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Daniell cell refers to an electrochemical cell that transforms chemical energy into electrical energy. The cell carries out various types of chemical reactions in order to produce electricity. The Daniell cell is composed of zinc and copper electrodes with zinc at anode and copper at cathode. Both of the metals are dipped in their respective salt solutions.
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What is a Daniell Cell?
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Daniell Cell consists of two different metals: Zinc (Zn) which works as an anode and Copper (Cu) as a cathode. Each of these electrodes remains dipped into chemical solutions made out of their own ions. For Zinc it is Zinc Sulphate and for Copper, it’s Copper Sulphate.
It is an improvement of the Voltaic cell and works by exploiting the two electrodes that are formed out of Zinc and Copper in order to produce a difference potential of 1.1 V. The circuit is charged by the cell in which the electrons circulate after being produced at the anode and being transferred to the cathode.
Important Links:
| Chapter Related Links | ||
|---|---|---|
| Reduction Potential | Nernst Equation | Batteries |
| Fuel Cells | Corrosion | Electrolytic Cell |
Diagram of Daniell Cell
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Daniell Cell is designed, based on the spontaneous reaction between Cupric ion and Zinc, to create electric energy. At the negative electrode, zinc metal is oxidized while at the positive electrode copper undergoes reduction. It consists of two half-cells that are joined by a salt bridge. The salt bridge prevents the solution to mix mechanically.
Daniell Cell Diagram
Daniell Cell Working
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Daniell Cell includes a copper vessel that contains saturated Copper Sulphate (CuSO4) and diluted Hydrogen Sulphate (H2SO4). Its working mechanism includes:
- A consolidated Zinc rod remains immersed into the Zinc Sulphate (Zn2SO4) solution.
- Below the copper vessel, a transparent layer can be seen where the Copper Sulphate crystals and solution are kept in contact with each other. This helps keep the solution saturated.
- A flow of electric current is generated through the external circuit.
- The zinc rod loses mass while copper gains it.
- The concentration of Zinc Sulphate increases while the concentration of Copper Sulphate decreases.
- In both the compartments, the solutions remain electrically neutral.
- While the separation resists polarization, the design allows a flow of ions between the metals through the electrolyte.
Daniell Cell Reactions on Anode and Cathode
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The chemical reaction within Daniell Cell can be summed up as:
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
This reaction can be further broken into two steps for better understanding. One part sums up how the cell reacts to the anode and the other to the cathode.
- At the anode, the solid zinc metal transforms into zinc ions due to the oxidation process while it releases 2 electrons. The equation can be written as:
Zn(s) → Zn2+(aq) + 2e-
- At the cathode, those copper ions get transformed into copper metal and further get deposited. The equation is:
Cu2+ + 2e → Cu(s)
- The electrons released by the zinc at the anode reach the cathode through an interconnecting metal wire. Electricity is produced as a result of this particular flow in the opposite direction of the electrons travelling from cathode to anode. The salt bridge completes the entire circuit.
- Daniell cell can be represented as Zn | Zn2+(aq) | | Cu2+(aq) | Cu where the “| |” represents the salt bridge, the left side symbolizes oxidation process, and the right one pertains to the reduction of half-cell.
Difference between Daniell Cell and Electrolytic Cell
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Electrochemical cells can be divided into two main types:
- Galvanic cells or Voltaic Cells
- Electrolytic Cells
The following table discusses the differences at a glance:
| Differing Factors | Galvanic Cells | Electrolytic Cells |
|---|---|---|
| Production of Electricity | Electricity can be produced with the help of chemical reactions. | Electric currents are used for chemical reaction transmission. |
| Conversion of Energy | Converts chemical energy into electrical ones and vice-versa. | Electrical energy gets transformed into chemical energy. |
| Occurrence of Reaction | Spontaneous reactions | Non-spontaneous reactions |
| Cathode/Anode | The cathode is the positively charged part. The anode is negative. | The anode is positively charged. The cathode is negative. |
| Electrical Energy | Electrical energies are caused by internal Chemical reactions. | An external source is required for the production of Chemical reactions. |
| Origination of Electrons | Originates internally | Originates from external sources such as a battery |
What is a Salt Bridge?
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Salt Bridge is a glass tube consisting of ammonium nitrate or potassium chloride in gelatin form. It is responsible for maintaining electrical neutrality in the two-compartment of the Daniell cell. It is used in electrochemical cells for connecting the oxidation and reduction half cells where a weak electrolyte is used. Salt Bridge can be understood as a junction connecting the anode and the cathode compartment in an electrolytic solution. It usually has a strong electrolyte that is further formed of ions.
Salt Bridge acts as a point of contact between the two compartments of the Daniell Cell
It allows the movement of anions towards anodic and cations towards cathodic sections. The gelatin form prevents any mixing while allowing ionic movement.
| Example: List the functions of Salt Bridge.
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Things to Remember
- Daniell cell is mainly used for generating electricity and storing electricity by electrode consumption
- Daniell cell works on the basis of Redox reaction. During the reaction time, electrons work as electrical energy flowing from the zinc to the copper metals.
- While oxidation happens, metal ions are extracted from the anode within the cell. This causes the electrons to accumulate on the anode and thus making it negative. On the other hand, the extraction of metal ions from the electrolyte makes use of mostly electrons from the cathode. Therefore, it remains fairly positive.
- Salt Bridge is a glass tube responsible for maintaining electrical neutrality in the two-compartment of the Daniell cell.
Sample Questions
Ques. Identify the Incorrect Statement? (1 mark)
i) In Galvanic cells, the passage of current drives a redox reaction
ii) Copper, while being used as an electrolytic cell, can be purified
iii) For electrolytic cells, an external power source is required
Ans. 1) In Galvanic cells, the passage of current drives a redox reaction.
Ques. What will be the change in the cell voltage while the ion concentration in the anode gets increased by a factor of 10? (3 marks)
Ans. Within the Daniell cell, zinc works as an anode and copper as a cathode. Therefore, Zn2+ and Cu2+ ions are respectively in the anode and cathode parts of the cell.
If we suppose that the original equation can be referred to as Zn2+ + Cu2+= 1M
Therefore, when the ion concentration gets increased by a factor of 10
Ecell=Eocell−(0.0592V2)log1012
Ecell=Eocell−0.03V
Hence, when the ion concentration gets increased by factor 10, the cell voltage gets decreased by 0.03V.
Ques. What is the difference between the Daniell cell and the voltaic cell? (1 mark)
Ans. A voltaic cell may or may not be reversible but the Daniell cell is a reversible cell if the current fed or drawn is small.
Ques. Highlight why the cathode in Daniell Cell positive? (2 Marks)
Ans. Once oxidation occurs, the metals are removed from the anode which results in the gathering of electrons on the anode that gradually makes it negative. Since there is a removal of metal ions from electrolyte, the electrons are consumed, making the cathode positive.
Ques. Write the cell reaction of the Daniell Cell? (2 marks)
Ans. Zn(s) + Cu2+(aq) -------> Zn2+(aq) + Cu(s)
Within the Daniell Cell, electrons move from the zinc to the copper electrode through the help of an external source or circuit. At the same time, through a salt bridge, the zinc metal ions flow from a half cell to the other. The flow of the current is from the copper electrode to zinc, from cathode to anode.
Ques. What is a salt bridge? State two functions. (3 marks)
Ans. Salt Bridge is a glass tube responsible for maintaining electrical neutrality in the two-compartment of the Daniell cell.
Functions
- Salt Bridge allows the movement of ions to maintain the electric current through the solution.
- Salt Bridge helps in maintaining the balance of charges.
Ques. What are some of the electrolytes that can be used in the salt bridge? (1 mark)
Ans. Potassium Chloride or Ammonium nitrate consisting agar-agar can be used as electrolytes in the salt bridge.
Ques. Is a Voltaic cell reversible? (1 mark)
Ans. The voltaic cell may or may not be reversible. However, a daniell cell is a reversible cell.
Ques. Highlight whether a Daniell Cell is reversible or not. (2 marks)
Ans. The Daniell cell comprises of electrons that travel around the outer circuit, from cathode to the anode. While, the positive ions are seen to travel from cathode to anode through the solution. Thus, the daniell cell is reversible, only if the current drawn in small.
Ques. Give a short note on Daniell cell and its working. (3 marks)
Ans. A galvanic cell can be understood as an electrochemical cell that generates or produces electrical energy from some spontaneous redox reactions that take place within the cell. It generally consists of two different metals, copper and zinc, which are connected by a salt bridge, or it has two individual half-cells that are separated by a porous membrane. Daniel cell is known as a galvanic cell.
Daniel cell has Zinc as anode and Copper as cathode. The reactions of Daniel cell are,
At anode (Oxidation),
Zn(s) → Zn2+(aq) + 2e−
At cathode(Reduction),
Cu2+ (aq) + 2e− → Cu(s)
Hence, the overall reaction is,
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Here the chemical energy is released when the redox reaction takes place in the Daniell cell and the chemical energy is converted into electrical energy.
Ques. What are the factors on which the conductivity of an electrolyte depends? (2 marks)
Ans. The conductivity of an electrolyte depends upon
i) The nature of the electrolyte
ii) Size of the ions produced
iii) Nature of solvent and its viscosity.
iv) Concentration of electrolyte.
v) Temperature
Ques. How is molar conductance related to the conductivity of an electrolyte? (2 marks)
Ans. Molar conductance is related to the conductivity of an electrolyte by the following equation,
\(\Omega m = \frac{k}{c}\)
where,
k = Conductivity in s/m
c = concentration in mol/m3
Ques. The conductivity of an aqueous solution of NaCl in a cell is 92\(\Omega^{-1}cm^{-1}\). The resistance offered by this cell is 247.8\(\Omega\). Calculate the cell constant? (2 marks)
Ans. Specific conductivity = \(\frac{Cell\ Constant}{Reistance}\)
Cell constant = Resistance x specific conductivity
= 92 x 247.8
= 22797.6 \(\Omega^{-1}\)
Ques. The conductivity of metals decreases while that of electrolytes increases with increases in temperature. Why? (2 marks)
Ans. With an increase in temperature, the K.E. of metal cation increases and obstructs the free flow of electrons decreasing the conduct of metal while in the case of electrolytes, increased temperature increases the mobility of ions which increases the conductance of ions.
Ques. State Faraday’s Laws of electrolysis. (2 marks)
Ans. Faraday’s Laws of electrolysis
First Law: The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolyte.
Second Law: The amount of different substances liberated by the same quantity of electricity passing through the electrolytic solution is proportional to their chemical equivalent weights.
Ques. What do you mean by primary and secondary battery? (2 marks)
Ans. A primary battery is a single-use battery which cannot be recharged. Here, the electrolysis reaction is not reversible. On the other hand, secondary batteries are rechargeable batteries where electrolysis reaction is reversible. This can be recharged many times.
Ques. Why does iron rust quicker in saline water than in ordinary water? (2 marks)
Ans. Saline water consists of a greater number of ions than normal water which increases the electrochemical reaction. This increases the rate of corrosion and thus iron rusts quicker in saline water.
Ques. What are the factors that affect corrosion? (2 marks)
Ans. Factors affecting corrosion are –
1) Water and air
2) Presence of electrolytes in water.
3) Presence of gases like CO2, SO2
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