CHEM 7L FINAL PRACTICE Exp. 4 Synthesis/Analysis of Iron (III) - Oxalate Complex

1.) Make an Iron (III)- Oxalate complex
2.) Prove that you made it thru Analysis -> find how much iron content you made! (Fe3+) (Percent Yield & Empirical Formula)

What is the goal of Exp. 4? (green crystals)

Coordination Compound:
Ligand (Oxalate) bound to Iron.

What complex are we making in Exp. 4? (green crystals)

It is a Bidentate Chelator since it is binding to 2 sites of the metal ion at the center (Fe3+)

What kind of Ligand is the Oxalate in this experiment?

It must deprotonate!
C2H2O4 + 2H2O -> C2O4 + 2H3O+
We want the C2O4! (not the C2H2O4 Oxalic Acid)

What needs to happen to the Oxalate before binding to our Fe3+?

The analyte (thing we are analyzing -> Fe3+ Complex) absorbs light that is measured by this process.

What is Spectrophotometry?

Absorbance is directly proportional to Concentration

How is Absorbance and Concentration related?

Red! (Since the crystals are green)

What color light is absorbed from the Fe (III) Oxalate complex?

T = [I]t / [I]0

What is the equation for Transmittance?

A = -log (T)

What is the equation for Absorbance?

Relates Absorbance (A) to Concentration (c):
A = 𝜀𝜆𝑏𝑐
(b is pathlength)
(𝜀 is molar asorbativity)
A is unitless!!!

What is the Beer-Lambert Law?

SYNTHESIS

SYNTHESIS

Oxidizing it!
Fe 2+ -> Fe3+ + e-

What are we doing to the Fe2+ given to form our complex? (For Synthesis part)

We are adding H2O2 (Hydrogen Peroxide) to it! The H2O2 will release heat when we do this:
Fe2+ + H2O2 +2H2+ -> Fe3+ 2H2O + HEAT

How are we Oxidizing our Fe2+ to Fe3+?

We are reducing it!
Fe 3+ + e- -> Fe2+

What are we doing to the Fe3+ (For Analysis part)

We are adding C6H8O6 (Absorbic Acid aka Vitamin C)!
2Fe3+ C6H8O6 -> 2Fe2+ + C6H6O6 + 2H+

How are we Reducing our Fe3+ to Fe2+?

Metal = Fe
Ligand = Oxalate (Potassium Oxalate for Basic Conditions (C2O4) // Oxalic Acid for Acidic Conditions (C2H2O4) when we want ligand present)

What is the ligand and what is the Metal of this experiment (4)?

Ammonium Iron (II) Sulfate aka Fe(NH4)2(SO4)2*6H2O

What do you start with in exp. 4? (green crystals)

Potassium Ferrioxalate aka Kw[Fex(C2O4)3]*zH2O

What do you end up making in exp. 4? (green crystals)

H2SO4 Sulfuric Acid!

What do you add to your starting compound (Fe(NH4)2(SO4)2*6H2O) to dissolve it?

Oxalic Acid (C2H2O4)!
It will turn the solution yellow
Solution is now acidic!

What do you add next?

It's washing off the Sulfuric Acid (H2SO4)! and Ammonium Sulfate (NH4)2(SO4)2 that existed in starting compound. So only Iron and Oxalate will be left (we want this!)

Next you will wash it 3 times with water, decanting/disposing of supernatant. Why do we do this?

Add K2C2O4 (Potassium Oxalate) to make the solution basic now!

Now we make the solution basic instead of acidic. How?

H2O2 (Hydrogen Peroxide) We want this in excess!

Next we heat this solution on a hotplate to exactly ~40 degrees Celsius. What do we add next that makes heat be released?

The H2O2 (Hydrogen Peroxide) will decompose aka become the limiting reagent instead and both Fe2+ AND Fe3+ will carry on thru the experiment :( We only want Fe3+!!!

What happens if your solution accidently heats to 50 degrees Celsius?

Rust! Fe3+(OH)3 (Brown Color)

What is the product of this addition of H2O2 on the hotplate?

Heat it! It will decompose it and take it out.

Now we don't need the Hydrogen Peroxide (H2O2) anymore. How do we get rid of it?

Add acid! Our acidic Oxalic Acid H2C2O4, this will redissolve any precipitate in our brown/rust solution and turn it green!

We still have the OH on our compound (Fe3+(OH)3) but we just want the Fe3+ part. How do we get rid of OH aka Hydroxide?

The green solution will now become cloudy aka precipitate out. This precipitate will become our green crystals as the solution cools to room temp!

Now we add EtOH to "crash" our product. Why?

To get rid of any impurities, we want a slow cool down to make sure only Fe 3+ comes out and not anything else.

Why do we redissolve our green crystal solution?

EtOH makes it precipitate, so too much EtOH means even if we heat the solution up it still won't go back to soluble.

What happens if you add too much EtOH to "crash" our product?

You'll burn off all the EtOH and it won't recrystallize in your drawer overnight.

What happens when you're redissolving the crystals and you boil it too long?

This removes any contaminates. We can't use water cuz it will redissolve everything!

After you come back to lab after 2 days to your green crystals that are formed you vacuum filtrate them and you wash them with EtOH. Why?

The heat will decompose all our stuff! Let it air dry.

Why can't we use the oven to dry the green crystals instead of the vacuum?

ANALYSIS

ANALYSIS

We turn both into (Fe(bpy)3)2+! So we can compare apples to apples.

We know the material we start with, Fe(NH4)2(SO4)26H2O but we don't know the material we just made out of it, Kw[Fex(C2O4)3]zH2O. How do we figure out what we don't know?

It's a strong light absorber! Will be nice to use with spectrophotometry.

Why do we wanna use bpy?

We need to take the Fe3+ back to Fe2+! (go backwards) since bpy like Fe2+ better.

What do we need to do with our newly formed Kw[Fex(C2O4)3]*zH2O to bind it with the bpy?

Absorbic Acid (Vitamin C)

How are we going to reduce our Fe3+ to Fe2+?

Ruby red

What color is our primary/secondary stock?

We will add CaCl2 (calcium loves Oxalate -> kidney stones). CaOx (Oxalate is C2O4) will precipitate out, leaving our Fe3+ free.

For our unknown, how will we get rid of the Oxalate attached to it?

CaOx will precipitate out (CaC2O4) & Fe3+
will be in supernatant

What will be in the supernatant and what will be in the precipitate?

Absorbic Acid (Vitamin C) & bpy!

Throw away the CaOx precipitate (kidney stone-like substance) and keep the Fe3+ supernatant. What do you add to this supernatant now?

Linear!

Calibration Curves should be...

Between 10% - 90% T

When is our Spectrophotometer most accurate?

Close to zero!

What should your Y-intercept be on your calibration curve?