1. NOTES: - Thermochemical Equations (17.2)

2. ENTHALPY
ENTHALPY = a type of chemical energy, sometimes referred to as “heat content”
Many reactions involve the production or absorption of heat. This heat is called the heat of reaction, or enthalpy (H).
Example:
CH O2  CO H2O kJ
Heat Term

3. Heat can be a product…
When the heat is a product, it appears on the RIGHT side of the equation, and the reaction is said to be EXOTHERMIC.
2Mg + O2  2MgO + Heat

4. Heat can also be a reactant….
When the heat is a reactant, it appears on the LEFT side of the equation, and the equation is said to be ENDOTHERMIC.
NH4NO3 + Heat  NH4+ + NO3-

5. H = -333 kcal (why negative?)
An equation involving heat can be written 2 ways:
CH4 + O2  H2O + CO kcal
-or-
CH4 + O2  H2O + CO2
H = -333 kcal (why negative?)

6. Reaction Diagram: Exothermic
Change of Heat (H)
Energy
Exothermic Reaction (H = -)

7. Reaction Diagram: Endothermic
Change of Heat (H)
Energy
Endothermic Reaction (H = +)

8. REVIEW: Exo vs. Endothermic
exothermic reactions:
q = ΔH < 0 (negative values)
surroundings get hot
heat appears on the right side of the equation (as a product)

9. REVIEW: Exo vs. Endothermic
endothermic reactions:
q = ΔH > 0 (positive values)
surroundings get cold
heat appears on the left side of the equation (as a reactant)

10. Thermochemical Equations
● A chemical equation that shows the enthalpy (H) is a thermochemical equation.
Example:
2H2(g) + O2(g)  2H2O(g) ∆H = -572 kJ

11. Rules of Thermochemistry
Rule #1 - The magnitude of H is directly proportional to the amount of reactant of product.

12. Rules of Thermochemistry
Example 1:
H2 + Cl2  2HCl H = kJ
Calculate H when 1.00 g of Cl2 reacts.

13. H Cl2  HCl kJ
ΔH = kJ (exothermic)

14. Rules of Thermochemistry
Example 2: When an ice cube weighing 24.6 g of ice melts, it absorbs 8.19 kJ of heat. Calculate H when 1.0 mol of ice melts.

15. H2O(s) kJ  H2O(l)

16. Rules of Thermochemistry
Example 3: Methanol burns to produce carbon dioxide and water:
2CH3OH + 3O2  2CO2 + 4H2O kJ
What mass of methanol is needed to produce 1820 kJ?

17. 2CH3OH + 3O2  CO2 + H2O kJ

18. Rules of Thermochemistry
Example 4: How much heat is produced when 58.0 liters of hydrogen (at STP) are also produced?
Zn + 2HCl  ZnCl2 + H kJ

19. Volume of H2  Moles of H2  Heat of reaction
Zn HCl  ZnCl H kJ
Volume of H2  Moles of H2  Heat of reaction

20. Example #5:
According to the following thermochemical equation for the combustion of glucose, (a) how many kilojoules of energy will be released when 12.5 g of glucose are combusted? (b) How many grams of glucose are needed to produce 3245 kJ of energy?
C6H12O6 + 6O2  6CO2 + 6H2O kJ

21. C6H12O6 + 6O2  6CO2 + 6H2O kJ
(a) How many kilojoules of energy will be released when 12.5 g of glucose are combusted?

22. C6H12O6 + 6O2  6CO2 + 6H2O kJ
(b) How many grams of glucose are needed to produce 3245 kJ of energy?