Electrochemistry

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• Leerjaar 5

Electrochemistry 

Transfer reactions

Electron transfer

Redox reactions

Will it react?

Chemical reactions

You should already be familiar with the reaction types:

Synthesis	A+B → AB	Ca + S → CaS
Dissociation	AB → A+B	CaS → Ca + S
Single-replacement	AB + C → AC + B	HCl + F- → Cl- + HF
Double-replacement	AB + CD → AC + BD	CaS + BeO → CaO + BeS

But there's more to it...

Let's zoom in on the following synthesis reaction:

2Fe+O​2​​→2FeO

But there's more to it...

Let's zoom in on the following synthesis reaction:

2Fe+O​2​​→2FeO

Fe​0​​

Fe​2+​​

O​2−​​

O​0​​

molecular oxygen

What's really happening here?

metallic iron

But there's more to it...

Let's zoom in on the following synthesis reaction:

2Fe+O​2​​→2FeO

Fe​0​​

Fe​2+​​

O​2−​​

O​0​​

molecular oxygen

Electrons are taken from iron atoms to make iron(II) ions.

Electons are added to oxygen atoms to make oxide ions.

metallic iron

Electrochemistry

Any reaction where electrons are transferred between atoms is called a REDOX reaction.

This type of reaction consists of 2 "half reactions": 

• one that produces electrons (RED):
• one that consumes electrons (OX):

Fe​0​​→Fe​2+​​+2e

O​2​0​​+4e→2O​2−​​

the "reductor" producers electrons

the "oxidizer" consumes electrons

This slide requires a bit of talking/ explaining/examples and assignments with some class interaction. Spend a few minutes on this!

REDOX reactions

Why do we define these "half reactions"?
So that we can mix and match them together! (Binas 48)

First, let's practice making our own half reactions!

On top of balancing the atoms (as with any reaction equation)

     Specifically for REDOX:

• When lacking H, add H+ (NOT H3O+!!)
• When lacking O, add H2O    

And make sure to balance the charge with electrons! 

Also, when lacking H, you 

Assignment 3a (REDOX I) after this slide.

But there's more to it...

Let's zoom in on the following dissociation reaction:

2Fe​2​​O​3​​→4FeO+O​2​​

But there's more to it...

Let's zoom in on the following dissociation reaction:

2Fe​2​​O​3​​→4FeO+O​2​​

Fe​3+​​

O​2−​​

Fe​2+​​

O​2−​​

O​0​​

(molecular oxygen)

What's really happening here?

But there's more to it...

Let's zoom in on the following dissociation reaction:

2Fe​2​​O​3​​→4FeO+O​2​​

Fe​3+​​

O​2−​​

Fe​2+​​

O​2−​​

O​0​​

(molecular oxygen)

Electrons are added to iron(III) ions to make iron(II) ions. 

Electons are taken from oxide ions to make molecular oxygen.

When will it happen?

I can tell you that this works: 

Sadly, this does not:

WHY??!!

2Na+Cl​2​​→2NaCl

2Au+I​2​​↛2AuI

When will it happen?

When the potential difference (OX minus RED) is positive!

2Na+Cl​2​​→2NaCl

1.36−(−2.71)=4.07

2Au+I​2​​↛2AuI

1.50−(0.80)=−0.70

Highly exothermic (and rather vigorous!)

Quite endothermic (will not happen by itself)

Of course, if you add energetic electrons (electricity of sufficient voltage) the endothermic reaction will work

Extra topics

Organic reductors/oxidizers

REDOX titrations

Organic REDOX

In organic chemistry, the following half reactions can occur:

These are the reductor half reactions

When going the other way, they are (of course) oxidizer half reactions

+H​2​​O

+2H​+​​+2e​−​​

+2H​+​​+2e​−​​

Primary, secondary en tertiary

1. Unsubstituted alcohol
2. Primary alcohol 
3. Secundary alcohol
4. Tertiary alcohol

alcohol as reductor 

• Step 1:  primary alcohol -> aldehyde

• In case of a secondary alcohol , the blue H is a CH3 group, and it becomes a ketone instead of an aldehyde
• With a tertiary alcohol, besides OH there is no 2nd H atom available, so this reaction cannot work...

alcohol as reductor 

• Stap 2: aldehyde -> carboxylic acid

• This cannot work with a secondary alcohol, because there is no blue H 

Yes, I know this is bad chemistry, I know the O from H2O can be either O atoms. I left that out so as not to confuse students.

Reaction mechanisms is a later module.

Redox titrations

These titrations do not utilize a color change of a pH indicator, but a color change (or precipitation) of a half reaction!

e.g.: MnO₄^- (permanganate ion) is purple and a powerful oxidizer

When you add this dropwise to a reductor solution, the solution will start to color purple once you have depleted the reductor!