<< Chapter < Page Chapter >> Page >

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

how does the earth revolute
Kevin Reply
What is the momentum
Given Reply
The product of the object's mass and it's velocity
Mvula
electric field def
FREENCO
how is ester formed
Aubrey Reply
how is n ester formed
Aubrey
Alcohol reacts with a carboxylic acid
Texas
and the reaction is catalysed by sulphuric acid
Vhulenda
an athlete with a mass of 70kg runs at a velocity of 45km . determine the athlete's momentum
Lesedi Reply
Is that a velocity or something else
msawenkosi
45km/h i guess
Texas
Change to m/s
Texas
45km/h = 12.5 m/s p=mv =70×12.5 =875 kg.m/s
Thato
what are the measures of the rates of reaction
Lesego Reply
Volume Concentration Temperature Pressure Surface Area
Thato
the principle of superposition of waves
Sfundo Reply
what is work
Kool Reply
is this a group chat
Nobuhle Reply
Hey can y'all define newton's 2nd law
mthebzification
If a resultant force act on an object...the object will accelerate in the direction of a resultant force,the acceleration of the object is directly proportional to the net force and inversely proportional to the mass of the object
mosa
how do you calculate tension force
Bulumko
use the formula Fnet=ma if there is tension connecting two objects
Sboniso
to calculate Tension, usually calculate acceleration first Draw separate free body diagrams for each body. Apply Fnet = ma to calculate Tension
Kevin
Hi people
Paul
how does temperature affect the equilibrium position
Blessing Reply
an increased temperature increases the average kinetic energy thus in turn increases the number of effective collisions........
Lwando
so...which reaction is favored between endothermic and exothermic .when temperature is increased..?
Blessing
exothermic reaction because energy is realised to the surroundings as heat and light energy ....graphical so much energy is realised as reactants to form product and because temperature is high rate of reaction is fast which means there is a successful collision
Code
INTEMENDO - INCREASE IN TEMPERATURE FAVOURS ENDOTHERMIC DETEMEXO - DECREASE IN TEMPERATURE FAVOURS EXOTHERMIC
Thato
an object will continue in a state of rest unless it is acted upon an unbalanced force
Junior Reply
Newton's Law 1
Code
First Newton's Law
Azola
Newton's first law
Surprise
newton first law
Thinavhuyo
Newton's first law
Blessing
when pressure is increased what happen to volume
Siphelo Reply
decreases
Code
care to explain?
Mpati
if pressure is applied to a pistol , the volume will decrease and particles will collide more frequently with the wall of a container .Each time they collide with the wall they exert a force on them .More collision means more force and the pressure will increase , that Boyle's Law
Code
Because the volume has decreased , the particle will collide more frequently with the wall of a container and each time they collide with the wall of a container they exert a force on them.More collision means more force so the pressure will increase , that Boyle's Law
Code
what is the difference between momentum and a change in momentum?
Chavonne Reply
Change in momentum is the impulse of the object
Rachel
Momentum is a vector quantity equal to the product of the mass of the particle and its velocity
Mmola
How to name a branched molecule from right or left?
Vadin Reply
What's Free Fall
Senzo Reply
Free Fall means there is no acting force on that object.
Dingaletu
only gravitational force
Dingaletu
no external force acting on an object
Sphiwe
by only force of gravite
Sello
but gravitational force
Sphiwe
true
Lucky
a motion in which the only force acting is gravitational force
Blessing
and an object experiencing free fall is referred as a projectile
Blessing
Do polymers form restrictedly only if compound is saturated, only?
milani Reply

Get the best Siyavula textbooks: gr... course in your pocket!





Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 12 physical science' conversation and receive update notifications?

Ask