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WHAT IS ELECTROCHEMISTRY?
- Electrochemistry is the branch of chemistry which deals with the study of production of electricity from energy released during the spontaneous chemical reactions and the use of electrical energy to bring about non- spontaneous chemical transformations.
- This is an interdisciplinary subject which let us know the reason behind various life processes like transmission of sensory signals to brain, use of hydrogen fuel cells, purification of metals, use of batteries, lithium ion batteries, chargeable and rechargeable batteries.
- So, let’s get started.
They are the devices in which chemical energy of spontaneous redox reaction is converted into electrical energy.
- How to represent an electrochemical cell?
A galvanic cell is represented by putting a vertical line between metal and electrolyte solution and double vertical line between two electrolytes connected by a salt bridge.
In this, anode is on left side and cathode on right.
I.e. for a Daniell cell,
Electrons flow from Anode to Cathode
Current flow from Cathode to Anode
- Salt bridge:- it is an inverted U-tube containing a salt in agar-agar. It contains only that salt for which cations and anions have equal transport no. i.e. (speed of ions)
- Role of Salt bridge:
- It makes the cathodic and anodic compartment of cell electrically neutral for continuous flow of electrons
- It completes the internal circuit of cell.
ELECTRODE AND ELECTRODE POTENTIAL
A metal rod dipped in metal salt solution is called a metal electrode.
When a metal is dipped in metal salt solution, two things happen
- Metal rod may undergo oxidation.(Due to this, the weight of metal rod decreases.)
- Metal may undergo reduction.(Due to this, the weight metal rod increases.)
- Since, an electrochemical cell is the combination of two electrodes in which one undergoes oxidation and the other reduction, so there exist a separation of charges between the electrode and electrolyte.
- A potential difference develops between the electrode and the electrolyte which is called electrode potential. i.e. The electrode potential is a measure of tendency of an electrode in a half cell to gain or lose electrons.
- It is of two types:-
- OXIDATION POTENTIAL: – It is the tendency of an electrode to lose electrons or get oxidized is called as OXIDATION POTENTIAL.
- REDUCTION POTENTIAL: – It is the tendency of an electrode to gain electrons or get reduced is called as REDUCTION POTENTIAL.
- According to IUPAC convention, electrode potential is represented as its reduction potential.
- STANDARD HYDROGEN ELECTRODE:- Since, the electrochemical cell can only work in pair, the individual electrode potential can be determined relatively with respect to standard hydrogen electrode, represented as
which is assigned zero potential at all temperatures corresponding to the reaction
- STANDARD ELECTRODE POTENTIAL: – The tendency of an electrode to be reduced with reference to hydrogen gas electrode in the standard state (1 atm pressure and 298K temp.) It is defined in terms of reduction.
It is the representation of standard electrode potential of different electrode in descending order
- NATURE OF ELECTRODE
The electrode placed at higher position acts as cathode and that placed at lower position act as Anode.
For example- In Zn-Cu cell, Zn act as anode while Cu act as cathode. As you can see in the table given below, Cu lies above Zn in the series.
- DETERMINATION OF
EMF of cell is defined as the potential difference between two electrodes when no or negligible current is allowed to flow in the circuit
For a Zn- Cu cell,
- SPONTANEITY / FEASIBILITY OF CELL REACTION
A reaction is said to be feasible only if the species having higher reduction potential gets reduced while one having lower reduction potential is oxidized.
For spontaneity of reaction,
Where, n- no. of electrons transferred during a balanced redox reaction
F- Faraday’s constant=96500 C
- DISPLACEMENT REACTION
The elements placed at lower position can displace another elements placed at higher position from their salts.
- EVOLUTION OF H2 GAS
The elements placed below hydrogen can evolve gas from water and dilute acids. So, the electrochemical series helps in predicting whether the metal can liberate H2 gas or not. In general, metals having –ve reduction potential liberate hydrogen. For example, metals like zinc, magnesium, nickel, etc. liberate hydrogen while copper and silver does not.
Following is the table showing electrochemical series, it indicates that fluorine gas has the highest tendency to get reduced to fluorine ions, thus, Fluorine gas is the strongest oxidizing agent and Fluorine ion is the weakest reducing agent.
It also indicates that Lithium has lowest electrode potential thereby indicating, Lithium is the strongest reducing agent and Lithium ion is the weakest oxidizing agent.
For detailed notes, please download the NCERT Complete notes of Electrochemistry PDF given above.