I. Standard Enthalpies of FormationA. Finding H’s

1. Calorimetry can give us enthalpy values for many reactions

2. Some reactions (Cg ----> Cd) are too slow to obtain H directly

3. We can get the H data from combining combustion and other reactions

B. Standard Enthalpy of Formation = Hof

1. Defined as H when one mole of a compound is made from its elements

2. The elements in the reaction must be in their Standard States

a. 25 oC, 1 atm, and 1 M (if in solution)

b. We must set a reference for energy changes

c. Pure elements in there standard states have Hof = 0

d. Hof of compounds from calorimetry and combining reactions

3. Examples: reactions always written for 1 mol of product

a. ½ N2(g) + O2(g) -------> NO2(g) Hof = +34kJ/mol

b. C(s) + 2H2(g) + ½ O2 -------> CH3OH(l) Hof = -239kJ/mol

c. Appendix 2 lists many more

C. Using Hof to calculate Enthalpies of other Reactions

1. Remember: Enthalpy is a state function, so any path is ok

2. CH4(g) + 2O2(g) -------> CO2(g) + 2H2O(l) Ho = ?

a. CH4(g) -------> C(s) + 2H2(g) Hof = +75kJ/mol

b. Oxygen is the other element needed Hof = 0

c. C(s) + O2(g) -------> CO2(g) Hof = -394kJ/mol

d. 2[H2(g) + ½ O2(g) -------> H2O(l)] Hof = -572kJ/mol

e. Add up all reactions: Hof =

-891kJ/mol

3. Algebraically, this is what we did:

4. Example: Ho = ? 4NH3(g) + 7O2(g) ----> 4NO2(g) + 6H2O(l)

a. Ho = 4(Hof NO2) + 6(Ho

f H2O) – 4(Hof NH3) – 7(Ho

f O2)

b. Ho = 4(34) + 6(-286) - 4(-46) – 7(0)

c. Ho = 136 -1716 + 184 -0 = -1396 kJ/mol

5. Example: Ho = ? 2Al(s) + Fe2O3(s) ----> Al2O3(s) + 2Fe(s)

Ho = 1(-1676) – (-826) = -850 kJ/mol

6. Example: Compare the H per gram of CH3OH and C8H18.

a. 2CH3OH(l) + 3O2(g) ----> 2CO2(g) + 4H2O(l)

Ho = 2(-394) + 4(-286) – 2(-239) – 3(0) = -1454 kJ/mol

b. 2C8H18(l) + 25O2(g) ----> 16CO2(g) + 18H2O(l)

Ho = 16(-394) + 18(-286) – 2(-269) – 25(0) = -10,900 kJ/mol

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II. Present Energy SourcesA. Fossil Fuels

1. Plants trap solar energy in carbon containing compounds

2. Wood, coal, petroleum, natural gas fuels let us release this energy

3. Industrialization changed our fuel use to petroleum dependence

B. Petroleum and Natural Gas = Hydrocarbon mixtures formed from the remains of ancient marine organisms

1. Natural gas is mostly methane CH4

2. Petroleum has hundreds of hydrocarbons

3. First “oil well” drilled in Pennsylvania in 1859

4. Fractional Distillation (boiling) of petroleum gives different products

a. Kerosene = C10-C18 was used as lamp oil

b. Gasoline = C5-C10 is now used in internal combustion engines

5. Cracking = heating longer hydrocarbons to break them into smaller ones

C. Coal = solid plant remains subjected to heat and high pressure

1. Cellulose (CH2O)n is the major molecular plant material

2. The carbon content (energy) in coal increases over time (anthracite = best)

3. 23% of US energy comes from coal, which is plentiful in US

4. Mining is dangerous and can be environmentally problematic

5. Coal contains S, which leads to acid rain when burned

D. Carbon Dioxide and Climate Change

1. CO2 is a product of burning fossil fuels

2. Greenhouse effect = various gases (including CO2) traps heat in the atmosphere that would normally radiate away

3. CO2 increased 16% from 1880-1980

4. Earth has correspondingly warmed

5. Local conditions may get warmer, cooler,

wetter, or dryer; the atmosphere is a complex

system

6. Global Climate Change may be the best

descriptor for what is clearly happening

III. New Energy SourcesA. Coal Conversion

1. Solid coal is less energy efficient and more difficult to ship/use

2. Coal Gasification produces Syngas and Methane

3. Uses of Syngas

a. Directly burned as a fuel

b. Converted to methanol: CO(g) + 2H2(g) ----> CH3OH(l)

i. Useful industrial chemical

ii. Used as a fuel itself or converted to gasoline

4. Coal Slurry = pulverized coal suspended in water

a. Can be used as a liquid fuel

b. Requires a lot of water

B. Hydrogen

1. Combustion of H2 as a fuel has advantages over petroleum

a. Highly energetic (2.5 x methane on a per gram basis)

b. No CO2 H2(g) + ½ O2(g) ----> H2O(l) Ho = -286 kJ/mol

2. Problems

a. Production

i. Treat natural gas with steam: CH4 + H2O ----> 3H2 + CO

ii. Ho = +206 kJ/mol is Endothermic (actually costs us energy)

iii. Using H2O as a source: current methods aren’t feasible yet.

b. Storage and Transportation

i. Pipeline: metal reacts with H2, which could lead to leaks

ii. Inefficient per unit volume (3 x volume of methane is needed)

iii. 277L H2(l) or 238,000L H2(g) is equivalent to 20gal of gasoline

iv. Liquid requires T = 20 K (expensive) or high P (hazardous)

v. H2 can be stored as Metal Hydride H2(g) + M(s) ----> MH2(s)

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C. Energy Alternatives1. Oil Shale: carbon material contained in rock

a. Large deposits in Western USb. Used to have to heat > 250 oC to get, produces a lot of waste c. New “Fracking” techniques use water/chemicals pumped in to release

2. Ethanol = product of plant (corn) fermentationa. Modern engines can burn in place of gasolineb. 10% Ethanol in gasoline in Midwest is commonc. Pure ethanol doesn’t vaporize well at low temperatures

3. Wind Powera. Wind turns a propeller attached to a turbine to produce electricityb. A windfarm surrounds Weatherfordc. Real farmers can earn more from wind than from crops: $8,000/acred. Wind power is expected to rise 60%/year in USe. Turbines have become more efficient making wind produced

electricity competitive with traditional sourcesf. Problem: need transmission lines from rural areas to cities

4. Nuclear Power (Chapter 21), Solar Power, Hydroelectricity are others