1. Ap Chemistry Due Next Class: Hand Warmer Pre-lab
Review Hand Warmer lab (become familiar with what the heck we are doing!)
Up Coming Due Dates:
MC CH. 5 due Sunday
MC Ch. 5 Due Tuesday 10/10
Ch. 5-7 Test 10/25

2. Warm up
In a constant-pressure process, ∆H = 0. What can you conclude about ∆E, q, and w?
If ∆H = 0, qp = ∆E and w = 0
If ∆H = 0, qp = ∆E = w
If ∆H = 0, qp = 0 and ∆E = 0, w = 0
If ∆H = 0, qp = w and ∆E = 0
If ∆H = 0, qp = 0 and ∆E = w

3. Answer
∆H = ∆E + ∆PV ∆H = q + w + ∆PV ∆H = q + -p ∆V + ∆PV ∆H = q ∆H = 0, then q = 0 If q = 0, and E = q + w Then ∆E = w

4. Lecture

5. Energy of Chemical Bonds
Energy must be added to break bonds; Energy released when bonds are formed!

6. Hess’s Law
H is well known for many reactions, so do not need to measure H for every reaction in which we are interested.
Instead, estimate H using published H values and the properties of enthalpy.

7. Hess’s Law
If a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.
H is a state function, so the total enthalpy change depends only on the initial state (reactants) and the final state (products) of the reaction.

8. Defined set of conditions at which a reaction occurs. Standard State:
Pure form of substance
Pressure = 1 atm (atmospheric pressure)
Temperature = 298 K or 25 ̊ C (room temperature)
Standard Enthalpy Change = ∆ H ̊
Indicates “standard state conditions”

9. Enthalpy of Formation
Hf the enthalpy change for the reaction in which one mole of a compound is made from its constituent elements in their elemental forms.
The most stable form of the element that exists under standard conditions is used as the “elemental form”
Ex: for carbon, graphite is more stable than diamond
The standard enthalpy of formation of the most stable elemental form of any element is zero, because no reaction is needed to form it, if it is already in its standard state.

10. Standard Enthalpy of Formation: Is it so?
One mole of substance is formed
Substance is formed from constituent elements in most stable elemental forms (formula & solid/gas/liquid state)
Reaction takes place under standard conditions: Pure, P=1 atm, T=298 K
BE SURE TO CHECK!

11. Standard Enthalpies of Formation
Standard enthalpies of formation, ∆Hf°, are measured under standard conditions (25 ºC and 1.00 atm pressure).
Also in App. C in the text!

12. Ozone, O3(g), is a form of elemental oxygen produced during electrical discharge. Is ΔHf for O3(g) necessarily zero?
Yes, because it is just another elemental form of oxygen.
No, because it is not the most stable form of the element oxygen at the given conditions.
Yes, because changing the subscripts of an elemental formula does not change the standard heat of formation.
No, because there is a temperature change when ozone is formed.
Answer: b

13. C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
Calculation of H
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
Step 1: Decomposition of propane to the elements: C3H8(g)  3 C(graphite) + 4 H2(g)

14. C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
Calculation of H
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
Step 1: Decomposition of propane to the elements: C3H8(g)  3 C(graphite) + 4 H2(g)
Step 2: Formation of CO2:
3 C(graphite) + 3 O2(g) 3 CO2(g)

15. C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
Calculation of H
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
Step 1: Decomposition of propane to the elements: C3H8(g)  3 C(graphite) + 4 H2(g)
Step 2: Formation of CO2:
3 C(graphite) + 3 O2(g) 3 CO2(g)
Step 3: Formation of H2O:
4 H2(g) + 2 O2(g)  4 H2O(l)

16. C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
Calculation of H
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
The sum of these equations is the overall equation!
C3H8(g)  3 C(graphite) + 4 H2(g)
3 C(graphite) + 3 O2(g) 3 CO2(g)
4 H2(g) + 2 O2(g)  4 H2O(l)
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)

17. Calculation of H
We can use Hess’s law in this way: H = nHf,products – mHf°,reactants where n and m are the stoichiometric coefficients.

18. C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
Calculation of H
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
H = nHf,products – mHf°,reactants
H = [3(−393.5 kJ) + 4(−285.8 kJ)] – [1(− kJ) + 5(0 kJ)] = − kJ

19. Energy in Foods
Most of the fuel in the food we eat comes from carbohydrates and fats.

20. Carbohydrates Proteins Fats
Which releases the greatest amount of energy per gram when metabolized?
Carbohydrates
Proteins
Fats
Answer: c

21. Energy in Fuels
The vast majority of the energy consumed in this country comes from fossil fuels.

22. Other Energy Sources
Nuclear fission produces 8.5% of the U.S. energy needs.
Renewable energy sources, like solar, wind, geothermal, hydroelectric, and biomass sources produce 7.4% of the U.S. energy needs.

23. Practice