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CHEMICAL REACTIONS

CHEMICAL REACTIONS. Precipitation Reactions (Ch 5, pp 50-62) Oxidation-Reduction Rxns (Ch 5, pp 35-49). PRECIPITATION. In a ppt rxn, we start with two solutions each containing water as the solvent and a soluble salt as the solute.

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CHEMICAL REACTIONS

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  1. CHEMICAL REACTIONS Precipitation Reactions (Ch 5, pp 50-62) Oxidation-Reduction Rxns (Ch 5, pp 35-49)

  2. PRECIPITATION • In a ppt rxn, we start with two solutions each containing water as the solvent and a soluble salt as the solute. • When the two aqueous solns are mixed, a solid will form if the product compound is insoluble or only slightly soluble in water. • How do we know what will dissolve in water? Solubility Rules for Salts in Water

  3. Soluble salt I (aq) + Soluble salt II (aq)  Solid cmp (s) + Soluble salt III (aq) • (aq) means that the compound is dissolved into its constituent ions: Ba(NO3)2 Ba2+(aq) + 2NO3- (aq) • This reaction involves ion interchange or metathesis (switch cation/anion partners)

  4. Fig 4.13 K2CrO4(aq) + Ba(NO3)2(aq)  2KNO3(aq) + BaCrO4(s)

  5. K2CrO4(aq) + Ba(NO3)2(aq)  2KNO3(aq) + BaCrO4(s) • This eqn shows the two soluble salts (i.e. reactants) and the insoluble solid on the product side plus another solution salt. • Note that it is balanced. • The overall or net eqn is Ba2+(aq) + CrO42- (aq)  BaCrO4(s) This eqn is also balanced by mass and charge.

  6. Figure 4.15 a&b The Reaction of K2CrO4 and Ba(NO3)2

  7. The Color of Seashells ( p 51) • The shells of some sea mollusks form via a precipitation reaction • Ca2+(aq) + CO32- (aq)  CaCO3(s) • The calcium ions are secreted from the mollusk and the carbonate ion comes from CO2 (greenhouse gas) dissolving in ocean.

  8. Acidification of the Oceans • As the level of CO2 increases in the atmosphere, more CO2 dissolves in the oceans. A series of rxns occur leading to increased acid levels in the water. (pH ↓) • In over 250 years since before the industrial Revolution, the pH has changed from 8.2 to 8.1. • In the next 50 years, it is expected to drop to 7.9.

  9. Chemical Equations • CO2 (g) + H2O(ℓ)  H2CO3 (aq) = carbonic acid = weak acid • H2CO3(aq)  H+(aq) + HCO3- (aq) = bicarbonate = also a weak acid • HCO3- (aq)  H+(aq) + CO32- (aq) = carbonate ion which reacts with the calcium ion to form the seashell.

  10. Another problem • As the pH decreases, the oceans become less habitable for mollusks and plants, thus upsetting the food chain. • Note competition for the carbonate ion H+(aq) + CO32- (aq)  HCO3- (aq) Ca2+(aq) + CO32- (aq) CaCO3 (s) = shell As [H+] ↑, the 1st rxn dominates and the 2nd is less likely, i.e. less shell formation.

  11. And the colors? • The variation in color is due to impurities from the mollusk due to diet, other mollusks, ions in the water, pollutants.

  12. Discoloration of Old Paintings (p 56) • To discuss discoloration, let us first discuss pigments which provide color. • White: 2PbCO3·Pb(OH)2, PbSO3·PbO, • Yellow or Orange: PbCrO4, PbSO4, PbO • Blue-Green: Fe4[Fe(CN) 6] 3 + PbCrO4 • Orange: PbCrO4 + PbMoO4+ PbSO4 • Red: Pb3O4, Cu2O

  13. Chemical Rxns of Pigments Lead to Discoloration • Reactions between pigments • Reactions between pigments and external chemicals (water) • Oxidation (copper-based pigments; see redox example) • Precipitation (blue-green + white)

  14. Glazes • Recall china glazes at Coalport China and toxicity of transition and heavy metal in the various glazes. • http://digitalfire.com/4sight/material/ • http://digitalfire.com/4sight/recipes/index.html • See also oxides and minerals

  15. Non-lead Based Pigments • Plant extracts: http://jcsparks.com/painted/pigment-chem.html • Synthetic organic chemistry compounds: http://www.handprint.com/HP/WCL/pigmt1d.html • Fall colors: http://scifun.chem.wisc.edu/CHEMWEEK/fallcolr/fallcolr.html

  16. Dr. Seuss’s Green Eggs and Ham ( p. 59) • Observation: Cooking an egg too long can cause the yolk to turn greenish. • The yolk contains Fe (0.590 mg) • The egg white proteins contain some amino acids with S. • Heating the egg too long decomposes the protein forming H2S (smelly, rotten eggs) • Fe + H2S  FeS (greenish, but not harmful) + H2

  17. S-containing Amino Acids

  18. OXIDATION-REDUCTION REACTIONS • A redox reaction involves the transfer of electrons between atoms in the reactants. • Electrons gained by one atom must equal electrons lost by another. (conservation of e-s) • Oxidation states or numbers are assigned to atoms and they change in a redox rxn. • Both oxidation and reduction must occur simultaneously. (or e-s would not be conserved) 2

  19. OXIDATION • If atom X in compound A loses electrons and becomes more positive (OX# increases), we say X (with charge) or A is oxidized. • Also, we say that A is the reducing agent (RA) or is the electron donor. • LEO the lion goes GER • LEO: Loses electrons = oxidation • GER: Gains electrons = reduction

  20. REDUCTION • If atom Y in compound B gains electrons and becomes more negative (OX# decreases, is reduced), we say Y (with charge) or B is reduced. • Also, we say that B is the oxidizing agent (OA) or is the electron acceptor.

  21. Fig 4.20 Summary of a Redox RxnNote the M is often a metal which tends to lose electrons and become positive. X is a nonmetal which tends to accept electrons and become negative.

  22. OXIDATION STATES OR NUMBERS (OX#) • Actual or imaginary charge on atom: single atom, atom in molecule or atom in polyatomic ion • We use these OX#s to keep track of electrons in redox rxns. • We will study rules for assigning OX# and then use this information to balance redox equations

  23. DETERMINING OX# (T4.2) • OX# of an atom in an element is 0 [Fe, O2] • If the species is neutral, sum of OX# is 0 [NaCl, MnO2] • If the species is charged, sum of OX# is value of overall charge (NH4+; SO42-) • OX# of a monatomic ions is its charge: 1A atoms have OX# = +1; 2A atoms have OX# = +2; 7A atoms have OX# = -1, etc

  24. OX# (2) • In molecular (covalent) cmps O has OX# = -2; sometimes -1 (with metal) • In molecular (covalent) cmps H has OX# = +1; sometimes -1 (peroxide) • F always has OX# = -1; other halides can have other OX#s • There are exceptions

  25. OXIDATION • If atom X in compound A loses electrons and becomes more positive (OX# increases), we say X (with charge) or A is oxidized. • Also, we say that A is the reducing agent (RA) or is the electron donor. • LEO the lion goes GER • Loses electrons = oxidation • Gains electrons = reduction

  26. Figure 4.20 A Summary of Oxidation-Reduction Process

  27. Hydrogen Peroxide, H2O2(p. 40) • Household product in low concentrations: antiseptic (3%) and bleach (6%). • Easily decomposes to water and oxygen H2O2  H2O + O2 • This reaction is accelerated by high temperatures (store in cool location), light (thus the dark bottle), metal ion catalysts (thus the plastic bottle, rather than glass)

  28. Redox Rxn of H2O2 • Recall that in a redox rxn, the Ox# of one element increases (oxidation due to loss of e-s) and the Ox# of another element decreases (reduction due to gain of e-s). • What happens in H2O2? H has Ox# = +1 and O has Ox# = -1 • The Ox# of O in H2O is -2 and in O2 it is 0. • So one O is reduced; the other oxidized.

  29. Redox Example:Weathering of Copper • http://www.copper.org/Applications/architecture/arch_dhb/finishes/finishes.html#ntrwthrng • Color changes from salmon pink  russet brown  chocolate brown  gray  gray-green or blue-green as copper reacts with oxygen, sulfur and then converts to a sulfate.

  30. Oxidation of Copper (Weber Hall, “Copper-top” Church) • 2Cu(s)-salmon + ½ O2(g)  Cu2O(s)-brown • Oxidation Number assignment • Cu has Ox# = 0 (element) • O in oxygen has Ox# = 0 (element) • After rxn, O in copper oxide has Ox# = -2, and Cu has Ox# = +1. Cu2O(s) is called copper(I) oxide. • Which is oxidized? Reduced?

  31. Oxidation of Copper • Cu2O(s)-brown reacts with sulfur in the atm to form Cu2S(s)-choc. brown and CuS(s) black. Assign Ox# to Cu, S. • Water and weather converts these sulfides to sulfate, CuSO4–blue/green or gray/green • What is the Ox# in the sulfate? • Process can take 5-10 years = f(S-pollution, water)

  32. Blue Jeans (p. 45) • Blue or indigo (deep purple) is an ancient dye from plants. This website describes how indigo was extracted from plants. • http://www.chriscooksey.demon.co.uk/indigo/hist.html

  33. Synthesis • In 1880, von Baeyer synthesized indigo and won the Nobel Prize in 1905. • Since the 1900’s, almost all indigo is synthesized. • http://www.chriscooksey.demon.co.uk/indigo/indust.html • C16 H10 N2 O2

  34. Indigo + Fabric of Jeans • Indigo is water-insoluble. • The dye-cotton bond is via hydrogen bonding so indigo-cotton bonding is not favored.

  35. Reduced Form of Indigo • Prior to dying, indigo is reduced by NaOH and Na2S2O4 to add OH groups to the molecule which then H-bond to the H atoms in cotton. The reduced form is colorless-yellow. • http://commons.wikimedia.org/wiki/Image:Leucoindigo_structure.png

  36. Oxidizing Indigo • Once the reduced form of the dye is applied to the jeans, exposure to air or to chromic acid oxidizes it to a dark blue color.

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