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The basic electrolytic reactions involved are as follows: At the cathode :

A l + 3 + 3 e - A l ( s ) ( 99 % purity )

At the anode :

2 O 2 - O 2 ( g ) + 4 e -

The overall reaction is as follows:

2 A l 2 O 3 4 A l + 3 O 2

The only problem with this process is that the reaction is endothermic and large amounts of electricity are needed to drive the reaction. The process is therefore very expensive.

Summary

  • An electrochemical reaction is one where either a chemical reaction produces an external voltage, or where an external voltage causes a chemical reaction to take place.
  • In a galvanic cell a chemical reaction produces a current in the external circuit. An example is the zinc-copper cell.
  • A galvanic cell has a number of components . It consists of two electrodes , each of which is placed in a separate beaker in an electrolyte solution. The two electrolytes are connected by a salt bridge . The electrodes are connected two each other by an external circuit wire.
  • One of the electrodes is the anode , where oxidation takes place. The cathode is the electrode where reduction takes place.
  • In a galvanic cell, the build up of electrons at the anode sets up a potential difference between the two electrodes, and this causes a current to flow in the external circuit.
  • A galvanic cell is therefore an electrochemical cell that uses a chemical reaction between two dissimilar electrodes dipped in an electrolyte to generate an electric current.
  • The standard notation for a galvanic cell such as the zinc-copper cell is as follows:
    Z n | Z n 2 + | | C u 2 + | C u
    where
    | = a phase boundary ( solid / aqueous ) | | = the salt bridge
  • The galvanic cell is used in batteries and in electroplating .
  • An electrolytic cell is an electrochemical cell that uses electricity to drive a non-spontaneous reaction. In an electrolytic cell, electrolysis occurs, which is a process of separating elements and compounds using an electric current.
  • One example of an electrolytic cell is the electrolysis of copper sulphate to produce copper and sulphate ions.
  • Different metals have different reaction potentials . The reaction potential of metals (in other words, their ability to ionise), is recorded in a standard table of electrode potential . The more negative the value, the greater the tendency of the metal to be oxidised. The more positive the value, the greater the tendency of the metal to be reduced.
  • The values on the standard table of electrode potentials are measured relative to the standard hydrogen electrode .
  • The emf of a cell can be calculated using one of the following equations: E ( c e l l ) 0 = E 0 (right) - E 0 (left) E ( c e l l ) 0 = E 0 (reduction half reaction) - E 0 (oxidation half reaction) E ( c e l l ) 0 = E 0 (oxidising agent) - E 0 (reducing agent) E ( c e l l ) 0 = E 0 (cathode) - E 0 (anode)
  • It is possible to predict whether a reaction is spontaneous or not, either by looking at the sign of the cell's emf or by comparing the electrode potentials of the two half cells.
  • It is possible to balance redox equations using the half-reactions that take place.
  • There are a number of important applications of electrochemistry. These include electroplating , the production of chlorine and the extraction of aluminium .

Summary exercise

  1. For each of the following, say whether the statement is true or false . If it is false, re-write the statement correctly.
    1. The anode in an electrolytic cell has a negative charge.
    2. The reaction 2 KClO 3 2 KCl + 3 O 2 is an example of a redox reaction.
    3. Lead is a stronger oxidising agent than nickel.
  2. For each of the following questions, choose the one correct answer.
    1. Which one of the following reactions is a redox reaction?
      1. H C l + N a O H N a C l + H 2 O
      2. A g N O 3 + N a I A g I + N a N O 3
      3. 2 F e C l 3 + 2 H 2 O + S O 2 H 2 S O 4 + 2 H C l + 2 F e C l 2
      4. B a C l 2 + M g S O 4 M g C l 2 + B a S O 4
      (IEB Paper 2, 2003)
    2. Consider the reaction represented by the following equation: B r 2 ( l ) + 2 I a q - 2 B r a q - + I 2 ( s ) Which one of the following statements about this reaction is correct?
      1. bromine is oxidised
      2. bromine acts as a reducing agent
      3. the iodide ions are oxidised
      4. iodine acts as a reducing agent
      (IEB Paper 2, 2002)
    3. The following equations represent two hypothetical half-reactions: X 2 + 2 e - 2 X - (+1.09 V) and Y + + e - Y (-2.80 V) Which one of the following substances from these half-reactions has the greatest tendency to donate electrons?
      1. X -
      2. X 2
      3. Y
      4. Y +
    4. Which one of the following redox reactions will not occur spontaneously at room temperature?
      1. M n + C u 2 + M n 2 + + C u
      2. Z n + S O 4 2 - + 4 H + Z n 2 + + S O 2 + 2 H 2 O
      3. F e 3 + + 3 N O 2 + 3 H 2 O F e + 3 N O 3 - + 6 H +
      4. 5 H 2 S + 2 M n O 4 - + 6 H + 5 S + 2 M n 2 + + 8 H 2 O
    5. Which statement is CORRECT for a Zn-Cu galvanic cell that operates under standard conditions?
      1. The concentration of the Zn 2 + ions in the zinc half-cell gradually decreases.
      2. The concentration of the Cu 2 + ions in the copper half-cell gradually increases.
      3. Negative ions migrate from the zinc half-cell to the copper half-cell.
      4. The intensity of the colour of the electrolyte in the copper half-cell gradually decreases.
      (DoE Exemplar Paper 2, 2008)
  3. In order to investigate the rate at which a reaction proceeds, a learner places a beaker containing concentrated nitric acid on a sensitive balance. A few pieces of copper metal are dropped into the nitric acid.
    1. Use the relevant half-reactions from the table of Standard Reduction Potentials to derive the balanced nett ionic equation for the reaction that takes place in the beaker.
    2. What chemical property of nitric acid is illustrated by this reaction?
    3. List three observations that this learner would make during the investigation.
    (IEB Paper 2, 2005)
  4. The following reaction takes place in an electrochemical cell:
    C u ( s ) + 2 A g N O 3 ( a q ) C u ( N O 3 ) 2 ( a q ) + 2 A g ( s )
    1. Give an equation for the oxidation half reaction.
    2. Which metal is used as the anode?
    3. Determine the emf of the cell under standard conditions.
    (IEB Paper 2, 2003)
  5. The nickel-cadmium (NiCad) battery is small and light and is made in a sealed unit. It is used in portable appliances such as calculators and electric razors. The following two half reactions occur when electrical energy is produced by the cell. Half reaction 1: Cd ( s ) + 2 OH - ( aq ) Cd ( OH ) 2 ( s ) + 2 e - Half reaction 2: NiO ( OH ) ( s ) + H 2 O ( l ) + e - Ni ( OH ) 2 ( s ) + OH - ( aq )
    1. Which half reaction (1 or 2) occurs at the anode? Give a reason for your answer.
    2. Which substance is oxidised?
    3. Derive a balanced ionic equation for the overall cell reaction for the discharging process.
    4. Use your result above to state in which direction the cell reaction will proceed (forward or reverse) when the cell is being charged.
    (IEB Paper 2, 2001)
  6. An electrochemical cell is constructed by placing a lead rod in a porous pot containing a solution of lead nitrate (see sketch). The porous pot is then placed in a large aluminium container filled with a solution of aluminium sulphate. The lead rod is then connected to the aluminium container by a copper wire and voltmeter as shown.
    1. Define the term reduction .
    2. In which direction do electrons flow in the copper wire? (Al to Pb or Pb to Al)
    3. Write balanced equations for the reactions that take place at...
      1. the anode
      2. the cathode
    4. Write a balanced nett ionic equation for the reaction which takes place in this cell.
    5. What are the two functions of the porous pot?
    6. Calculate the emf of this cell under standard conditions.
    (IEB Paper 2, 2005)

Questions & Answers

formula of heat reaction
Anelisiwe Reply
final energy-initial energy of substances
Chantel
what is the difference between ethene and ethane
Joy Reply
Ethane is a saturated organic molecule while ethene is unsaturated Ethane has 2 carbon atoms and 6 hydrogen atoms, while ethene has 2 Carbons and 4 hydrogens Ethane has single bond between the 2 Carbon atoms while ethene has double bond
Rito
How do you know that the reaction is in acid medium or basic medium
Miss Reply
What formular is used when source and Observer are moving towards to each other
Esihle Reply
put a + on top n - on the bottom
Sanele
what is doppler effect?
Ansiwe Reply
doppler effect is the perceived change in frequency of a sound caused by either the source or the listener
Qaqamba
moving relative to each other
Qaqamba
A Catalyst is a chemical substance that speeds up a chemical reaction.
Sheron Reply
It does this by lowering the activation energy.
Sheron
what is exothermic
Nambile Reply
reaction that releases energy
Beauty
what is a catalyst
Joseph
Catalyst is to speed up the reaction
CELO
catalyst is a substance added into a solution to speed up the process
Joseph
As it speeds up the reaction by lowering the activation energy, it does not take part in the reactants or products formed
Rito
reaction that release energy
Qaqamba
what is exothermic
Nambile Reply
reaction that gives off heat to the surroundings
Thabile
formula for calculating the heat of reaction
Anelisiwe
I need help in electrostatic
Muthuphei Reply
in exactly in electrostatic
Beauty
send the question on electrostatic maybe I can help
Beauty
factors affecting the rate of reaction
thabo Reply
hi
Xiluva
hi
Mashweme
hey
Musa
I'm xiluva ...do we help those who are having challenges on their studies?
Xiluva
I mean on physical science
Xiluva
I believe it's supposed to be like that. Helping those who are having educational challenges
Rito
Yep physical sciences more specifically
Rito
ok ....nna I need some help in physics electricity ...just hints how we do what n how to solve some of the problem... just hints
Xiluva
The first thing you can do is to understand ohm's law and the difference between a series and parallel connection concerning current, resistance and potential difference
Thendo
Hi Xiluva,not sure if this will be helpful but to understand electricity better I suggest you download Siyavula physical science books there's both a learner book and a teacher's guide I was struggling too they really did help you might be able to do some exercises after reading siyavula material
LUTHO
what diode mean
Ansiwe
diode s a part of electrical current tht allows only moves to one direction ..only ...
Frank
allows current .....to flow to one direction I meant .
Frank
ok thanks
Ansiwe
ok thanks guys
Xiluva
temperature surface area concentration addition of a catalyst
Joseph
thank you mr
Mzukisi
How to calculate e.p
zama Reply
ep=mgh
mudodzwa
Ep=mgh
Maximurm
What is polymer
Kwinda Reply
chemical compound with molecules bonded together in long ,repeating chains
Bukani
hey guys i need help with the topic of forces and their application
Monica Reply
A force is a pull or push and it can be applied in rocket launchers moving cars & stoping (breaking them).
Duncan
A force is that Agency which changes the object's state of rest or motion. A push or pull are the effects of force.
Katiso
are you sure u wantd to ask tht or u wantd to say " how many bonds are there in alkane nd their names?" ?
Frank Reply
I also think that's what she meant 'coz I don't really understand her question
LINDOKUHLE
yeah nd if that's th case , there are only single bonds wch I could say one of em tht I know is methane
Frank
I mean methane,ethane they end up to what
Sylvia
well I also assume ths is wah u lookin for .. methane , ethane , propane, butane, pentane, hexane , Heptane, octane , nonane , decane . is ths wah u wanted ?
Frank
how many alkane are there n there names
Sylvia Reply

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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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