ELECTROCHEM by Lizelle Swanepoel

1. GALVANIC CELL:

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The flow of electrons between species in a redox reaction can be harnessed through a wire connection, as shown below.

This is the setup for a Galvanic cell.
It converts chemical energy into electrical energy.

This makes a simple battery.

We call it a GALVANIC CELL
It is also called a VOLTAIC CELL

2. ELECTROLYTIC CELL: E electrical —> E chemical. Using electricity to make a chemical reaction occur

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ELECTROLYSIS
ELECTROLYTIC CELLS

TYPICAL EXAM QUESTION

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THE GALVANIC CELL

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USES OF ELECTROLYTIC CELLS - ELECTROPLATING

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Electroplating = a method of using an electric current to deposit a thin layer of metal over another from a solution containing a dissolved salt of the selected metal.

NOTE:
The anode is connected to the positive terminal:

The anode is generally composed of the material that we want to deposit.
E.g. Copper in the case of copper plating, nickel in the case of nickel plating.
It is also possible to plate using “inert” anodes like carbon rods in certain cases.

The cathode connected to the negative terminal:

The cathode is the component on which we intend to deposit the metal.
In a plating workshop this is commonly referred to as the “work-piece”.

The English physicist Michael Faraday (1791 – 1867) provided us with the important rules that govern electro-deposition:

He showed that the amount of metal deposited at the cathode and the amount dissolved at the anode are directly proportional to the quantity of electricity passed, in other words, to the current and time period of the electro-deposition.
Thanks to Faraday's efforts and Faraday's Constant (F = 96 500 C), we can calculate the deposit mass of any metal that we are plating based on its atomic mass and the number of electrons involved in the electrochemical reaction. The formula use is:
n = q/F (where n=number of moles of e^- transferred between electrodes, q=charge, F=96500 C)

Extraction of aluminium

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Copper purification

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Electroplating questions

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Stoichiometry of electrochem

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Counting electrons moving across and calculating mass of metal deposited

Faraday constant exercise

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Chlor-alkali industry

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Exercise

Membrane cell (Chlor-alkali industry)

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DIAPHRAGM CELL (Chlor-alkali industry)

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Cu vs Nitrate ions on redox table

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NO₃^-ions would be reduced in preference over Cu²⁺ions in the electrolyte solution.

So, to electroplate a medal with Copper, do not use copper(II) nitrate as electrolyte.
Use CuSO₄(aq)

CELLS COMPARISON

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Electrochemical cell vs. Electrolytic cell

VOLTAIC (GALVANIC) CELLS VS. ELECTROLYTIC CELLS

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Spontaneous reaction between Zn and CuSO4 solution.

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Reaction is SPONTANEOUS when the stronger reducing agent react with the stronger oxidizing agent.

WHAT IS ELECTROCHEMISTRY?

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Electrochemistry = The study of the relationship between chemical reactions and electricity.

Two main ways in which they interact:

E_chem-> E_electr.Some chemical reactions can generate electricity.
E_electric-> E_chem.Electricity can also cause a chemical reaction to happen that would not occur spontaneously.

Then came ALESANDRO VOLTA

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THE CHLOR ALKALI INDUSTRY

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There are three major types of cells used in the electrolysis of brine:

the mercury cell
the diaphragm cell
the membrane cell

The PURPOSE of the chlor alkali industry is to make:

Chlorine
Hydrogen gas
Sodium hydroxide (caustic soda)

Electrolytic Cell Question

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Q: Aluminium extraction:
1. What energy conversion takes place in the type of cell used in aluminium extraction?
2. Write the half reaction occurring at the anode.
3. Write the half reaction occurring at cathode.
4. Explain what happens with the graphite anodes during operation, and write the reaction to state this.
Also write the overall cell reaction down.
5. What is the advantage to using cryolyte here?
6. Give a disadvantage of using cryolite in this process.
7. Al2O3 is insoluble in water. You decide to extract aluminium from aqueous AlCl3 and electrolyse the solution.
Would you be successful in extraction Aluminium? w
Would aluminum ions or water be reduced at the cathode? Explain.

USE FARADAY'S CONSTANT TO CALCULATE MASS OF METAL DEPOSITED AT CATHODE

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Use:

Q = It
n = q/F
m = n.Mr

See below:

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INCREASING CURRENT AND EMF OF A CELL

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The CURRENT that an electrochemical cell can deliver is increased by:

INCREASING THE RATE OF THE FORWARD REACTION, WHICH RELEASES MORE ELECTRONS AT THE ANODE AND ACCEPTED AT THE CATHODE.
increasing the surface area (size) of the electrodes
increasing the diameter of the salt bridge and decreasing its length (making it wider and shorter)
increasing the concentration of the salt solution in the salt bridge.

The EMF (cell Voltage) is increased by:

Increasing the concentration of REACTANTS.
Decreasing the concentration of PRODUCTS.
This favours the forward reaction, which transfers more electrical energy in the form of more electrons moving from anode to cathode
HOWEVER - EMF IS NOT AFFECTED BY:
* Increased electrode surface area 
* Wider, shorter, more conductive salt bridge
             * Because those changes do not change the          
               concentration of species in solution.
             * Therefore it does not affect the equilibrium               
               position to favour either fwd or reverse 
               reaction in the nett ionic reaction.

The SHE

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Standard hydrogen electrode

The Mercury Cell

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Answer this

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Electroplating animation

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Exam questions

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The Membrane Cell

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Operation explained with animation.

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Electrolysis of brine - UCT Pack animations of all chlor-alkali cells

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Three cells used in industry to electrolyse brine:

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Mercury Cell
Diaphragm Cell
Membrane Cell

The Diaphragm Cell

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Animation

Extra Questions

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REDUCTION POTENTIAL TABLE

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INVESTIGATING THE REDUCTION OF METAL IONS

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REDOX INTRO

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Recap of previous section on electrochemistry.

NEW CONCEPT INTRO: TWO TYPES OF ELECTROCHEMICAL CELLS:

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1. Galvanic cell
2. Electrolytic cell

LET US STUDY THE TWO CELLS:

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First: We will study Galvanic cells.
Second: We will study Electrolytic cells.

Oxidation-reduction

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GALVANIC CELLS - EXERCISE 1 ANSWERS

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GALVANIC CELLS - EXERCISE 2 ANSWERS

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ELECTRONS ARE TRANSFERRED FROM ANODE TO CATHODE IN ELECTROCHEMICAL CELLS

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Can we work out the mass of metal produced at the cathode, if we know how many moles of electrons have moved across?

YES!
We call this QUANTITATIVE ELECTROCHEM.

TODAY'S LESSON

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INDUSTRIAL ELECTROLYSIS OF BRINE

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When a current is run through a brine solution (a high-concentration of NaCl and H₂O solution) in an electrolysis process, we can extract these products:

chlorine gas
hydrogen gas
sodium hydroxide (caustic soda)

BRINE SOLUTION - IONS AND MOLECULES PRESENT

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BRINE = AN ELECTROLYTE
ELECTROLYTE = Solution containing free ions and which can conduct electricity

Why do you think brine can conduct electricity?

DISSOCIATION PROCESS ANIMATION

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UCT PACK - ANIMATION OF THE CHLOR-ALKALI CELLS

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