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)