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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 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. • 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
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)
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.
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.
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.
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.
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.
And the colors? • The variation in color is due to impurities from the mollusk due to diet, other mollusks, ions in the water, pollutants.
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
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)
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
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
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
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
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
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.
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.
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
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
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
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
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)
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.
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.
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?
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)
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
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
Indigo + Fabric of Jeans • Indigo is water-insoluble. • The dye-cotton bond is via hydrogen bonding so indigo-cotton bonding is not favored.
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
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.