Chemical Questions

Chemical Questions Some were chosen for their timeliness; why?

Organ Pipe Deterioration (p. 113) • Observation: The pipes of some 19th century organs of N. Europe deteriorated. • Analysis: These pipes were made of tin. • At T > 55F or 13 oC, white tin or β-tin is most stable. It is solid, shiny, dense, has a body-centered tetragonal unit cell. • At T < 55F or 13 oC, grey tin or α-tin is most stable. It is powdery, loose, has a cubic unit cell.

β-Sn to α-Sn • β-Sn and α-Sn are two different allotropes or forms of tin (like O2 and O3 or graphite and diamond). • As temperature decreases, β-Sn α-Sn spontaneously and auto-catalytically (i.e. it speeds up once it starts). • The organ at New College (Oxford U) http://www.newcollegechoir.co.uk/organ.htm

Lime and Acid Rain (p.46) • Acid rain is a product of water and coal-fired power plant emissions • SO2(g) + H2O (ℓ) + ½ O2 (g)  H2SO4 • This acid ends up in the atmosphere but precipitates as rain and snow. • NO and NO2 + water produce nitric acid • Unpolluted rain has pH of 5.2; acid rain pH can be as low as 2.4.

Prevention of Sulfuric Acid Formation • Strategies to prevent SO2 from being released into atmosphere are many and varied. + CaO(s, lime)  CaSO3(s) + CaCO3(s, limestone)  CaSO3(s) + CO2(g) + Ca(OH)2(s, slaked lime)  CaSO3(s) + H2O + NaOH(aq) + 1/2 O2  Na2SO4(s) + H2O + Na2SO3 + 1/2 O2  2HSO3-(aq) + zeolites  chemisorption (chemical) or adsorption (physical) • Recover S as appropriate.

Leavening Agents (p 67)Puff Pastry, Bread, Yeast… • What causes puff pastry to “puff” up, bread and cakes to rise? • A gas is produced throughout the dough via various mechanisms. • As the item is baked, the volume of the gas increases with temperature (whose law?) and the dough rises and then the gas bubbles are baked into the dessert.

Liquid vs Vapor Volume • 18 mL liquid water expands to • 24 L = 24,000mL of gas at STP • 30 L gas at 100 oC or 212F • 42 L gas at 200 oC or 400F.

What Produces the Gas Molecules • Mixing or beating a cake mix with eggs, oil and water incorporates air bubbles into the batter. • Yeast added to bread ferments the sugar (composition and compound type?) in the dough producing CO2 gas and water. • Steam leavening occurs when water in the puff pastry vaporizes.

One More Mechanism • Baking soda or NaHCO3 + H+(aq)  Na+ (aq) + H2O(g) + CO2(g) Source of acid?

Thermochemistry • “Thermo”: Heat or energy associated with • “Chemistry”: Chemical reactions • Thermochemistry asks is heat given off by a reaction or is heat required for the reaction to proceed? In either case, how much heat? • The chemical industry is concerned with the cost of heat needed and also how to recycle heat given off.

Exothermic Reactions • Some reactions give off heat when R  P • R  P + heat • We observe the reaction container getting hot or heat being generated by the rxn. • Chemical Examples: burning of fuel, hot packs • Phase (Physical) Changes: condensation from gas to liquid or liquid to solid.

Endothermic Reactions • Other reactions absorb heat when R  P • R + heat  P • We observe this type of rxn by the container getting colder as the heat from the container is absorbed or we have to provide heat • Chemical Examples: cold packs (p 134), • Phase (Physical) Change: evaporating a liquid to a gas or melting a solid to a liquid.

Spraying Orange Orchards to Prevent Their Freezing (p. 107) • When temperatures drop below freezing, citrus fruits are especially vulnerable to freezing (water content freezes). • Strategies to prevent this: • Use heaters to heat orchard air. • Spray warm water on tree trunks; as water cools, heat is released to heat orchard air. • Spray water on fruit if T < 0 oC so liquid water freezes (exothermic). Also ice insulates.

Why Does a Setting Cast get Warm? (p. 133) • Plaster of Paris (like flour) + water  Gypsum (hard solid cast) • CaSO4•½ H2O(s) + 3/2 H2O(ℓ)  CaSO4•2H2O(s) • Exothermic or Endothermic? • Calculation says that the temp will increase from 25 oC to 117 oC!

Making Snow on Spirit Mountain (p. 131) • A snow making machine spews water vapor and compressed air through a nozzle. • The water vapor expands as it passes through the nozzle and then freezes to a solid or snow. • Recall that when we heat a gas at constant T, its volume increases (expansion). We have to provide heat in this case. • If we insulate the gas from the heat source but still allow the gas to expand, another energy source has to be found to expand the gas.

Snow Making • Now recall that molecules are always in motion (KMT) and the higher the temp, the faster they move and the more energy they have. • So if we cool the molecules, they will lose energy. This is the energy used to expand the water vapor. So as the water vapor passes through the nozzle, its temp drops as it expands. And the temp drop is sufficient to cause the formation of snow.

Northern Lights (p. 145) • Solar flares create fast moving ions and electrons. • These enter our atm and ionize and/or excite and/or dissociate oxygen, nitrogen, etc. Ie non-ground states are created. • When the ion regains its electron and/or de-excites and/or associates (return to ground state), light is emitted.

Colors of the Aurora Borealis • O2+ + e-  O2λ = 630 nm red • 2O  O2λ = 557.3 nm green-yellow and λ = 630 nm red • N2+ + e-  N2λ = 391.4 - 470 nm violet, blue

Chemical Questions