2. Do Now 2NaHCO 3 + 85 kJ  Na 2 CO 3 + H 2 O + CO 2 Is this an endothermic or exothermic reaction? Calculate the amount of heat transferred when 36 g of NaHCO 3 is decomposed.

3. Thermochemical Equations

4. Enthalpy (H) Heat content of a system at constant pressure ΔH rxn = H final – H initial

5. Changes of State Many processes other than chemical reactions absorb or release heat

6. Changes of State ΔH vap Molar heat of vaporization : the heat required to vaporize one mole of liquid ΔH fus Molar heat of fusion: the heat required to melt one mole of a solid substance

7. What happens in the reverse process? ΔH vap = -ΔH cond ΔH fus = -ΔH solid Molar enthalpy of condensation Molar enthalpy of solidification

8. Changes of State H 2 O(l)  H 2 O(s) Δh solid =-6.01 kJ H 2 O(s)  H 2 O(l) ΔH fus =6.01 kJ

9. Example: How many grams of ice at 0°C will melt if 2.25 kJ of heat at added?

10. Changes of State H 2 O(l)  H 2 O(g) ΔH vap =40.7 kJ H 2 O(g)  H 2 O(l) ΔH cond =-40.7 kJ

11. Example: How much heat is absorbed when 24.8 g of H 2 O (l) and 101.3 kPa is converted to H 2 O(g) at 100°C?

12. Changes of State H 2 O(l)  H 2 O(g) ΔH vap =40.7 kJ H 2 O(g)  H 2 O(l) ΔH cond =-40.7 kJ H 2 O(s)  H 2 O(l) ΔH fus =6.01 kJ H 2 O(l)  H 2 O(s) ΔH solid =-6.01 kJ

13. Calculate the heat required to melt 25.7 g of solid methanol at its melting point. ΔH fus = 3.22 kJ/mol

14. Hess’s Law If you add 2 or more thermochemical equations to give a final equation, the you can also add the heats of reaction to give the final heat of reaction.

15. What is the enthalpy change in kJ for the following reaction? 2Al(s) + Fe 2 O 3 (s)  2Fe(s) + Al 2 O 3 (s) Use the enthalpy changes for the combustion of aluminum and iron: 2Al(s) + 3/2O 2 (s)  Al 2 O 3 (s); ΔH = -1676.0 kJ 2Fe(s) +3/2O 2 (s)  Fe 2 O 3 (s); ΔH = -822.1 kJ

16. 15 Determine the heat of reaction for the reaction: 4NH 3 (g) + 5O 2 (g)  4NO(g) + 6H 2 O(g) Using the following sets of reactions: N 2 (g) + O 2 (g)  2NO(g)  H = 180.6 kJ N 2 (g) + 3H 2 (g)  2NH 3 (g)  H = - 91.8 kJ 2H 2 (g) + O 2 (g)  2H 2 O(g)  H = -483.7 kJ

17. 16 4NH 3 (g) + 5O 2 (g)  4NO(g) + 6H 2 O(g) Using the following sets of reactions: a. N 2 (g) + O 2 (g)  2NO(g)  H = 180.6 kJ b. N 2 (g) + 3H 2 (g)  2NH 3 (g)  H = - 91.8 kJ c. 2H 2 (g) + O 2 (g)  2H 2 O(g)  H = -483.7 kJ Goal: NH 3 : O2 O2 : NO: H 2 O: Reverse and x 2 4NH 3  2N 2 + 6H 2  H = + 183.6 kJ Found in more than one place, SKIP IT (its hard). x2 2N 2 + 2O 2  4NO  H = 361.2 kJ x3 6H 2 + 3O 2  6H 2 O  H = -1451.1 kJ

18. Add it up!! Hess StyLe b. 4NH 3  2N 2 + 6H 2  H = + 183.6 kJ a. 2N 2 + 2O 2  4NO  H = 361.2 kJ c. 6H 2 + 3O 2  6H 2 O  H = -1451.1 kJ 4NH 3 + 2N 2 + 6H 2 + 5O 2  2N 2 + 6H 2 + 4NO + 6H 2 O ΔH = -906.3 kJ 4NH 3 + 5O 2  4NO + 6H 2 O ΔH = -906.3 kJ Is this reaction exothermic or endothermic?

19. 18 Determine the heat of reaction for the reaction: C 2 H 4 (g) + H 2 (g)  C 2 H 6 (g) Use the following reactions: C 2 H 4 (g) + 3O 2 (g)  2CO 2 (g) + 2H 2 O(l)  H = -1401 kJ C 2 H 6 (g) + 7/2O 2 (g)  2CO 2 (g) + 3H 2 O(l)  H = -1550 kJ H 2 (g) + 1/2O 2 (g)  H 2 O(l)  H = -286 kJ Consult your neighbor if necessary.

20. 19 Determine the heat of reaction for the reaction: Goal: C 2 H 4 (g) + H 2 (g)  C 2 H 6 (g)  H = ? Use the following reactions: C 2 H 4 (g) + 3O 2 (g)  2CO 2 (g) + 2H 2 O(l)  H = -1401 kJ C 2 H 6 (g) + 7/2O 2 (g)  2CO 2 (g) + 3H 2 O(l)  H = -1550 kJ H 2 (g) + 1/2O 2 (g)  H 2 O(l)  H = -286 kJ C 2 H 4 (g) :use 1 as is C 2 H 4 (g) + 3O 2 (g)  2CO 2 (g) + 2H 2 O(l)  H = -1401 kJ H 2 (g) :# 3 as is H 2 (g) + 1/2O 2 (g)  H 2 O(l)  H = -286 kJ C 2 H 6 (g) : rev #2 2CO 2 (g) + 3H 2 O(l)  C 2 H 6 (g) + 7/2O 2 (g)  H = +1550 kJ C 2 H 4 (g) + H 2 (g)  C 2 H 6 (g)  H = -137 kJ

21. 20