transcript
- Slide 1
- Slide 2
- Conduction of electricity conductor is a solid which allows
electricity to pass through but is not chemically changed during
the conduction presence of freely moving valence electrons e.g. all
metals and graphite conducting rods & wires are used in
electrolysis cells non-conductor is a solid which does not allow
electricity to pass through, a.k.a insulator valence electrons are
held in fixed positions e.g.. sulfur, phosphorus, diamond, solid
state crystalline salts, wood and glass etc.
- Slide 3
- Electrolysis electrolysis is the process of breaking up an
ionic compound into simpler substances, usually elements, using
electricity electrolysis occurs when the ionic compound is in
aqueous solution or molten because it allows electric current to
pass through electrolysis is carried out in an electrolysis cell
electricity is passed from a cell or battery through a liquid which
may be a aqueous solution or molten solid a molten ionic compounds
or aqueous solution of ionic compounds that allows electricity to
pass through is called an electrolyte the rods which carry the
electric current to and from the electrolyte are called electrodes
Cell (Source of electricity) Beaker Cathode Electrode Anode
Electrode Electrolyte Conducting wire
- Slide 4
- Electrodes electrodes are rods that carry the electric current
to and from an electrolyte they are usually; platinum, copper,
graphite Cathode: electrode that is connected to the negative
terminal of the cell or battery. positively charged ions, cations,
moved towards the cathode Anode: electrode that is connected to the
positive terminal of the cell or battery. negatively charged ions,
anions, moved towards the anode
- Slide 5
- Electrolytes and non-Electrolytes electrolytes: molten ionic
compounds or aqueous solution of ionic compounds that allows
electricity to pass through and are decomposed in the process e.g..
acids, alkali, salts dissolved in water, molten salts
non-electrolytes: does not allow passage of electricity e.g..
distilled water, alcohol, turpentine, oil, paraffin, organic
solvents
- Slide 6
- Electrolysis when electricity is passed through an electrolyte,
chemical decomposition occurs this involves the splitting up of the
electrolyte since all electrolytes are ionic, composed of
positively charged ions (cations) & negatively charged ions
(anions) when an electric current pass through the electrolyte,
ions in the solution or liquid migrate towards the oppositely
charged electrode discharge of ions at the electrodes results in
the chemical decomposition of the electrolyte to form its elements
at the anode, negatively charged ions lose their electron(s) to the
anode to form neutral atoms the negatively charged ions are said to
be oxidized and discharged at the anode - oxidation occurs at the
anode at the cathode, positively charged ions gain electron(s) from
the cathode to form neutral atoms the positively charged ions are
said to be reduced and discharged at the cathode - reduction occurs
at the cathode metals or hydrogen are formed at the negative
electrode (cathode), and that non-metals (other than hydrogen) are
formed at the positive electrode (anode).
- Slide 7
- Electrolysis of molten Compounds many ionic compounds are
binary compounds i.e. compounds containing only 2 elements e.g. KI,
PCl 2, NaCl binary ionic compound contains a metal cation and a
non-metal anion electrolysis of a molten binary compound will yield
a metal and a non-metal as products metal will be formed at the
cathode while non-metal will be formed at the anode
- Slide 8
- Electrolysis of molten Compounds Compound Electrolysed Product
formed at; Cathode (-)Anode(+) Copper Bromide Zinc Chloride Sodium
Chloride Aluminium Oxide Copper Zinc Sodium Aluminium Bromine
Chlorine Oxygen
- Slide 9
- Electrolysis of Molten Lead(II) Bromide
- Slide 10
- At the cathode: Pb 2+ ions gain electrons from the electrodes
to become lead atoms Pb 2+ ions have been discharged and molten
greyish globules of lead metal are formed below the electrolyte
electrode reaction at the cathode; Pb 2+ (l) + 2e - Pb (l) At the
anode: Br - ions lose electrons to electrode to become bromine
molecules - Br - are oxidized Bromine atoms combine to form bromine
molecules forming an effervescence of pungent, red-brown bromine
gas electrode reaction at the anode; 2 Br - (l) Br 2(g) + 2e -
- Slide 11
- Predicting products in the electrolysis of aqueous solutions
using inert electrodes e.g. graphite;
- Slide 12
- Electrolysis of Aqueous Solutions an aqueous solution of an
ionic compound is a mixture of 2 electrolytes one of which is water
although water is molecular, in aqueous solutions, a small % of it
ionizes into hydrogen ions & hydroxide ions; H 2 O (l) H + (aq)
+ OH - (aq) e.g. aqueous copper (II) sulphate; contains water &
copper (II) sulphate water ionises into; hydrogen ions &
hydroxide ions copper (II) sulphate ionises into; copper (II) ions
and sulphate ions ions discharged at the electrodes depends on the
position of the ions in the electrochemical series
- Slide 13
- Rules for Predicting Selective Discharge of Cations cations
from the metals lowest in the reactivity series are discharged at
the cathode in preference to any other cation present in the
solution cations of less reactive metals e.g. Cu 2+, Au +, Ag + are
preferentially discharged otherwise, H + ions from water will be
discharged to form H 2 gas 2H + (aq) + 2e - H 2(g) cations of very
reactive metals (Na +, K +, Ca 2+ ) cannot be discharged in the
presence of water, so they remain in solution
- Slide 14
- Rules for Predicting Selective Discharge of Anions OH - ions
from water are preferentially discharged when the solutions are
dilute, to form O 2 4OH - (aq) O2 (g) + 2H 2 O (l) + 4e - anions
such as Cl -, Br - and I - can be preferentially discharged when
their concentrations are high enough when compared to OH - when SO
4 2- and NO 3 - are present in water, it is the OH - from water
which is preferentially discharged
- Slide 15
- Electrolyte Cations & anions present Product formed at;
Cathode (-)Anode(+) Concentrated sodium chloride Na +, H +, Cl -
& OH - HydrogenChlorine Dilute sulfuric acid Dilute sodium
chloride Concentrated hydrochloric acid Dilute Silver nitrate
Dilute Copper nitrate Dilute hydrochloric acid H +, SO 4 2- &
OH - Na +, H +, Cl - & OH - H +, Cl - & OH - Ag +, H +, NO
3 - & OH - Cu 2+, H +, NO 3 - & OH - H +, Cl - & OH -
Hydrogen Silver Copper Hydrogen Oxygen Chlorine Oxygen
- Slide 16
- Electrolysis of dilute sodium chloride solution at the anode;
anions OH - & Cl - moves to the anode OH - ions are selectively
discharge according to the position of ions in the electrochemical
series gas bubbles are formed which re-lights a glowing wooden
splinter -this gas is oxygen 4OH - (aq) 2H 2 O (l) + O 2(g) + 4e -
at the cathode; cations H + & Na + moves to the cathode H +
ions are selectively discharge according to the position of ions in
the electrochemical series gas bubbles are formed, when a lighted
wooden splinter is placed near the test tube, a 'pop' sound is
produced this gas is hydrogen gas 2 H + (aq) + 2e - H 2(g)
- Slide 17
- Electrolysis of dilute sulfuric acid
- Slide 18
- Electrolysis of dilute sulphuric acid dilute sulphuric acid can
be electrolysed using the apparatus shown at the cathode: H + ions
attracted to the cathode electrode reaction at the cathode(-); 2H +
(aq) + 2e - H 2(g) hydrogen gas is produced at the cathode (gas B)
at the anode: both OH - and SO 4 2- will be attracted to the anode
but OH - ions are preferentially discharged electrode reaction at
the anode(+); 4 OH - (aq) O 2(g) + 2H 2 O (l) +4e - oxygen gas is
produced at the anode (gas A)
- Slide 19
- overall Reaction; 2H 2 O (l) 2H 2(g) + O 2(g) this reaction is
sometimes known as electrolysis of acidified water sulphuric acid
is added to increase the number of mobile ions to help conduct
electricity in this process, the amount of acid remains the same,
but amount of water decreases - hence, the concentration of
sulphuric acid increases how do you test for gas A (oxygen) &
gas B (hydrogen)?
- Slide 20
- Electrolysis of aqueous copper(II)sulphate using carbon
electrodes copper (II) sulfate solution contains the following
ions; blue Cu 2+, SO 4 2- and H + & OH - from water Cu 2+ &
H + migrate to the cathode while SO 4 2- & OH - migrate to the
anode at the cathode; Cu 2+ ions are preferentially discharged and
deposited on the cathode as a brown deposit of solid copper Cu 2+
(aq) + 2e Cu (s) at the anode; OH - ions are preferentially
discharged and oxygen bubbles are given off 4 OH - (aq) O 2(g) + H
2 O (l) +4e - the blue colour of copper (II) sulfate solution fades
as Cu 2+ ions are discharged
- Slide 21
- Electrolysis of aqueous copper(II)sulphate using copper
electrodes copper (II) sulfate solution contains the following
ions; blue Cu 2+, SO 4 2- and H + & OH - from water Cu 2+ &
H + migrate to the cathode while SO 4 2- & OH - migrate to the
anode at the cathode; Cu 2+ ions are preferentially discharged and
deposited on the cathode as a brown deposit of solid copper thus
the cathode grows thicker Cu 2+ (aq) + 2e - Cu (s) at the anode;
electrodes are not inert, copper loses electrons forming copper
ions which goes into solution Cu (s) Cu 2+ (aq) + 2e - the anode
gets thinner Cu 2+ ions formed at the anode move to the cathode
where they are deposited the blue colour of copper (II) sulfate
solution does not fade because Cu 2+ ions removed from the solution
are replaced by those from the anode
- Slide 22
- Electroplating electroplating is the application of a thin
layer of metal to a metallic or other conducting surface through
electrolysis the object to be plated is made the cathode (- ve
electrode) of an electrolysis cell the anode is usually a bar of
the plating metal the object is immersed in an aqueous solution
containing cation of the plating metal during electrolysis, the
plating metal (anode) dissolves forming cation which are deposited
on the object (cathode) as metal atoms
- Slide 23
- Electroplating steel key with copper to which terminal of the
battery is the copper foil & steel key connected to? copper
foil is connected to the + ve terminal to become the anode &
while the steel key is connected to the - ve terminal to become the
cathode suggest a suitable substance to be used as an electrolyte
aqueous solution of copper (II) sulfate/nitrate/chloride what
observations would be made at the electrodes when electric current
is passed through the electrolyte for some time? anode;- copper
foil gradually become thinner Cu (s) Cu 2+ (aq) + 2e - cathode;-
steel key gradually get coated with copper Cu 2+ (aq) + 2e - Cu (s)
giving a reason, predict what observation(s) will be made in the
electrolyte during electrolysis the colour of the copper sulfate
electrolyte remains the same as the Cu 2+ ions removed from the
solution at the cathode are replaced by Cu 2+ ions formed at the
anode
- Slide 24
- Uses of Electroplating nickel, silver, tin & gold are the
most commonly used metals for electroplating articles are
electroplated to; improve their appearance;- plating gives the
object a shiny surface that does not get dull, - gold plating is
used for jewelry to provide a protective coating;- steel objects
are coated with a layer of tin which protects the metal underneath
from air and water thus protecting it from rusting to give them
special surface properties to give them engineering or mechanical
properties
- Slide 25
- Manufacture of Cl 2, H 2 & NaOH from brine brine is a
concentrated aqueous solution of sodium chloride i.e. sodium
chloride (NaCl) dissolved in water (H2O) brine is obtained from sea
water or from rock salt underground electrolysis of brine is used
to manufacture chlorine, sodium hydroxide & hydrogen on a large
scale chlorine is used for making disinfectants & bleaches,
manufacture of hydrochloric acid and for water treatment sodium
hydroxide is used for making soap & in extraction of aluminium
hydrogen is used for manufacture of ammonia, making margarine &
as a pollution-free fuel
- Slide 26
- Electrolysing brine brine is electrolyzed in a diaphragm cell
shown in the diagram aqueous sodium chloride contains; Na +, H +,
Cl - & OH - ions Na + & H + migrates to the cathode while
Cl - & OH - migrates to the anode at the anode; Cl - ions are
preferentially discharged forming chlorine gas 2Cl - (aq) Cl 2(g) +
2e - at the cathode; H + ions are preferentially discharged forming
hydrogen gas 2H + (aq) + 2e - H 2(g) Na + & OH - ions remain in
the solution forming an aqueous sodium hydroxide (NaOH)
solution
- Slide 27
- Manufacture of aluminium from aluminium oxide extraction of
aluminium from aluminium oxide is done by electrolysis first the
aluminium oxide must be molten to allow electricity can pass
through it aluminium oxide has a very high melting point (2,070C),
so it would be expensive to melt it instead, aluminium oxide is
dissolved in molten cryolite, sodium aluminium fluoride (Na 3 AlF 6
), which has a lower melting point (about 1000C) use of cryolite
reduces some of the energy costs involved in extracting aluminium
and also improves electrical conductivity aluminium is the most
abundant metal in Earths crust it occurs as metal ore called
bauxite which contains about 50% Al 2 O 3 bauxite is purified to
yield a white powder, aluminium oxide, from which aluminium can be
extracted
- Slide 28
- electrolysis is carried out using inert graphite electrodes in
steel electrolysis cell the electrolyte is molten aluminium oxide
Al 2 O 3(s) 2Al 3+ (l) + 3O 2- (l) at the cathode; aluminium ions
(Al 3+ )gain electrons forming aluminium metal Al 3+ (l) + 3e - Al
(l) molten aluminium is siphoned out of the steel electrolysis cell
at the anode; oxide ions (O 2- ) lose electrons forming oxygen
atoms which combine to form oxygen gas 2O 2- (l) O 2(g) + 4e -
oxygen reacts with hot carbon anodes forming carbon dioxide thus
the anodes needs to be replaced from time to time overall equation
for the electrolysis; 2Al 2 O 3(s) 4Al (l) + 3O 2(g)
- Slide 29
- Summary Factors affecting discharge of Ions Uses of
electrolysis relative positions of the ions in the electrochemical
series concentration of the ions in the electrolyte nature of the
electrode inert or reactive purification of metals electroplating
extraction of metals manufacture chlorine, sodium hydroxide &
hydrogen from brine
- Slide 30
- Electricity and chemistry Revision
- Slide 31
- End of Topic C 5 Read your textbook pages 70 to 81 and answer
the questions on page 84 & 85 Conductors and insulators (pp82
83) is not part of your syllabus but it wont harm your brain if you
read through! Best regards ~ Dnl