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Electrolytic and Galvanic cells are the types of electrochemical cells that find varied applications in our everyday lives. The key difference between them is that an electrolytic cell converts electrical energy into chemical energy whereas a galvanic cell undergoes a reverse process.
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Keyterms: Electrolytic cell, Galvanic cells, electrochemical cells, chemical energy, electrical energy, electrolysis, chemical compound, decompose, Electrochemistry
Electrolytic Cell
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Electrolytic cells are where electrolysis takes place. Here the electrical energy from a power source is converted into chemical energy. This chemical energy is used to decompose a given chemical compound. The first electrolysis was conducted by Sir Humphrey Davey, in 1808.
Here the cathode and anode are immersed in a single electrolyte. Normally, the aqueous solution of the salt of the metal is taken as the electrolyte. Since these types of cells convert electrical energy to chemical energy, an external power source is required to employ the desired voltage.
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Working Principle Of Electrolytic Cell
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Electrolytic cells are where non-spontaneous reactions occur. Electrolysis is the decomposition of elements by the force of current. It is normally used in the process of electroplating. Let us consider the example where molten sodium chloride is taken as the electrolyte and a pair of electrodes are immersed into the electrolyte connected through a battery to form a circuit.
When the circuit is in an on state, the cathode acquires a negative charge and undergoes a reduction process. The positively charged sodium cations get attracted to the cathode and liberate sodium metal. On the other hand, the anode with a positive charge undergoes an oxidation process where it loses electrons. As a result, chlorine gas gets liberated. This can be expressed in the form of equation as:
Na+ + e- = Na
2 Cl- = Cl2 + 2e-
Uses Of Electrolytic Cell
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- The process of electrolysis finds its application in electroplating. This is used to coat a fine layer of one metal over another. Eg: Gold plating
- It is used in the synthesis of oxygen and hydrogen gas from water
- They are used in the electrolytic refining of metals. Eg: To extract pure aluminium from Bauxite.
- Industrial grade extraction of pure metals such as copper, aluminium and zinc are done by the use of electrolytic cells.
Galvanic Cell
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Galvanic or a voltaic cell is where a spontaneous oxidation and reduction reaction occurs. It converts chemical energy to electrical energy. This is the working principle behind the batteries that we use in our everyday lives.
It consists of two half cells and in each half cell, a suitable electrode is immersed. The two half cells are connected through a salt bridge. The need for the salt bridge is to keep the oxidation and reduction processes running simultaneously. Without it, the electrons liberated at the anode would get attracted to the cathode thereby stopping the reaction on the whole.
Working Principle Of Galvanic Cell
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Here, two beakers containing electrolytic solutions of copper sulphate and zinc sulphate are taken. It is connected via a salt bridge containing an aqueous solution of potassium chloride. Zinc and copper electrodes are immersed in the respective electrodes and connected through a voltmeter to measure the electrical potential.
Zinc which acts as the anode readily undergoes an oxidation process and acquires a negative charge. The electrons travel through the salt bridge and undergo a reduction process at the copper cathode. Thus the cathode would acquire a positive charge. This flow of electrons from the anode to the cathode induces a flow of electric current in the opposite direction which shall be measured by the voltmeter.
Uses Of Galvanic Cell
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Since galvanic cells convert chemical energy to electrical energy, these are used in the commercial production of batteries. This can be used to charge remotes, clocks, flashlights and even electric cars. There are two types of galvanic cells mainly: Primary (disposable) and secondary ( rechargeable) batteries.
Primary batteries: These are non-rechargeable and single-use batteries where the electrode reactions are irreversible.
Secondary batteries: These are rechargeable batteries that have an insoluble element sticking to the electrodes. While in use they behave as regular galvanic cells and while recharging, they behave as electrolytic cells.
Differences based on Electrolytic & Galvanic Cells
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| Characteristics | Electrolytic cell | Galvanic Cell |
|---|---|---|
| Energy Conversion | Electrical energy is converted to chemical energy. | Chemical energy is converted to electrical energy. |
| Redox reaction | Electrical energy powers the redox reaction to take place. | The redox reaction generates electrical energy. |
| Components | A Salt bridge is not needed. | A Salt bridge is needed. |
| Polarity | Anodes are positive and cathodes are negative. | Anodes are negative and cathodes are positive. |
| Chemical reaction | Reactions are non-spontaneous | Reactions are spontaneous |
| Current flow | Anode to cathode | Cathode to anode |
Things To Remember
- In an electrolytic cell, an electric potential is necessary to carry out the redox reaction.
- In a galvanic cell, the spontaneous redox reactions generate electrical energy.
- The polarity of the electrodes is interchanged in an electrolytic and galvanic cell.
- The major use of electrolytic cells is in the process of electroplating and electrolytic refining.
- Galvanic cells find its application in the production of batteries.
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Sample Questions
Ques. How would you determine the standard electrode potential of the system Mg2+1 Mg? (3 marks)
Ans. A cell will be set up consisting of Mg/MgSO4 (1 M) as one electrode and standard hydrogen electrode Pt, H, (1 atm)H+/(l M) as the second electrode, measure the EMF of the cell and also note the direction of deflection in the voltmeter. The direction of deflection shows that e-1 s flow from mg electrode to hydrogen electrode, i.e., oxidation takes place on magnesium electrode and reduction on hydrogen electrode. Hence, the cell may be represented as follows :
Ques. Why does the conductivity of a solution decrease with dilution? (2 marks)
Ans. The conductivity of a solution is linked with the number of ions present per unit volume. With dilution, these decrease and the corresponding conductivity or specific conductance of the solution decreases.
Ques. Suggest a list of metals that can be extracted electrolytically. (2 marks)
Ans. The highly reactive metals having large -ve E° values, which can themselves act as powerful reducing agents can be extracted electrolytically. The process is known as electrolytic reduction. For details, consult Unit-6. For example, sodium, potassium, calcium, magnesium etc.
Ques. Write the chemistry of recharging the lead storage battery, highlighting all the materials that are involved during recharging. (3 marks)
Ans. A lead storage battery consists of an anode of lead, the cathode of a grid of lead packed with lead dioxide (PbO2) and 38% H2SO4 solution as electrolyte. When the battery is in use, the reaction taking place is:
On charging the battery, the reverse reaction takes place, i.e., PbSO4 deposited on electrodes is converted back to Pb and PbO2 and H2SO4 is regenerated.
Ques. What are fuel cells? Explain the electrode reactions involved in the working of the H2 – O2 fuel cells. (3 marks)
Ans. Ans. Fuel cells are the devices that convert the energy produced during the combustion of fuels like H2, CH4, etc. directly into electrical energy.
The electrode reaction for H2 – O2 fuel cell:
Ques. What is corrosion? Explain the electrochemical theory of rusting of iron and write the reactions involved in the rusting of iron. (3 marks)
Ans. Corrosion is defined as the deterioration of a substance because of its reaction to its environment. Corrosion is an electrochemical phenomenon. At a particular spot of an object made of iron, oxidation takes place and that spot behaves as an anode and the reaction is
At Anode: 2Fe → 2Fe²+ + 4e-
Electrons released at an anodic spot move through the metal and go to another spot on the metal and reduce oxygen in presence of H+. This spot behaves as a cathode
At Cathode: O2 + 4H+ + 4e-
Overall reaction: 2Fe + O2 + 4H+ → 2Fe²+ + 2H2O
Ques. Define Primary and Secondary batteries. (2 marks)
Ans. Primary batteries: In primary batteries, when the reactants have been converted into
products, no more electricity is produced. The cell reaction cannot be reversed and the battery becomes dead.
Secondary batteries: In secondary batteries (or cells), the cell reaction can be reversed by passing electricity through the battery (charging). It means that the battery can be used again and again through a large number of discharging and charging cycles.
Ques. Write the name of the cell which is generally used in inverters. Write the reactions taking place at the anode and the cathode of this cell. (2 marks)
Ans. The lead storage battery is used in inverters.
At Anode: Pb(s) + SO4²-(aq) → PbSO4 (s) + 2e-
At Cathode: PbO2 (s) + SO4²-(aq) → 4H+ (aq) + 2e-
Ques. What is an emf? (2 marks)
Ans. The passage of current from one electrode to the other indicates the existence of a potential difference between them. This difference of potential which causes current to flow from the electrode of higher negative potential is called the electromotive force (emf).
Ques. Define galvanic cells. (2 marks)
Ans. A galvanic cell consists of two half cells. Each half cell contains an electrolytic solution and a metallic electrode. The electrode at which- oxidation takes place is called an anode and the electrode at which reduction takes place is called the cathode. The half-cells are separated from each other by means of a porous pot or a salt bridge.
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