Transcript Lab 11
Job’s Method of Continuous Variation
Lab 11
Outline
Purpose Coordination Complexes Reaction Determining “n” Graph of Abs vs. Mole Fraction of SCN Procedure Safety Concerns Waste Submit Lab 11 Reports on…
Purpose
Students will become familiar with Job’s method of Continuous variation, which is a technique for determining the empirical formula of a coordination complex.
Coordination Complexes
A coordination complex consists of a central metal ion bound by surrounding ligands. Transition metals are positively charged and will readily accept electrons.
Ligand molecules contain atoms with non-bonding electrons, which can be donated to the electron deficient metal ion in order to form a complex ion.
This can be viewed as Lewis acid – base chemistry because of the electron involvement.
Reaction
Fe 3+ + nSCN [Fe(SCN) n ] 3-n iron (III) thiocyanate iron (III) thiocyanate metal ion ligand complex ion We will experimentally determine the value of “n.”
Related to the value of “n”:
The coordination number of a metal ion is the number of bonds it can form with ligands – typically 1, 2, 3, 4, 5, or 6. The coordination number of Fe 3+ is 6.
Iron (III) can therefore bond to as many as 6 monodentate ligands or three bidentate ligands.
X Ligand vs. X SCN-
For the purpose of this lab: X ligand refers to the mole fraction of SCN each solution (of SCN + Fe 3+ ).
in X SCN refers to the mole fraction of SCN the compound [Fe(SCN) n ] +3-n .
in
To determine “n”:
We will prepare a series of solutions, each with a different mole fraction of ligand:
Mol Fraction of Ligand
moles of SCN
The absorbance of the complex at directly proportional to the concentration of the complex.
= 470 nm is
Abs vs. Mole Fraction of SCN
1.5
1.2
y = 2.6188x + 0.1608
R² = 0.9965
0.9
0.6
0.3
0 0.00
0.20
0.40
0.60
y = -2.5963x + 2.7203
R² = 0.9987
Mole Fraction of Ligand 0.80
1.00
Graph
The point of intersection of the two linear extrapolations corresponds to the mole fraction of SCN- in the complex ion.
Remember to limit your graph slopes to the appropriate significant figures and y-intercepts to the appropriate digits of precision before attempting any calculations.
Solving for the intersection we are solving for x where: y 1 = y 2 m 1 x + b 1 b 1 – b 2 = m = m 2 2 x + b x – m 1 2 x = x (m 2 – m 1 )
Graph
x =
b 1 m 2
b 2 m 1
SCN
Then, X Fe3+ = 1 – X SCN And, n =
SCN
Fe3
Procedure
Prepare your solutions following the directions in the manual.
Measure the absorbance of each solution at a wavelength of 470 nm. Apply Job’s Method to your data to determine the empirical formula of your complex.
Safety Concerns
• • • • Reagents: Ferric Nitrate (Fe(NO 3 ) 3 HNO 3 (1.0N) Potassium Thiocyanate (0.1N) Sulfamic Acid Eye Contact: • corneal damage . Skin Contact: • Severe irritation, burns, r edness, pain, stains and ulcers.
Inhalation: • Destructive to mucosa and upper respiratory tract. May cause burning, coughing, choking, wheezing, laryngitis, shortness of breath, headache, nausea, vomiting, m ethemoglobinemia, cyanosis, convulsions, tachycardia, and death.
Ingestion: • Pain and burns of the mouth, throat, esophagus and gastrointestinal tract. Gastrointestinal irritation with nausea, vomiting, diarrhea, methemoglobinemia, cyanosis, convulsions, systemic toxic effects on the heart, liver, and kidneys and death.
Waste
Conserve chemicals and distilled water.
If you spill, clean it up.
All solutions in this experiment are acidic. Dispose of all excess and waste solutions in provided acid waste containers in the fume hood(s).
Please DO NOT unplug the hotplates!
Submit Lab 11 Reports:
Your instructor will let you know when to submit your Lab 11 reports for preliminary grading.