Standard Enthalpies of Formation Finding D H’s

1. Standard Enthalpies of Formation • Finding DH’s • Calorimetry can give us enthalpy values for many reactions • Some reactions (Cg ----> Cd) are too slow to obtain DH directly • We can get the DH data from combining combustion and other reactions • Standard Enthalpy of Formation = DHof • Defined as DH when one mole of a compound is made from its elements • The elements in the reaction must be in their Standard States • 25 oC, 1 atm, and 1 M (if in solution) • We must set a reference for energy changes • Pure elements in there standard states have DHof = 0 • DHof of compounds from calorimetry and combining reactions • Examples: reactions always written for 1 mol of product • a. ½ N2(g) + O2(g) -------> NO2(g) DHof = +34kJ/mol • C(s) + 2H2(g) + ½ O2 -------> CH3OH(l) DHof = -239kJ/mol • Appendix 2 lists many more

2. Using DHof to calculate Enthalpies of other Reactions • Remember: Enthalpy is a state function, so any path is ok • CH4(g) + 2O2(g) -------> CO2(g) + 2H2O(l) DHo = ? • CH4(g) -------> C(s) + 2H2(g) DHof = +75kJ/mol • Oxygen is the other element needed DHof = 0 • C(s) + O2(g) -------> CO2(g) DHof = -394kJ/mol • 2[H2(g) + ½ O2(g) -------> H2O(l)] DHof = -572kJ/mol • Add up all reactions: DHof = -891kJ/mol

3. Algebraically, this is what we did: • Example: DHo = ? 4NH3(g) + 7O2(g) ----> 4NO2(g) + 6H2O(l) • DHo = 4(DHof NO2) + 6(DHof H2O) – 4(DHof NH3) – 7(DHof O2) • DHo = 4(34) + 6(-286) - 4(-46) – 7(0) • DHo = 136 -1716 + 184 -0 = -1396 kJ/mol

4. Example: DHo = ? 2Al(s) + Fe2O3(s) ----> Al2O3(s) + 2Fe(s) • DHo = 1(-1676) – (-826) = -850 kJ/mol • Example: Compare the DH per gram of CH3OH and C8H18. • 2CH3OH(l) + 3O2(g) ----> 2CO2(g) + 4H2O(l) • DHo = 2(-394) + 4(-286) – 2(-239) – 3(0) = -1454 kJ/mol • 2C8H18(l) + 25O2(g) ----> 16CO2(g) + 18H2O(l) • DHo = 16(-394) + 18(-286) – 2(-269) – 25(0) = -10,900 kJ/mol

5. Present Energy Sources • Fossil Fuels • Plants trap solar energy in carbon containing compounds • Wood, coal, petroleum, natural gas fuels let us release this energy • Industrialization changed our fuel use to petroleum dependence • Petroleum and Natural Gas = Hydrocarbon mixtures formed from the remains of ancient marine organisms • Natural gas is mostly methane CH4 • Petroleum has hundreds of hydrocarbons

6. First “oil well” drilled in Pennsylvania in 1859 • Fractional Distillation (boiling) of petroleum gives different products • Kerosene = C10-C18 was used as lamp oil • Gasoline = C5-C10 is now used in internal combustion engines • Cracking = heating longer hydrocarbons to break them into smaller ones • Coal = solid plant remains subjected to heat and high pressure • Cellulose (CH2O)n is the major molecular plant material • The carbon content (energy) in coal increases over time (anthracite = best) • 23% of US energy comes from coal, which is plentiful in US • Mining is dangerous and can be environmentally problematic • Coal contains S, which leads to acid rain when burned

7. Carbon Dioxide and Climate Change • CO2 is a product of burning fossil fuels • Greenhouse effect = various gases (including CO2) traps heat in the atmosphere that would normally radiate away • CO2 increased 16% from 1880-1980 • Earth has correspondingly warmed • Local conditions may get warmer, cooler, • wetter, or dryer; the atmosphere is a complex • system • Global Climate Change may be the best • descriptor for what is clearly happening

8. New Energy Sources • Coal Conversion • Solid coal is less energy efficient and more difficult to ship/use • Coal Gasification produces Syngas and Methane • Uses of Syngas • Directly burned as a fuel • Converted to methanol: CO(g) + 2H2(g) ----> CH3OH(l) • Useful industrial chemical • Used as a fuel itself or converted to gasoline • Coal Slurry = pulverized coal suspended in water • Can be used as a liquid fuel • Requires a lot of water

9. Hydrogen • Combustion of H2 as a fuel has advantages over petroleum • Highly energetic (2.5 x methane on a per gram basis) • No CO2 H2(g) + ½ O2(g) ----> H2O(l) DHo = -286 kJ/mol • Problems • Production • Treat natural gas with steam: CH4 + H2O ----> 3H2 + CO • DHo = +206 kJ/mol is Endothermic (actually costs us energy) • Using H2O as a source: current methods aren’t feasible yet. • Storage and Transportation • Pipeline: metal reacts with H2, which could lead to leaks • Inefficient per unit volume (3 x volume of methane is needed) • 277L H2(l) or 238,000L H2(g) is equivalent to 20gal of gasoline • Liquid requires T = 20 K (expensive) or high P (hazardous) • H2 can be stored as Metal Hydride H2(g) + M(s) ----> MH2(s)

10. Energy Alternatives • Oil Shale: carbon material contained in rock • Large deposits in Western US • Used to have to heat > 250 oC to get, produces a lot of waste • New “Fracking” techniques use water/chemicals pumped in to release • Ethanol = product of plant (corn) fermentation • Modern engines can burn in place of gasoline • 10% Ethanol in gasoline in Midwest is common • Pure ethanol doesn’t vaporize well at low temperatures • Wind Power • Wind turns a propeller attached to a turbine to produce electricity • A windfarm surrounds Weatherford • Real farmers can earn more from wind than from crops: $8,000/acre • Wind power is expected to rise 60%/year in US • Turbines have become more efficient making wind produced electricity competitive with traditional sources • Problem: need transmission lines from rural areas to cities • Nuclear Power (Chapter 21), Solar Power, Hydroelectricity are others