REVISION ELECTROCHEMISTRY. OXIDATION is a loss of electrons An increase in oxidation number...
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Transcript of REVISION ELECTROCHEMISTRY. OXIDATION is a loss of electrons An increase in oxidation number...
REVISIONELECTROCHEMISTRY
OXIDATIONis a loss of electrons
An increase in oxidation number
REDUCTIONis a gain of electrons
A decrease in oxidation number
CATHODEThe electrode where reduction takes place
ANODEThe electrode where oxidation takes place
ELECTROLYTEsolution/liquid/dissolved substance that conducts
electricity through the movement of ions.
TABLE OF STANDARD REDUCTION POTENTIALS
STANDARD CONDITIONS
• C• 25• Pressure of 1atm (101,3 kPa)• Metal electrodes• When a half-cell consists of a gas, Pt is used to
provide the surface area for the reaction
2
2
The two equations represent the halfreactions that
occur in an electrochemical cell.
Pt + 2e Pt E = + 1.20V
Sr + 2e Sr E = -2.89V
Which one of the follow
2 2
ing is TRUE when the cell is
working?
The mass of the Sr electrode will decrease.
B Pt oxidizes easier than Sr
C Pt reduces easier than Sr .
D In the external circuit, electrones will flow from
t
C
he P
A
t-electrode to the Sr-electrode
GALVANIC CELL
a cell in which chemical energy is converted into electrical energy.
A galvanic (voltaic) cell has self-sustaining electrode reactions
SPONTATEOUS REACTIONS
CELL POTENTIALS• Driving force of a cell to make electrons move -
away from the anode to the cathode.• . • = -• =reduction-oxidation
CELL NOTATION
• The H2|H+ half-cell is treated just like any other half-cell. . • Cell terminals (electrodes) are written on the outside of the cell
notation.
• Active electrodes reducing agent | oxidised species || oxidising agent | reduced species
• Inert electrodes (usually Pt or C): Pt | reducing agent | oxidised species || oxidising agent | reduced species | Pt
Example: Pt | Cℓ-(aq) |Cℓ2(g) || F2(g) | F-(aq) | Pt
SALT BRIDGE
Its function is to maintain ELECTRICALneutrality
During an experiment the following magnesium-lead electrochemical cell is built.
Both half cells contain nitrate salt solutions. M en N represent electrodes
stel elektrodes voor. A lamp wick drenched in
an electrolyte is used as a salt bridge
Write down the values of the standard conditions
Which electrode is the anode - give a reason for your answer
Write down the half reaction that occurs at elect
a
b
c
rode N
Write the cell notation for this cell
Calculate the emf of the cell when it is working
The lamp wick is now replaced with a convensional salt bridge (glass tube)
Does this change the emf of the
d
e
f cell? Explain
0 0
2
2 2
0
M is the anode, because electrons are flowing away form M
Mg is the anode, because Mg is being oxidised
Pb ( ) 2 '
Mg | ( )|| | ( )
, the
0.13 2.
e
36
2.2
f
3
m
sel katode anodeE
b
c aq e Pb s
d s Mg aq Pb aq Pb s
e
f o
E
N
E
V
is only dependent on the concentration of the reactants
Magnesium is used to protect underground iron pipes against rusting.
The diagram below shows an iron pipe connected to a magnesium bar.
Use the Table of Standard Reduction Potentials to explaina why
m
agnesium can be used to protect an iron pipe against rusting
The iron pipe in contact with the magnesium bar forms an electrochemical
cell. What serves as the salt bridge of this cell?
Give a r
b
c eason
why the magnesium bar must be replaced after some time
Write down a half-reaction to support your answerd to c
2
2
is a stronger reducing agent than
grondwater
magnesium plate is the anode and will
be oxidised.
oxidised to ( )
Mg ( ) 2 '
a Mg Fe
b The
c The
Mg s is Mg aq
d s Mg aq e
ELECTROLYTIC CELLa cell in which electrical energy is converted into
chemical energy.
ELECTROLYSISThe chemical process in which electrical energy is converted to
chemical energy OR
the use of electrical energy to produce a chemical change.
ELECTROLYTIC CELL
ELECTROPLATING
An attractive silver appearance can be created by electroplating artefacts made from cheaper
metals, such as nickel, with silver. The simplified diagram below represents an arrangement that
can be use
d to electroplate a nickel artefact with silver.
Which electrode cathode/anode will the nickel artefact represent?
Name the metal represented by electrode Y
Write down the half-reaction responsib
a
b
c le f
or the change that occurs at the surface of the artefact
In industry some plastic articles are sometimes electroplated.
Explain why plastic must be coated with graphite before electropl
d
ating
ELECTROPLATING
Ag ( ) 1 '
cells needs electrodes that
are made form METAL
a cathode
b Silver
c aq e Ag s
d Elektrochemical
REFINING OF COPPER
Electrolysis is an important industrial process used to decompose compounds, extract metals
from their ores and to purify metals like gold or copper. The simplified diagram below
represents an electr
olytic cell used to purify copper.
Define the term electrolysis
hich electrode, P or Q, consists of the impure copper?
Explain how you arrived at your answer.
Write down the half-reaction that t
a
b
c akes
place at electrode Q
During purification, metals such as silver and platinum form sludge at the bottom of the
container. Refer to the relative strengths of reducing agents to explain why these
d
two
me
tals do not form ions during the purification process.
Explain why the concentration of the copper II sulphate solution remains constant.
Assume that the only impurities in the copper are silver
d
and platinum
REFINING OF COPPER
2
,
the anode consists of the impure copper
- it is the electrode that is connected to the positive terminal
of the battery
Cu 2 '
Cu-ions are reduced when it plates the cathode
Cu-ions are strong
a P
c aq e Cu s
d
2
oxidising agents.
The other metals (Ag and Pt) are strong redusing agents
and will therefore oxidise easily
The oxidation half reaction at the anode produces
at the same rate as the reduction
d Cu ions
2 half reaction 'used' the Cu ions
CHLOOR-ALKALI INDUSTRIE
RECOVERY OF ALUMINIUM