Have you ever wondered how a simple chemical reaction can generate electrical energy? The answer is a Galvanic cell, specifically a Daniell cell provides us a perfect example of this. Daniell cell not only revolutionised the way we generate electricity but also laid the groundwork for understanding modern battery technology. Daniel's cell generates electrical energy through a redox (reduction-oxidation) reaction. It was invented by John Daniell in 1836 and is one of the simplest examples of a working battery.
The greatest example of a galvanic cell that turns chemical energy into electrical energy is a Daniell cell.
The Daniell cell is made up of two electrodes made of different metals, Zn and Cu, which are in contact with a solution of their respective ions, zinc sulphate and copper sulphate.
A conventional galvanic cell is meant to generate an electric current by using the spontaneous redox reaction between zinc and cupric ions.
A copper vessel makes up this cell. In this case, a saturated CuSO4 solution is used as a depolariser and diluent. Fill with H2SO4, which works as an electrolyte. Zn2SO4 is used to submerge a zinc rod that has been amalgamated. CuSO4 crystals are kept in touch with CuSO4 solution by a transparent layer just below the upper surface of copper vessels, ensuring that the solution is always saturated.
Daniel cell working is discussed below.
$\mathrm{Zn}(\mathrm{s})+\mathrm{Cu}^{2+}(\mathrm{aq}) \rightarrow \mathrm{Zn}^{2+}(\mathrm{aq})+\mathrm{Cu}(\mathrm{s})$
In a Daniell cell, electrons flow through an external circuit from the zinc electrode to copper electrode to copper electrode, while metal ions move from one half cell to the other via the salt bridge.
Through an external circuit, current flows from the copper electrode to the zinc electrode, which is the cathode, to the anode.
A voltaic cell can be reversible or irreversible, whereas a Daniell cell is reversible.
Chemical Reaction of Daniell Cells
$\mathrm{Zn}+\mathrm{Cu}^{2+} \rightleftharpoons \mathrm{Zn}^{2+}+\mathrm{Cu}$
Daniel cell representation
$\mathrm{Zn} / \mathrm{Zn}^{2+} \| \mathrm{Cu}^{2+} / \mathrm{Cu}$
The above reaction can be divided into two parts:
Half-cell anode reaction
$\mathrm{Zn} \rightarrow \mathrm{Zn}^{2+}+2 \mathrm{e}^{-}$
Zinc metal is oxidised once two electrons are liberated.
Reduction half-cell reaction
$\mathrm{Cu}^{2+}+2 \mathrm{e}^{-} \rightarrow \mathrm{Cu}$
Copper metal is formed when copper ions are reduced to copper metal
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It's made up of a copper container filled with a concentrated copper sulphate solution. One porous cylindrical pot filled with diluted sulphuric acid is immersed in the concentrated copper sulphate solution inside the container. In a porous pot, one amalgam zinc rod is immersed in dilute sulphuric acid. Sulphuric acid in its diluted state has positive hydrogen ions and negative sulphate ions, which is a feature of dilute electrolytes.
When sulphate ions come into contact with zinc rods, they release electrons and generate zinc sulphate as a result of the oxidation reaction. As a result, the zinc rod acquires a negative charge and takes on the role of a cathode.
Positive hydrogen ions can pass through the porous wall of the pot and into the copper sulphate solution, where they combine with copper sulphate electrolyte sulphate ions to generate sulphuric acid. The positive copper ions in the copper sulphate electrolyte come into contact with the copper container's inner wall, where they are reduced and produce copper atoms, which are then deposited on the wall.
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For a better understanding, let us go over the cell's operating concept step by step. In diluted sulfuric acid solution, there are H+ and $\mathrm{SO}_4^{--}$ ions. The H+ ions come out to the copper sulphate solution through the wall of the porous pot. The sulphate ions of diluted sulfuric acid react with the zinc rod, where Zn++ ions get attached with $\mathrm{SO}_4^{--}$ ions and form zinc sulphate (ZnSO4). During this oxidation reaction, each zinc atom leaves two electrons in the zinc rod. Hence, the zinc rod becomes negatively charged, which means it behaves as the cathode of the battery.
The hydrogen ions (H+) in the copper sulphate solution form sulfuric acid (H2SO4), and copper ions ($\mathrm{Cu}^{++}$) come to the wall of the outer copper container. By absorbing electrons from the container, copper ions deposit as copper metal on the container's wall. As a result, the copper container becomes positively charged, indicating that it serves as the Daniell Cell's anode. When an external load is connected between the central zinc rod and the peripheral copper container wall, electrons begin to flow from the zinc rod to the copper container.
Voltaic Cell Conditions
Because the external source's emf is greater than that of the voltaic cells, current can flow from the external source into the voltaic cell, reversing the cell reaction. Current flows from the voltaic cell to the external source if the emf of the voltaic cell is greater than that of the external source.
Salt bridge helps to maintain the electrical neutrality in two compartments by allowing movement of anions towards the anodic compartment and cations towards the cathodic compartment. A salt bridge is usually made by filling a U-tube or a tube with a gel or agar-agar containing an inert electrolyte, typically a salt like potassium nitrate ($\mathrm{KNO}_3$) or sodium chloride (NaCl). It's a gelatin-coated glass tube containing potassium chloride or ammonium nitrate.
Between the two half-cells, a salt bridge serves as an electrical contact.
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The Daniell cell has several advantages, including:
Some limitations of the Daniell cell are:
The Daniell cell is based on the redox reaction theory. Electrons can be transmitted as useful electrical current during the reaction cycle.
Daniell Cell Process Steps
Zn(s)+Cu+2? Zn+2(aq)+Cu+2(s)
In a Daniell cell, electrons flow through an external circuit from zinc electrode to copper electrode to copper electrode, while metal ions move from one half cell to the other via the salt bridge.
Through an external circuit, current flows from the copper electrode to the zinc electrode, which is the cathode to the anode.
A voltaic cell can be reversible or irreversible, whereas a Daniell cell is reversible.
A Daniell cell is a type of electrochemical cell that produces electricity through a redox reaction between zinc and copper. It consists of a zinc electrode in a zinc sulfate solution and a copper electrode in a copper sulfate solution, separated by a salt bridge. The zinc oxidizes, releasing electrons that flow through an external circuit to the copper electrode, where copper ions are reduced. This flow of electrons generates an electric current.
The Daniell cell is considered a galvanic cell because it spontaneously produces electricity from a chemical reaction. In galvanic cells, the redox reaction occurs naturally without any external energy input, converting chemical energy into electrical energy.
The salt bridge in a Daniell cell serves three crucial functions: 1) It completes the electrical circuit by allowing ions to flow between the two half-cells, 2) It maintains electrical neutrality in both half-cells by allowing anions and cations to move freely, and 3) It prevents direct mixing of the two electrolyte solutions, which would cause unwanted reactions.
In a Daniell cell, the anode is the zinc electrode where oxidation occurs (Zn → Zn2+ + 2e-). It is the negative terminal and the source of electrons. The cathode is the copper electrode where reduction takes place (Cu2+ + 2e- → Cu). It is the positive terminal and receives electrons from the external circuit.
The zinc electrode gradually dissolves because it undergoes oxidation, losing electrons and forming zinc ions (Zn → Zn2+ + 2e-). These zinc ions enter the solution, effectively "dissolving" the electrode over time. This process is essential for the cell's operation as it provides the electrons that flow through the external circuit.
The concentration of electrolytes affects the voltage of a Daniell cell through the Nernst equation. Higher concentrations of reactants (Zn2+ and Cu2+) increase the cell potential, while higher concentrations of products (Zn and Cu) decrease it. This is because concentration differences affect the tendency of electrons to flow, thus influencing the cell's voltage.
Replacing the copper electrode with silver would increase the cell voltage. Silver has a higher standard reduction potential (+0.80 V) than copper (+0.34 V). The cell would still function similarly, but with silver ions being reduced at the cathode instead of copper ions. The overall cell potential would increase to about 1.56 V (0.80 V - (-0.76 V)).
Temperature affects a Daniell cell's performance in several ways: 1) It influences reaction rates, generally increasing them at higher temperatures, 2) It affects the solubility of electrolytes, potentially changing ion concentrations, and 3) It impacts the cell potential according to the Nernst equation. Usually, increasing temperature slightly decreases the cell potential but can increase overall power output due to faster reaction kinetics.
The size of the electrodes affects the performance of a Daniell cell primarily through the surface area available for reaction. Larger electrodes provide more surface area for electron transfer, potentially increasing the current output of the cell. However, the voltage of the cell remains constant regardless of electrode size, as it depends on the nature of the materials, not their quantity. Larger electrodes can also extend the cell's lifespan by providing more material for the redox reactions.
Connecting multiple Daniell cells in series would increase the overall voltage of the system. Each cell contributes its voltage (about 1.1 V) to the total. For example, three cells in series would produce approximately 3.3 V. The current, however, would remain the same as that of a single cell. This arrangement is useful when higher voltages are needed while maintaining the same current output.
The lifespan of a Daniell cell is determined by several factors: 1) The amount of zinc available for oxidation, 2) The concentration of copper ions available for reduction, 3) The rate of side reactions like hydrogen evolution at the zinc electrode, 4) The maintenance of the salt bridge's effectiveness, 5) The prevention of copper ion diffusion to the zinc electrode, and 6) The external load or current draw, which affects how quickly the reactants are consumed.
The internal resistance of a Daniell cell affects its performance by reducing the voltage available at the terminals when current flows. This resistance arises from factors like electrolyte conductivity, electrode surface conditions, and the salt bridge's efficiency. Higher internal resistance leads to greater voltage drop under load, reducing the cell's efficiency and maximum power output. Minimizing internal resistance is crucial for optimizing cell performance.
The Nernst equation is crucial for understanding the Daniell cell's behavior under non-standard conditions. It relates the cell potential to the standard cell potential and the concentrations of reactants and products. This equation allows us to predict how changes in ion concentrations affect the cell voltage. For example, it explains why the cell voltage decreases as the reaction progresses and ion concentrations change, providing insight into the cell's dynamic behavior during operation.
Increasing the concentration of zinc sulfate in the anode half-cell would affect the cell potential according to the Nernst equation. It would slightly decrease the overall cell potential because it increases the concentration of a reactant (Zn2+) on the anode side. This change makes the oxidation of zinc less favorable, reducing the driving force for electron flow. However, it might increase the current capacity of the cell by providing more zinc ions for sustained reaction.
The Daniell cell demonstrates the relationship between Gibbs free energy (ΔG) and cell potential (E) through the equation ΔG = -nFE, where n is the number of electrons transferred and F is Faraday's constant. The spontaneous nature of the cell reaction (negative ΔG) corresponds to a positive cell potential. The magnitude of the cell potential (1.1 V) directly relates to the amount of Gibbs free energy released, showcasing how the favorability of the reaction translates into electrical energy output.
Using a voltmeter with high internal resistance is important when measuring the voltage of a Daniell cell to minimize current draw from the cell. A low-resistance voltmeter would allow significant current flow, causing a voltage drop due to the cell's internal resistance and polarization effects. This would result in an inaccurately low voltage reading. A high-resistance voltmeter ensures minimal current flow, allowing measurement of the cell's true open-circuit voltage without significantly affecting its state.
The standard electrode potential is crucial in understanding the Daniell cell's behavior. It represents the tendency of a half-reaction to occur relative to a standard hydrogen electrode. In a Daniell cell, the difference between the standard electrode potentials of copper (+0.34 V) and zinc (-0.76 V) determines the cell's overall voltage (1.10 V), indicating the direction and strength of electron flow.
A Daniell cell can't use the same electrolyte solution for both half-cells because this would lead to direct reaction between the zinc and copper ions, bypassing the external circuit. The separate solutions are crucial for maintaining the distinct oxidation and reduction environments at each electrode, ensuring that electron transfer occurs through the external circuit to generate usable electricity.
The Daniell cell demonstrates the electrochemical series by showing how metals with different standard electrode potentials interact. Zinc, being higher in the series (more negative potential), acts as the anode and is oxidized, while copper, lower in the series (more positive potential), acts as the cathode and is reduced. This illustrates how the relative positions of elements in the electrochemical series predict their behavior in electrochemical cells.
The composition of the salt bridge is significant because it must allow ion movement without contaminating the half-cells. Typically, an inert electrolyte like potassium chloride or sodium nitrate is used. The salt must be chemically unreactive with the cell components, have good ionic conductivity, and not interfere with the electrode reactions. The choice of salt can affect the internal resistance of the cell and its overall efficiency.
The Daniell cell relates to the Standard Hydrogen Electrode (SHE) as a practical application of electrode potentials. While the SHE is the reference point (0.00 V) for measuring standard electrode potentials, the Daniell cell uses these measured potentials in a real-world application. The cell's voltage can be predicted by comparing the standard reduction potentials of zinc and copper relative to the SHE.
A standard Daniell cell cannot be recharged because the electrochemical reactions are not easily reversible. The zinc anode dissolves over time, and copper is deposited on the cathode. Attempting to reverse the process would require a higher voltage than the cell produces and would likely result in different reactions occurring, rather than simply reversing the original process. This is why Daniell cells are classified as primary cells, not secondary (rechargeable) cells.
Electronegativity plays a crucial role in the Daniell cell's function. The difference in electronegativity between zinc and copper drives the electron transfer. Copper, being more electronegative, has a stronger tendency to attract electrons than zinc. This difference creates the potential that allows electrons to flow from the zinc anode to the copper cathode through the external circuit, generating an electric current.
A porous barrier or salt bridge is crucial in a Daniell cell to prevent direct mixing of the two electrolyte solutions while allowing ion movement. Direct contact would lead to immediate reaction between zinc and copper ions, short-circuiting the cell. The barrier ensures that the redox reactions occur at the electrodes, forcing electrons to flow through the external circuit where they can do useful work. It also maintains charge balance in each half-cell by allowing selective ion migration.
The Daniell cell illustrates spontaneous redox reactions by demonstrating how a favorable electron transfer can occur naturally between two different metals. The spontaneous oxidation of zinc and reduction of copper ions occurs because the overall process decreases the system's free energy. This spontaneity is reflected in the positive standard cell potential, indicating that the reaction will proceed without external energy input, converting chemical energy directly into electrical energy.
The Daniell cell demonstrates the conversion of chemical energy to electrical energy through a spontaneous redox reaction. The chemical energy stored in the zinc-copper system is released as zinc atoms lose electrons (oxidation) and copper ions gain electrons (reduction). This electron transfer is harnessed by forcing the electrons to flow through an external circuit, where they can perform electrical work. The amount of energy converted is related to the change in Gibbs free energy of the reaction.
The Daniell cell is composed of two half-cells: the zinc half-cell (anode) and the copper half-cell (cathode). Each half-cell represents one half of the overall redox reaction. The zinc half-cell is where oxidation occurs (Zn → Zn2+ + 2e-), while the copper half-cell is where reduction takes place (Cu2+ + 2e- → Cu). When connected, these half-cells form a complete cell, with electrons flowing from the zinc half-cell to the copper half-cell through the external circuit.
Electrical neutrality is maintained in the Daniell cell through the movement of ions in the salt bridge. As zinc oxidizes, producing Zn2+ ions in the anode half-cell, negative ions from the salt bridge move into this half-cell to balance the charge. Simultaneously, as Cu2+ ions are reduced at the cathode, positive ions from the salt bridge enter the cathode half-cell. This ion movement ensures that neither half-cell accumulates an excess of positive or negative charge, maintaining overall electrical neutrality in the system.
Water cannot be used as the sole electrolyte in both half-cells of a Daniell cell because it doesn't provide the necessary ions for the redox reactions. The cell requires Zn2+ ions in the anode half-cell and Cu2+ ions in the cathode half-cell. Pure water is a poor conductor of electricity and doesn't contain these metal ions. Using appropriate metal sulfate solutions ensures the presence of the required ions and enables the cell to function effectively.
The Daniell cell demonstrates the principle of oxidation numbers through the changes that occur at each electrode. At the zinc anode, the oxidation number of zinc increases from 0 to +2 as it loses electrons (Zn → Zn2+ + 2e-). At the copper cathode, the oxidation number of copper decreases from +2 to 0 as it gains electrons (Cu2+ + 2e- → Cu). These changes in oxidation numbers clearly show the transfer of electrons in the redox reaction, illustrating how oxidation numbers track electron movement in electrochemical processes.
The Daniell cell illustrates redox coupling by showing how the oxidation of zinc is coupled with the reduction of copper ions. These two half-reactions are complementary and must occur simultaneously for the cell to function. The electrons released by zinc oxidation are consumed by copper reduction, demonstrating how one species' oxidation drives another's reduction. This coupling is essential for the continuous flow of electrons and the generation of electrical energy in the cell.
It's crucial to keep the zinc and copper electrodes from touching in a Daniell cell to prevent a short circuit. If the electrodes touch, electrons would flow directly between them instead of through the external circuit. This would bypass the useful work the cell is designed to do and could lead to rapid, uncontrolled reaction between the metals. Keeping the electrodes separate ensures that electron flow occurs only through the intended path, allowing the cell to generate usable electricity.
If you used a zinc electrode in a copper sulfate solution and a copper electrode in a zinc sulfate solution, the Daniell cell would not function as intended. Instead, a rapid, direct reaction would occur at the zinc electrode, where zinc would spontaneously reduce copper ions without generating a useful electric current. This setup would essentially short-circuit the cell, producing heat rather than electrical energy, and quickly deplete the reactants without creating a sustained voltage.
Standard reduction potentials help predict the behavior of a Daniell cell by indicating the direction of electron flow and the magnitude of the cell voltage. The more positive reduction potential (copper: +0.34 V) indicates the species more likely to be reduced, while the more negative potential (zinc: -0.76 V) indicates the species more likely to be oxidized. The difference between these potentials (1.10 V) predicts the cell's standard voltage and confirms that the reaction will be spontaneous.
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