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Chemistry 5
Chapter-5Reactions in Aqueous Solutions
Part-1
27 September 2002
Some Cool Chemistry Research
Chemistry Research, cont’d.
100
80
60
40
20
0
Con
duct
ance
cha
nge
(nS)
6543210PSA concentration (pg/ml)
• Quantitative detection of protein concentration to femtomolar (10-15 M) level
• Potential for single molecule detection.
Chemistry Research, cont’d.
Blood Sample
Detect simultaneously in real-time:• all serum proteins & disease marker proteins• viruses and pathogens• screen genomic DNA for large or complete set of SNPs
106 sensor array
Properties of Aqueous Solutions
Structure of H2O liquid?
Water as a solvent?
Oxidation State of element in H2O?Polarity of H2O molecule enables it to interact strongly with other polar molcules (including itself) and ions!
-0.4
+0.2
+0.2
Cations in aqueous solution are stabilized by interactions with the oxygen (partial negative charge) atoms of water molecules.
Na+ Cl-
Anions in aqueous solution are stabilized by interactions with the hydrogen (partial positive charge) atoms of water molecules.
Electrolytes & Nonelectrolytes
nonelectrolyte
strong electrolyte
weak electrolyte
What happens when dissolve a chemical compound in solution?
Nonelectrolytes are substances that do not ionize in aqueous solution.
Examples: CH3OH, CH3CH2OH
Strong electrolytes are substances that for all intents are considered to be completely ionized in aqueous solution.
Examples: almost all soluble ionic compounds.
Weak electrolytes are substances that are only partially ionized in aqueous solution.
Examples: most molecular compounds such as organic acids, organic bases & proteins.
Electrolytes & ConcentrationStrong electrolytes dissociate completely upon dissolution in water. What are the concentrations of cations and anions when 0.05 M solutions of salts are made?
KBr
CaCl2
Na2SO4
Ions in Nature
K+ + Br-
Ca2+ + 2Cl-
2Na+ + SO42-
.05 M .05 M
.05 M .1 M
.1 M .05 M
Precipitation Reactions
Precipitation reaction
overall equation
net ionic equation
A precipitation reaction is a chemical reaction in which cations and anions combine to form an insoluble ionic compound called a precipitate.Typically, precipitation reactions are carried out by mixing solutions of strong electrolytes.
We can describe the mixing of two solutions by a chemical equation; for example:
Pb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + 2KNO3(aq)
Pb2+(aq) +2NO3-(aq) + 2K+(aq) + CrO4
2-(aq) PbCrO4(s) + 2K+(aq) 2NO3
-(aq)
The overall ionic equation can be simplified by eliminating ‘spectator ions’, thereby including only those species directly involved in the reaction; that is a net ionic equation:
Pb2+(aq) + CrO42-(aq) PbCrO4(s)
General Solubility Trendspredicting precipitation: periodic table as guide!
general trends (+ some exceptions)
Acid-Base ReactionsAcid and bases and their reactions are ubiquitous in chemistry and biology!
Acids—what are key features?• taste• reactivity
• reaction in water:
strong acids
weak acids
The proton interacts strongly with H2O molecules in aqueous solution, and exists in a variety dynamic structures: H3O+, H5O2
+, H9O4+, H43O21
+
sour!Recognize reactions with metals & carbonates, which often produce gas.
Key feature for us– acid is substance that provides hydrogen ion in aqueous solution.
is a molecular compound that completely dissociates in aqueous solution; for example:
HNO3 H+(aq) + NO3-(aq)H2O
is a molecular compound that does not completely dissociate in aqueous solution:
CH3COOH(aq) H+(aq) + CH3COO-(aq)
Acid-Base ReactionsBases—what are key features?• qualitative features• reaction in water:
strong base
weak base
Neutralization Reactions--• strong acid + strong base
• strong acid + weak base
• weak acid + strong base
bitter taste & slippery feel
Base is substance that produces hydroxide ion, OH-, in aqueous solution [base can also react with acid]
is a molecular compound that completely dissociates in aqueous solution; for example:
NaOH(s) Na+(aq) + OH-(aq)H2O
is a molecular compound that does not completely dissociate in aqueous solution:
NH3(aq) + H2O NH4+(aq) + OH-(aq)
the reaction between an acid and base to form H2O and an aqueous solution of the corresponding ions (a salt).e.g., HCl(aq) + NaOH(aq) H2O + Na+(aq) + Cl-(aq)
the reaction between an acid and weak base to form the protonated form of base and corresponding anions of the acid.e.g., HCl(aq) + NH3(aq) NH4
+(aq) + Cl-(aq)
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