Transcript Aluminum and Amphoteric Behavior

Amphoteric Behavior
of Alum
The amphoteric behavior of alum
is shown by varying the pH of its
solution
7/20/2015
Purpose
To observe the solubility of alum in a
changing pH environment
7/20/2015
Demonstration
An acidic solution of “alum” --- aluminum sulfate,
Al2(SO4)3 --- is neutralized and thereby produces a
precipitate. The neutralized solution is then made
gradually basic and the solid again dissolves!
What you will see:
A red, acidic solution of alum
A blue, neutral solution of aluminum hydroxide
precipitate which is cloudy
A purple, basic solution of soluble
tetrahydroxoaluminate complex
7/20/2015
Concepts
pH
Indicators
Precipitates
Amphoteric Behavior
7/20/2015
pH
pH measures the concentration of hydronium ions (H3O+) in a solution
The pH scale is based on acids and bases which is based on the dissociation of
water
H2O + H2O <-------> H3O+ OHHydronium and hydroxide ions are in balance
A greater balance of H3O+ ions in a solution will make the solution acidic
A greater balance of OH- ions in a solution will make it basic
Fewer H3O+ ions in a solution will make it basic while fewer hydroxide ions will
make the solution acidic
The pH scale ranges from 0 to 14
At pH 7 the the concentrations of hydroxide and hydronium ions equal one
another
pH affects the solubility of Aluminum Hydroxide
Aluminum Hydroxide is soluble in acidic and basic, but insoluble in neutral
acidic
neutral
basic
0
7
14
pH Scale
7/20/2015
Indicators
Indicators are weak organic acids whose colors vary
with pH
The color and range of an indicator are unique to that
specific indicator
The colors that indicators exhibit are intense and
unique
The presence of an indicator will not effect the
experiment
A universal indicator has many different indicators
mixed together and can detect changes along the
entire pH scale
7/20/2015
Precipitates
Solid products of a reaction that occur in
solution are termed “precipitates”
Aluminum hydroxide precipitates out of
solution as the sodium hydroxide increases
the basicity of the solution
Aluminum hydroxide precipitate itself has the
interesting property that it dissolves in strong
acid and strong base but exists as insoluble
precipitate at more intermediate pH values
7/20/2015
Amphoteric
Behavior
The ability of a compound to be soluble
in acid and base environments and yet
be insoluble solid (precipitate) in neutral
Metal hydroxides can exhibit amphoteric
behavior
Aluminum Hydroxide is amphoteric
Aluminum Hydroxide is a solid in the pH
range of 4-10 and a liquid in acid and
base
Aqueous <----> Solid <----> Aqueous
pH 0 to 4
pH 4-10
pH 10 to 14
7/20/2015
Amphoteric Behavior
Alum is soluble in acidic and basic environments
Formation of a solid (pH 4-10)
[Al(H2O)6]3+ + 3OH- <-----> Al(OH)3 + 6H2O
(liquid)
(solid)
Solid dissolving (pH > 10)
Al(OH)3 + OH- -----> {Al(OH)4](solid)
(liquid)
1
7/20/2015
2
Conclusions
The amphoteric nature of aluminum can be
demonstrated by varying the pH of the
solution in the presence of an indicator
Amphoteric behavior is the ability of a
compound to be soluble in both high and low
pH but not in between the two
The solid that forms in a solution and makes
the solution cloudy is termed a “precipitate”
Indicators serve to visualize changes in pH by
changing color with changing pH values
7/20/2015
Comments
Other metal hydroxides demonstrate
amphoteric behavior including hydroxides of
zinc, lead, chromium and tin
The amphoteric behavior of metal hydroxides
is beneficial in extracting the metal ions from
aqueous solutions
Indicators are accurate to within .5 pH
The rates of many reactions can be altered
by adjusting the pH
7/20/2015