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)

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

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.

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

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) + CO3

2- (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

S-containing Amino Acids

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 Rxn

Note 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

Figure 4.20 A Summary of Oxidation-Reduction Process

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.