Dr Anand Gupta Mr. Mahesh Kapil Dr. Anand Gupta

84 Pages·2009·4.5 MB·English

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Dr Anand Gupta Mr. Mahesh Kapil
Dr. Anand Gupta
09356511518 09888711209
[email protected] [email protected]
Electrochemistry
EElleeccttrroollyyssiiss
Electric energy Chemical energy
GGaallvvaanniicc cceellll
2
Electrochemistry
The study of the interchange of chemical and
electrical energy.
Chemical Energy Electrical energy
I: A science studying the relationship between
chemical energy and electrical energy and the rules
of conversion of two energies.
II: Electrochemistry is the study of solutions of
electrolytes and of phenomena occurring at
.
electrodes immersed in these solutions
EEEllleeeccctttrrrooonnn TTTrrraaannnsssfffeeerrr RRReeeaaaccctttiiiooonnnsss
(cid:1)Electron transfer reactions are oxidation-reduction
or Redox reactions.
(cid:1)Results in the generation of an electric current
(electricity) or be caused by imposing an electric
current.
(cid:1)Therefore, this field of chemistry is often called
ELECTROCHEMISTRY.
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Electrochemical processes are oxidation-reduction
reactions in which:
• the energy released by a spontaneous reaction is
converted to electricity or
• electrical energy is used to cause a nonspontaneous
reaction to occur
0 0 2+ 2-
2Mg (s) + O (g) 2MgO (s)
2
2Mg 2Mg2+ + 4e- Oxidation half-reaction (lose e-)
O + 4e- 2O2- Reduction half-reaction (gain e-)
2
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TTTeeerrrmmmiiinnnooolllooogggyyy fffooorrr RRReeedddoooxxxRRReeeaaaccctttiiiooonnnsss
(cid:1)(cid:1)OOXXIIDDAATTIIOONN——lloossss ooff eelleeccttrroonn((ss)) bbyy aa ssppeecciieess;;
(cid:1)OXIDATION—loss of electron(s) by a species;
iinnccrreeaassee iinn ooxxiiddaattiioonn nnuummbbeerr;; iinnccrreeaassee iinn ooxxyyggeenn..
increase in oxidation number; increase in oxygen.
(cid:1)(cid:1)RREEDDUUCCTTIIOONN——ggaaiinn ooff eelleeccttrroonn((ss));; ddeeccrreeaassee iinn
(cid:1)REDUCTION—gain of electron(s); decrease in
ooxxiiddaattiioonn nnuummbbeerr;; ddeeccrreeaassee iinn ooxxyyggeenn;; iinnccrreeaassee iinn
oxidation number; decrease in oxygen; increase in
hhyyddrrooggeenn..
hydrogen.
(cid:1)(cid:1)OOXXIIDDIIZZIINNGG AAGGEENNTT——eelleeccttrroonn aacccceeppttoorr;; ssppeecciieess iiss
(cid:1)OXIDIZING AGENT—electron acceptor; species is
rreedduucceedd.. ((aann aaggeenntt ffaacciilliittaatteess ssoommeetthhiinngg;; eexx.. TTrraavveell
reduced. (an agent facilitates something; ex. Travel
aaggeennttss ddoonn’’tt ttrraavveell,, tthheeyy ffaacciilliittaattee ttrraavveell))
agents don’t travel, they facilitate travel)
(cid:1)(cid:1)RREEDDUUCCIINNGG AAGGEENNTT——eelleeccttrroonn ddoonnoorr;; ssppeecciieess iiss
(cid:1)REDUCING AGENT—electron donor; species is
ooxxiiddiizzeedd..
oxidized.
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Another way to remember
(cid:1)
OILRIG
ee
xx ss oo ss aa
dd
ii ss
ii
uu
dd ee
nn
cc
aa
tt
tt
ii
ii
oo
oo
nn
nn
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Review of Oxidation numbers
The charge the atom would have in a molecule (or an
ionic compound) if electrons were completely transferred.
1. Free elements (uncombined state) have an oxidation
number of zero.
Na, Be, K, Pb, H , O , P = 0
2 2 4
2. In monatomic ions, the oxidation number is equal to
the charge on the ion.
Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2
3. The oxidation number of oxygen is usually –2. In H O
2 2
and O 2- it is –1.
2
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4. The oxidation number of hydrogen is +1 except when
it is bonded to metals in binary compounds. In these
cases, its oxidation number is –1.
5. Group IA metals are +1, IIA metals are +2 and fluorine
is always –1.
6. The sum of the oxidation numbers of all the atoms in
a molecule or ion is equal to the charge on the
molecule or ion.
HCO -
3
Oxidation numbers of all O = -2 H = +1
the atoms in HCO - ?
3
3x(-2) + 1 + ? = -1
C = +4
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Galvanic Cells
anode cathode
oxidation reduction
-
+
spontaneous
redox reaction
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