Transcript 07. Electrochemistry

LECTURE 7:
Electrochemistry. Types of electrodes
and their using.
ass. prof. Yeugenia B. Dmukhalska
Definition
• The branch of science, which deals with the
study oxidation-reduction reaction to
produce the interconversion of chemical and
electricl energy. of transition chemical
energy to electrical energy is known as
electrochemistry.
М = Мn+ + neМn+ + nе- = М
(i) Мn+ ions reflected back after colliding without any
change;
(ii) Мn+ ions gaining electrons to form М (i.е. Мn+ get
reduced);
(iii) Metal atoms losing electrons to form Мn+ (i.е. М
gets oxidized)
Nernst’s equation
The dependence of cell voltage upon concentration
can also be described quantitatively. The freeenergy change G for any reaction is:
G =G 0+ RT ln Q
• Where: Q represents the mass-action expression
for an oxidation-reduction reaction
• G = - nFE, and G0 = - nFE0
• - nFE = - nFE0+ RT ln Q
• E = E0 - RT/ nF x ln Q
• R = 8.315 J/K .mol
• F = 96,485 С /mol
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Мn++nе = М
Then the Nernst eqn. is applied as follows:
E = E0 – (RT/ nF) ln ([M]/ [Mn+])
where Е = electrode potential under given
concentration of Мn+ ions and temperature
Т
Е0 – standard electrode potential
R – gas constant
Т – temperature in К
n – number of electrons involved in the
electrode reaction.
Standard (normal)
hydrogen electrode
Pt, Н2 (g)/Н+ (Concentration)
H2 = 2H+ + 2е2H+ + 2е- = H2
E = E0 – (RT/ 2F) ln (pH2/ [H+]2), E0H+/H2 = 0V.
In the standard hydrogen gas electrode, hydrogen at
atmospheric pressure is passed into 1 М НС1 in which
foil of the platinized platinum remains immersed through
which inflow or outflow of electrons takes place.
• Since а cathode reaction is а reduction, the
potential produced at such an electrode is called а
reduction potential. Similarly, the potential
produced at an anode is called an oxidation
potential. These are known as standard reduction
potentials or standard electrode potentials. They
are usually tabulated for 25 С.
Types of electrodes
1. Metal-metal ion electrodes
2. Gas-ion electrodes
3. Metal-insoluble salt-anion electrodes
4. Inert "oxidation-reduction" electrodes
5. Membrane electrodes
Electrodes of the first kind.
• An electrode of the first kind is а piece of pure metal
that is in direct equilibrium with the cation of the
metal. А single reaction is involved. For example, the
equilibrium between а metal Х and its cation Х+n is:
• Х+n + ne- = X (s)
• for which
•
•
0.0592
1
0.0592
Еnd = Е0X+n – -------- log ---- = Е0X+n + ---------- log aX+n
n
aX+n
n
• The metal - metal ion electrode consists of а
metal in contact with its ions in solution. An
example: silver metal immersed in а solution of
silver nitrate
• As a cathode: the diagram: Ag+(aq)  Ag(s)
• half-reaction equation is: Ag+ (aq) + e-Ag(s)
• as an anode: the diagram: Ag(s)  Ag+(aq)
• half-reaction equation is: Ag(s)  Ag+(aq) + е• Nernst’s equation:
• E = E0 – (RT/ nF) ln ([Ag]/ [Agn+])
Electrodes of the Second Kind.
• Metals not only serve as indicator electrodes for their
own cations but also respond to the concentration of
anions that form sparingly soluble precipitates or stable
complexes with such cations.
AgCl + e- = Ag (s) + Cl-
E0AgCl = 0.222 V
• The Nernst expression for this process is:
• EAgCl = E0AgCl – 0.0592 log [Cl-] = 0.222 + 0.0592 pCl
• In
the
metal-insoluble
salt-anion
electrode, а metal is in contact with one of
its insoluble salts and also with а solution
containing the anion of the salt. An
example is the so-called silver - silver
chloride electrode, written as а cathode as:
• Cl- (aq)  AgCl(s)  Ag(s)
• for which the cathode half-reaction is:
• AgCl (s) + е-  Ag(s) + Cl- (aq)
• Nernst’s equation:
• E = E0 – (RT/ 1F) ln ([Ag] [Cl-]/ [AgCl])
• An inert oxidation-reduction electrode consists of а
strip, wire, or rod of an inert materiel, say, platinum, in
contact with а solution, which contains ions of а
substance is two different oxidation states. In the
operation of this electrode the reactant not supplied by
the electrode itself, nor is it introduced from outside the
cell. And the product neither plates out nor leaves the
cell. Instead, both reactant and product are present in
solution. Thus, for the ferric - ferrous ion electrode
functioning as а cathode,
• Fe3+, Fe 2+(aq)  Pt(s)
• the iron(III), or ferric, ion, Fe+3(aq), is reduced to the
iron(II), or ferrous, ion, Fe+2(aq):
• Fe+3(aq) + е- Fe+2(aq)
• Nernst’s equation:
• E = E0 – (RT/ 1F) ln ([Fe+2]/ [Fe+3])
• а membrane electrode - the glass
electrode.
• This can be depicted as:
• Pt(s) Ag(s)  AgC1(s)  HC1(aq,1M)  glass 
• Cell can be depicted as:
reference electrode  salt bridge  analyte solution  indicator electrode
• Ecell = Eind + Eref + Ej
Cell potential or EMF of a cell.
• The difference between the electrode
potentials of the two half cell is known as
electromotive force (EMF) of the cell or
cell potential or cell voltage.
• The EMF of the cell depends on the nature
of the reactants, concentration of the
solution in the two half cells, and
temperature.
• Reference electrode is electrode potential
which stabile
• А hydrogen electrode is seldom used as а
reference electrode for day-to-day
potentiometric measurements because it is
somewhat inconvenient and is also а fire
hazard.
• Calomel Electrodes. A calomel electrode can be
represented schematically as
• Hg Hg2Cl2 (saturated), КС1 (saturate)
• The electrode reaction in calomel half-cells is:
• Нg2Cl2(s) + 2 е- = 2 Нg (1) + 2 Cl• The "saturated" in а saturated calomel electrode
refers to the KCl concentration. All calomel
electrodes are saturated with Hg2CI2 (calomel). At
250С, the potential of the saturated calomel
electrode versus the standard hydrogen electrode
is 0.244 V;
• Silver/silver chloride electrodes. А system
consists of а silver electrode immersed in а
solution that is saturated in both potassium
chloride and silver chloride:
• Аg АgС1(saturated),KC1(saturated)
• The half-reaction is AgC1(s) + е = Аg (s) + Сl• The potential of this electrode is 0.199 V at 25 0С.
• An ideal indicator electrode responds
rapidly and reproducibly to changes in the
concentration of an analyte ion (or group of
ions). Although no indicator electrode is
absolutely specific in its response, а few
are now available that are remarkably
selective. There are two types of indicator
electrodes: metallic and membrane.
• Metallic indicator electrodes:
• Electrodes of the first kind.
• Electrodes of the Second Kind.
• Membrane Electrodes
• The relationship between pH and the voltage of the hydrogen
elect
• calomel electrode cell at 250С can be written as
•
Ecell
Ecalomel
1
• pH = --------- - (---------- + ---- log pH2)
•
0.0592
0.0592
2
•
Ecell
• pH = ---------- = constant
•
0.0592
• 1.Glass electrode – indicator electrode;
diagram which is: Ag(s)  AgC1(s)  HC1(aq,1M)  glass 
• 2.Bulb of glass electrode.
• 3.Solution of unknown pH.
• 4.Silver-silver chloride electrode - reference electrode;
diagram which is: Cl- (aq)  AgCl(s)  Ag(s)
5.Amplifying potentiometer.