1. 15.3 Thermochemical Equations
Pages 529 – 533

2. A. Thermochemical Equations
A thermochemical equation is a balanced chemical equation that includes the physical states of all reactants and products, and energy change
In a thermochemical equation, the enthalpy of change for the reaction can be written as either a reactant or a product

3. A. Thermochemical Equations
How to write thermochemical equations:
Endothermic (positive ΔH)
2NaHCO3(s)+ 129kJ Na2CO3(s) + H2O(g) + CO2(g)
Exothermic (negative ΔH)
CaO(s) + H2O(l) Ca(OH)2(s) kJ

4. B. Thermochemical Equation Practice
Write the thermochemical equation for the addition of oxygen gas to solid iron (III) if the ΔHrxn= kJ
Fe(s) O2(g)→ Fe2O3(s) kJ
Write the thermochemical equation for the decomposition of sulfur trioxide gas into sulfur dioxide gas and oxygen gas if the ΔHrxn= 198 kJ
SO3(g) kJ → SO2 (g) + O2 (g)
Exothermic 4 3 2
Endothermic 2 2

5. 15.4 Hess’s Law
Pages 534 – 540

6. A. Hess’s Law
Hess’s law states that if you can add two or more thermochemical equations to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction
2S(s) + 3O2(g)  2SO3(g) H = ?

7. A. Hess’s Law

8. B. Hess’s Law Practice N2(s) + 2O2(g)  2NO2(g) H = ? Known:
N2 (g) + O2(g)  2NO(g) H = -180 kJ
2NO(g) + O2(g)  2NO2(g) H = -112 kJ
Build your equation:

9. B. Hess’s Law Practice 2H2O2(l)  2H2O(l) + O2(g) H = ?
Known:
2H2(g) + O2(g)  2H2O(l) H = -572 kJ
H2(g) + O2(g)  H2O2(l) H = -188 kJ
Build your equation:

10. B. Hess’s Law Practice 2S(s) + 3O2(g)  2SO3(g) H = ? Known:
S(s) + O2(g)  SO2(g) H = -297 kJ
2SO3(g)  2SO2(g) + O2(g) H = 198 kJ
Build your equation:

11. B. Hess’s Law Practice 2B(s) + 3H2(g)  B2H6(g) H = ?
Known:
2B (s) + 3/2O2(g)  B2O3(s) H = kJ
B2H6(g) + 3O2(g)  B2O3 (s) + 3H2O (g) H = kJ
H2 (g) + ½ O2(g)  H2O (l) H = -286 kJ
H2O (l)  H2O (g) H = 44 kJ
Build your equation: