fusion) = heat required to melt one mole of a solid substance (solid to liquid)

9. ∆H fus and ∆H vap Both are endothermic, so their values are positive. Table 16-6 on page 502 Does it take more heat to vaporize one mole of water or one mole of ammonia? Water (H vap = 40.7 kJ/mol). (Ammonia is only 23.3 kJ/mol)

10. STOICHIOMETRY  It’s back!!  Remember 1 mol = 6.02 x 10 23 r.p. 1 mol = 22.4 L (gas at STP) 1 mol = 22.4 L (gas at STP) 1 mol = molar mass (periodic table) NOW ADD: 1 mol = __kJ (from ∆H)

11. How do you use it?  Write the given with UNIT.  Use unit multipliers to cancel units until you get your answer.  Look up correct ∆H on table 16-5 or 16-6, that number of kJ = 1 mol.  Round to the correct significant digits.

12. EXAMPLE  Calculate the heat required to melt 25.7 g of solid methanol at its melting point. (∆H= 3.22kJ/mol) 25.7 g CH 3 OH x 1 mol x 3.22 kJ = 32.05 g 1 mol 32.05 g 1 mol 2.58 kJ