chemistry Flashcards
how many types of electrodes are there? name them.
There are 5 types of electrode. They are
1.metal-metal ion electrode
2.metal-metal insoluble salt electrode
3.gas electrode
4.ion selective electrode
5.redox electrode
6.amalgam electrode
Define single electrode potential.
The potential developed between a metal rod and its own ions in a
solution at equilibrium is called electrode potential. It is denoted by E and expressed in volts.
Define standard electrode.
: The potential developed when a metal is in equilibrium with
1M solution of its own ions at 298K is called standard electrode potential. It is represented by E’ It is also called as standard reduction potential (SRP).
explain what is metal-metal ion electrode and give examples
The electrode system consisting of metal in contact with a solution
containing its own metal ion.
Example: Cu|Cu2+ . Zn|Zn2+
explain what is metal-metal insoluble salt electrode and give examples
The electrode system consisting of metal in with a
sparingly soluble salt of the same metal dipped in a solution containing anion if the salt. contact
Example: Calomel electrode (Hg|Hg2Cl2|Cl-) , Ag-AgCl electrode (Ag|AgCl|Cl-)
explain what is gas electrode. give example
The electrode system consists of gas in contact with an inert metal wire which
is dipped in an ionic solution of the gas molecule.
Example: Standard hydrogen electrode (SHE)
explain what is ion selective electrode and give examples
These are the electrodes which are sensitive to particular ionic species
and will develop a potential when a membrane is in contact with an ionic solution.
Example: Glass electrode
explain what is redox electrode and give examples
Electrode in which potential arises due to the presence of oxidized and
reduced forms of the same substance in solution.
Example: Platinum electrode, Pt | Fe2+, Fe3+
explain what is amalgam electrode and give examples
It is a modification of metal-metal ion electrode. Amalgam is the alloy
of any metal with mercury. In the place of metal, its solution in mercury is used.
Example: Zinc -amalgam electrode, where Zn(Hg) | Zn2+
What is a reference electrode. What are the types of reference electrode.
Reference electrodes: These are the electrodes of constant potential which are used to determine
electrode potential of other electrodes. There are two types of reference electrodes.
❖ Primary reference electrode. Eg: Standard hydrogen electrode (SHE).
❖ Secondary reference electrode. Eg: Calomel electrode, Ag/AgCl electrode
the construction of calomel electrode
- Calomel electrode is a metal- metal insoluble salt electrode and is a secondary reference
electrode. - It consists of a glass tube, a small amount of pure mercury is placed at the bottom of the glass
tube and it is covered by a paste of Hg and calomel (mercurous chloride, Hg2Cl2). - The remaining portion of the glass tube is filled with KCl solution of known concentration
(saturated, 1N or 0.1N) - A platinum wire is dipped into the mercury and is used to provide external electrical contact.
The porous plug at the bottom acts as a salt bridge.
Representation: Pt|Hg|Hg2Cl2|KCl(x) Where x = sat or 1N or 0.1N.
explain the working of calomel electrode
Calomel Electrode can act as anode or cathode depending on the nature of the other
electrode.
When acts as anode: 2Hg + 2Cl- Hg2Cl2 + 2e-When acts as cathode: Hg2Cl2 + 2e-
2Hg + 2Cl-At equilibrium: Hg2Cl2 + 2e-
2Hg + 2Cl-
The electrode potential at 298K is given by
Ecal= E0
cal – 0.0591 log [Cl-]
From the above equation it is clear that as the concentration of Cl-ion increases, the electrode
potential decreases
explain the advantages of calomel electrode
Advantages:
❖ It is easy to construct and maintain.
❖ Its potential can remain constant and it can easily be reproduced.
❖ It can be used in the presence of oxidizing agents.
What are electrolyte concentration cells?
: The galvanic cell in which electrode and electrolyte present in
both half cells are same but only the concentration of electrolyte is different is called electrolyte
concentration cell
explain the applications of calomel electrode
Applications:
❖ It is used as reference electrode in all potentiometric titrations.
❖ It is used as reference electrode with glass electrode in pH determination.
❖ It is used as a secondary reference electrode in the measurement of single electrode potential.
Derive an expression for emf of an electrolyte
concentration cell.
DO IT IN THE BOOK
Define Reference electrode
These are the electrodes of constant potential which are used to determine
electrode potential of other electrodes.
Define Concentration cell
concentration cells are basically the galvanic cell which converts chemical energy to electrical energy which consists of 2 same electrodes and 2 same electrolytes but the concentration of the electrolytes differ from each other
example-zn cell
define Standard
electrode potential
it is defined as the potential developed at the interface of a metal and its ion when it is in contact with 1M concentration of its own solution at stp
Define EMF of the cell
the potential between 2 electrodes when there is a flow of current from higher potential to lower potential
Define ion selective electrode.
These are the electrodes which are sensitive to particular ionic species
and will develop a potential when a membrane is in contact with an ionic solution. The potential
developed at these electrodes is a function of the concentration of that ion in the solution.
Explain the construction of glass membrane electrode
Glass electrode is an ion selective electrode in which a thin glass membrane is present which is
permeable to H+
ion and not to other ions in the solution
A glass electrode consists of a thick–walled glass tube with a very thin walled glass membrane
(bulb) at the bottom. Its thickness varies from 0.01-0.03 mm.
* This glass membrane made up of special glass of low melting point and high electrical
conductivity. The approximate composition of this glass is 72%SiO2, 22%Na2O and 6%CaO.
* This glass membrane allows only H+
ions up to a pH range from 1 to 9.
* The glass bulb is filled with 0.1M HCl as internal reference solution in which a silver–silver
chloride electrode is dipped as an internal reference electrode which provides external electrical
contact
representation: ag|agcl|HCl(0.1M)|glass membrane
explain the working of glass membrane electrode
When the glass electrode is dipped in the test solution, the Na+
ions of the glass membranes are
exchanged for H+
ions of the test solution.
Na+Gl- + H+ ↔ H+Gl- + Na+
When a glass electrode is placed between two H+ ion solutions of different concentrations (C1 and
C2), boundary potential is developed across the membrane. The boundary potential is given by the
equation
{DERIVATION}
Eb =0.0591/nlog[C2/C1]
Eb=0.0591/nlog[C2]-0.0591nlog[C1]–{n=1 for H+ ions}
Eb=0.0591log[C2]-0.0591log[C1]
lets take,
L=-0.0591log[C1]
L is constant because C1=0.1M HCl
therefore,
Eb=L+0.0591log[C2]
since C2 is the unknown concentration of H+
we can write,
Eb=L+0.0591log[H+]
we know,
PH=-log[H+]
Eb=L-0.0591pH
It is found that the potential of the glass electrode (EG) has three components:
* Boundary potential (Eb)
* Asymmetric potential (Eassy)
* Potential of the internal reference Ag – AgCl electrodes (EAg/AgCl)
∴ EG = Eb + Eassy + EAg/AgCl
Substitute equation (1) in (2), we get
EG= (L- 0.0591 pH) + Eassy + EAg/AgCl
EG= E0G - 0.0591 pH
Where E0G = L+ Eassy + EAg/AgCl .
E0G is the combination of three constant terms.
Explain how glass electrode can be used in determination of pH of unknown solution?
The cell consists of a glass electrode as indicator electrode and a saturated calomel electrode (SCE)
as reference electrode, the cell is immersed in a solution whose pH is to be determined. The emf
cell is measured using vacuum tube voltmeter (VTVM).
The cell assembly is represented as
Pt|Hg|Hg2Cl2 |KCl(sat)||solution of unknown pH| glass membrane|HCl (0.1M) |AgCl|Ag
The emf of the cell is given by
Ecell = Ecathode – Eanode
Ecell = EG – ESCE
E cell = (E0G – 0.0591 pH) – ESCE—(Where EG =E0
G – 0.0591 pH)
𝒑𝑯 =𝑬𝑮𝟎 − 𝑬𝒄𝒆𝒍𝒍 − 𝑬𝑺𝑪𝑬/0.0591
E0G value of a glass electrode can be determined by using a solution of known pH.
