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The electrolytic cell

In "The Galvanic Cell" , we saw that a chemical reaction that involves a transfer of electrons, can be used to produce an electric current. In this section, we are going to see whether the 'reverse' process applies. In other words, is it possible to use an electric current to force a particular chemical reaction to occur, which would otherwise not take place? The answer is 'yes', and the type of cell that is used to do this, is called an electrolytic cell .

Electrolytic cell

An electrolytic cell is a type of cell that uses electricity to drive a non-spontaneous reaction.

An electrolytic cell is activated by applying an electrical potential across the anode and cathode to force an internal chemical reaction between the ions that are in the electrolyte solution. This process is called electrolysis .

Electrolysis

In chemistry and manufacturing, electrolysis is a method of separating bonded elements and compounds by passing an electric current through them.

Demonstration : the movement of coloured ions

A piece of filter paper is soaked in an ammonia-ammonium chloride solution and placed on a microscope slide. The filter paper is then connected to a supply of electric current using crocodile clips and connecting wire as shown in the diagram below. A line of copper chromate solution is placed in the centre of the filter paper. The colour of this solution is initially green-brown.

The current is then switched on and allowed to run for about 20 minutes. After this time, the central coloured band disappears and is replaced by two bands, one yellow and the other blue, which seem to have separated out from the first band of copper chromate.

Explanation:

  • The cell that is used to supply an electric current sets up a potential difference across the circuit, so that one of the electrodes is positive and the other is negative.
  • The chromate (CrO 4 2 - ) ions in the copper chromate solution are attracted to the positive electrode, while the Cu 2 + ions are attracted to the negative electrode.

Conclusion:

The movement of ions occurs because the electric current in the outer circuit sets up a potential difference between the two electrodes.

Similar principles apply in the electrolytic cell, where substances that are made of ions can be broken down into simpler substances through electrolysis.

The electrolysis of copper sulphate

There are a number of examples of electrolysis. The electrolysis of copper sulphate is just one.

Demonstration : the electrolysis of copper sulphate

Two copper electrodes are placed in a solution of blue copper sulphate and are connected to a source of electrical current as shown in the diagram below. The current is turned on and the reaction is left for a period of time.

Observations:

  • The initial blue colour of the solution remains unchanged.
  • It appears that copper has been deposited on one of the electrodes but dissolved from the other.

Explanation:

  • At the negative cathode, positively charged Cu 2 + ions are attracted to the negatively charged electrode. These ions gain electrons and are reduced to form copper metal, which is deposited on the electrode. The half-reaction that takes place is as follows: C u 2 + ( a q ) + 2 e - C u ( s ) (reduction half reaction)
  • At the positive anode, copper metal is oxidised to form Cu 2 + ions. This is why it appears that some of the copper has dissolved from the electrode. The half-reaction that takes place is as follows: C u ( s ) C u 2 + ( a q ) + 2 e - (oxidation half reaction)
  • The amount of copper that is deposited at one electrode is approximately the same as the amount of copper that is dissolved from the other. The number of Cu 2 + ions in the solution therefore remains almost the same and the blue colour of the solution is unchanged.

Conclusion:

In this demonstration, an electric current was used to split CuSO 4 into its component ions, Cu 2 + and SO 4 2 - . This process is called electrolysis .

The electrolysis of water

Water can also undergo electrolysis to form hydrogen gas and oxygen gas according to the following reaction:

2 H 2 O ( l ) 2 H 2 ( g ) + O 2 ( g )

This reaction is very important because hydrogen gas has the potential to be used as an energy source. The electrolytic cell for this reaction consists of two electrodes (normally platinum metal), submerged in an electrolyte and connected to a source of electric current.

The reduction half-reaction that takes place at the cathode is as follows:

2 H 2 O ( l ) + 2 e - H 2 ( g ) + 2 O H - ( a q )

The oxidation half-reaction that takes place at the anode is as follows:

2 H 2 O ( l ) O 2 ( g ) + 4 H + ( a q ) + 4 e -

A comparison of galvanic and electrolytic cells

It should be much clearer now that there are a number of differences between a galvanic and an electrolytic cell. Some of these differences have been summarised in [link] .

A comparison of galvanic and electrolytic cells
Item Galvanic cell Electrolytic cell
Metals used for electrode Two metals with different reaction potentials are used as electrodes The same metal can be used for both the cathode and the anode
Charge of the anode negative positive
Charge of the cathode positive negative
The electrolyte solution/s The electrolyte solutions are kept separate from one another, and are connected only by a salt bridge The cathode and anode are in the same electrolyte
Energy changes Chemical potential energy from chemical reactions is converted to electrical energy An external supply of electrical energy causes a chemical reaction to occur
Applications Run batteries, electroplating Electrolysis e.g. of water, NaCl

Electrolyis

  1. An electrolytic cell consists of two electrodes in a silver chloride (AgCl) solution, connected to a source of current. A current is passed through the solution and Ag + ions are reduced to a silver metal deposit on one of the electrodes.
    1. Give the equation for the reduction half-reaction.
    2. Give the equation for the oxidation half-reacion.
  2. Electrolysis takes place in a solution of molten lead bromide (PbBr) to produce lead atoms.
    1. Draw a simple diagram of the electrolytic cell.
    2. Give equations for the half-reactions that take place at the anode and cathode, and include these in the diagram.
    3. On your diagram, show the direction in which current flows.

Questions & Answers

can you mix kc nd pressure
Khanyisile Reply
equation for dissociation
Amanda Reply
under which topic
Phila
acids and bases
Amanda
Newton's law
Anna
How does a catalyst affect the rate of chemical reaction
Rebone Reply
it speeds up the reaction without being by the reaction. in other words it increases the Reaction rate by lowering the activation energy of a reaction
Keabetsoe
hey
Siphokazi
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Keabetsoe
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Holy
it only speed up the reaction not to affect the reaction
John
what is a compound?
Sithembiso Reply
define Doppler effect
Emihle Reply
It is the change in frequency of a wave in relation to an observer who is moving relative to the wave source.
Junior
Doppler effect is the change in frequency or pitch detected by a listener because the velocity of the sound source is different from that of a listener relative to the medium of sound propagation
John
explain how color is an unreliable clue to.the identify minerals
Camille Reply
Hi I want to know where the topic for electric circuits are?
Zahra Reply
hello.. How to deal with Reactions rate?. i dont Understand them
Simphiwe Reply
Firstly you must know how energy changes during chemical reactions...secondly the collision theory.... Rate and extent of reactions.... And factors that affect the rate of chemical reactions... If u capture all the topic then this chapter will be easy for you the is no way you can fail it...
Pearly
thank you
Petty
Heii.. I'm struggling with chemical equilibrium.
Ronny
how do I attempt chemical equilibrium?
Ronny
doppler effect question
Mabikinyane Reply
How do I differentiate from the formula of a listener who is moving away and who is moving towards the source
Njabulo
if the its away you will subtract on the numerator and add in the denominator but if its towards its the opposite
Ashley
how cn I calculate sound of a source if it's unkown
Simphiwe Reply
Use what you were given. What were you given?
Bandile
where can I find acids and bases
muravha Reply
how to calculate avarage power?
Bongani Reply
good result for bursary
Soluiswayi Reply
Good day Sir
noma
Good day Mam
Aobakwe
good day
clifort
How are you
Aobakwe
4. A builder of mass 75 kg is carrying bricks up a flight of stairs. There are 40 steps, each 25 cm high and he takes 10 s to climb the stairs, carrying 15 kg of bricks at a time. Calculate the total power output of the builder during his climb at constant speed.
Asanda Reply
I want to know how to calculate.
Tuauaa
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CHRISTOPHER
How it was calculated 😕
Nthabeleng
1stly describe Hydrocarbon for me please
Magadi Reply
Compounds that consists of carbon and hydrogen atoms only.
Learner
Compounds that consists of carbon and hydrogen atoms only.
Learner

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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