Electrochemistry

Transfer reactions

Electron transfer

Redox reactions

Will it react?

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This lesson contains 18 slides, with text slides.

Lesson duration is: 45 min

Items in this lesson

Electrochemistry

Transfer reactions

Electron transfer

Redox reactions

Will it react?

Slide 1 - Slide

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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

Slide 2 - Slide

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But there's more to it...

Let's zoom in on the following synthesis reaction:

2 F e + O^{​ 2 ​ ​} \to 2 F e O

Slide 3 - Slide

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But there's more to it...

Let's zoom in on the following synthesis reaction:

2 F e + O^{​ 2 ​ ​} \to 2 F e O

F e^{​ 0 ​ ​}

F e^{​ 2 + ​ ​}

O^{​ 2 - ​ ​}

O^{​ 0 ​ ​}

molecular oxygen

What's really happening here?

metallic iron

Slide 4 - Slide

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But there's more to it...

Let's zoom in on the following synthesis reaction:

2 F e + O^{​ 2 ​ ​} \to 2 F e O

F e^{​ 0 ​ ​}

F e^{​ 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

Slide 5 - Slide

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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):

F e^{​ 0 ​ ​} \to F e^{​ 2 + ​ ​} + 2 e

O^{​ 2 ​ 0 ​ ​} + 4 e \to 2 O^{​ 2 - ​ ​}

the "reductor" producers electrons

the "oxidizer" consumes electrons

Slide 6 - Slide

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

Slide 7 - Slide

Assignment 3a (REDOX I) after this slide.

But there's more to it...

Let's zoom in on the following dissociation reaction:

2 F e^{​ 2 ​ ​} O^{​ 3 ​ ​} \to 4 F e O + O^{​ 2 ​ ​}

Slide 8 - Slide

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But there's more to it...

Let's zoom in on the following dissociation reaction:

2 F e^{​ 2 ​ ​} O^{​ 3 ​ ​} \to 4 F e O + O^{​ 2 ​ ​}

F e^{​ 3 + ​ ​}

O^{​ 2 - ​ ​}

F e^{​ 2 + ​ ​}

O^{​ 2 - ​ ​}

O^{​ 0 ​ ​}

(molecular oxygen)

What's really happening here?

Slide 9 - Slide

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But there's more to it...

Let's zoom in on the following dissociation reaction:

2 F e^{​ 2 ​ ​} O^{​ 3 ​ ​} \to 4 F e O + O^{​ 2 ​ ​}

F e^{​ 3 + ​ ​}

O^{​ 2 - ​ ​}

F e^{​ 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.

Slide 10 - Slide

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When will it happen?

I can tell you that this works:

Sadly, this does not:

WHY??!!

2 N a + C l^{​ 2 ​ ​} \to 2 N a C l

2 A u + I^{​ 2 ​ ​} ↛ 2 A u I

Slide 11 - Slide

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When will it happen?

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

2 N a + C l^{​ 2 ​ ​} \to 2 N a C l

1.36 - (- 2.71) = 4.07

2 A u + I^{​ 2 ​ ​} ↛ 2 A u I

1.50 - (0.80) = - 0.70

Highly exothermic (and rather vigorous!)

Quite endothermic (will not happen by itself)

Slide 12 - Slide

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

Extra topics

Organic reductors/oxidizers

REDOX titrations

Slide 13 - Slide

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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

+ 2 H^{​ + ​ ​} + 2 e^{​ - ​ ​}

+ 2 H^{​ + ​ ​} + 2 e^{​ - ​ ​}

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Primary, secondary en tertiary

Unsubstituted alcohol
Primary alcohol
Secundary alcohol
Tertiary alcohol

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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...

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alcohol as reductor

Stap 2: aldehyde -> carboxylic acid

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

Slide 17 - Slide

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!

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Electrochemistry

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