Transcript CHEMICAL RXNS IN AQUEOUS SOLNS

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) + 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.ht
ml
• See also oxides and minerals
Non-lead Based Pigments
• Plant extracts:
http://jcsparks.com/painted/pigmentchem.html
• Synthetic organic chemistry compounds:
http://www.handprint.com/HP/WCL/pigmt1
d.html
• Fall colors:
http://scifun.chem.wisc.edu/CHEMWEEK/f
allcolr/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)
2
• 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)
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
OxidationReduction
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/archite
cture/arch_dhb/finishes/finishes.html#ntrwt
hrng
• 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(Spollution, 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/indi
go/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/indi
go/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.