1. Drill
350 J are released as ice ( Specific Heat = 2.1 J / (g oC) ) cools from oC to -32 oC.
What is the mass of ice?

2. Thermochemical Equations/ Hess’ Law

3. Objective
SWBAT:
Correctly manipulate thermochemical equations to predict the enthalpy of reaction (Hess’s Law)

4. Thermochemical Equations
Include enthalpy (heat) accompanying the reaction
Coefficients represent moles – it is possible to have fractions – Example: ½ means half a mole of the substance
State of matter is specified (because they influence the overall amount of energy as heat gained or lost)

5. Examples of Thermochemical equations
Δ H is directly proportional to the amount of substance produced or reacting in a reaction
H2(g) + ½ O2(g)  H2O(l)
Δ H = kJ
2 H2(g) + O2(g)  2 H2O(l)
Δ H = kJ

6. Laws of Thermochemistry
Δ H for a reaction is equal in magnitude but opposite in sign from the reverse reaction
HgO(s)  Hg(l) + ½ O2(g)
Δ H = 90.7 kJ
Hg(l) + ½ O2(g)  HgO(s)
Δ H = kJ

7. Thermochemical Equations Practice
Complete the practice at your desk

8. Hess’s Law Notes

9. Hess’s Law
The overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process.

10. Hess’s Law Example
Determine ∆H of the reaction Sn(s) + 2 Cl2(g)  SnCl4(l)
Sn(s) + Cl2(g)  SnCl2(s) ∆H= kJ
SnCl2(s) + Cl2(g)  SnCl4(l) ∆H = kJ

11. Answer
∆H = kJ

12. Hess’s Law Example 2

13. Solution
Determine what we must do to the three given equations to get our target equation:
first eq: multiply it by two to get 2C
second eq: do nothing
third eq: flip it so as to put CH4 on the product side
Rewrite all three equations with changes applied: kJ

14. Drill
Explain Hess’s Law in your own words.

15. Hess’s Law Practice Problems
Complete the practice problems at your desk. Problems not completed in class will become your homework