How Do Concentration Cells Work? Understanding Mechanism & Types
What is a Concentration Cell?
A concentration cell is formed when two half cells with the same electrodes and different concentrations are connected. In a concentration cell, the concentration of the more dilute solution increases, and the concentration of the concentrated solution decreases. The process of the concentration and dilution of the solutions continue to happen until a state of equilibrium of concentration is obtained. This means that the shift in concentration stops only after the concentration of solutions in both the half cells are equal.
A Sneak Peek Into Concentration Cells
Standard electrode potential which is commonly called Cell, in scientific terms is defined as the measures of individual potential of a reversible electrode at standard state with ions at a concentration of 1 mol/ dm³. The electrolyte concentration cell consists of 2 half cells or galvanic cells.
What is Half Cell?
A half cell is an electrochemical arrangement. It consists of two electrode rods made of any conductive material which is dipped in a conductive electrolyte. The electrodes are connected to a source of energy, most likely a 9 volt DC battery. This facilitates the motion of electrons between both the half cells until a state of equilibrium is obtained. Hence, using conductive electrolytes and electrode rods is essential, as this facilitates the movement of electrons and aids in obtaining a state of equilibrium.
More Interesting Facts About Concentration Cells
Half cells are used in concentration Cells. Concentration Cells, which is our topic today is a special case of electrolysis. Generally, the two electrode rods used and electrolytes used are different. However, in electrolytic concentration cells, both the electrodes and electrolytes used are the same but they differ in concentration. The method is used to make two solutions of equal concentration.
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The diagram above illustrates a half cell. Now, keenly observe the elements that it consists of. It clearly shows the use of electrodes and electrolytes. In this case, two different electrolytes are used.
Representing an electrolytic concentration cell through half cell reactions.
Pt|H₂(1 atm)|H⁺||H⁺|H₂(1 atm)|Pt
The above-mentioned reaction signifies a concentration cell reaction where the concentration of H+ ions differs in each half cell. The electrodes used in the reaction are made of platinum. Once the battery source is turned on the concentration of H+ will shift from high to low. The electrodes used don't have any role to play, instead of platinum, any rods made of conductive material could be used.
Types of Concentration Cells
Electrolyte Concentration Cells: The method is used to obtain two same electrolyte solutions of the same concentration. In this method, two conducting electrodes are dipped into the electrolytes, and the current is passed through the cell. Once the current is passed, the excessive electrons steadily move from higher concentration to lower concentration. The process continues until the concentration of both solutions is equal. For instance ,Fe/Fe²⁺(0.1M) is the cathode and Fe²⁺(0.01M)/Fe is the anode. In this case, we can notice that Fe²⁺ in the cathode is more concentrated whereas the anode is dilute. Hence, electrons would move the cathode to anode until the concentration of Fe²⁺ in both the half cells is equal, in this case, 0.55M.
Electrode Concentration Cell: Electrode concentration cells are the opposite of electrolyte concentration cells. In this case, identical solutions are used in both the half cells, cathode, and anode respectively. The electrodes in electrode concentration cells differ in concentration. Once the circuit is electrocuted, the concentration effect is negated and electrons move from the concentrated electrode to the diluted electrode.
An example of an Electrode Concentration Cell is a cell consisting of two hydrogen electrodes which are subjected to varied pressure.
Both these types of concentration cells are concentration cells with transference as they include the transfer of electrons from one half to another. Example of concentration cells without transference include
Pt,H₂, HCl/ AgCl, Ag
The two electrolytes don't come in direct contact with each other and hence are concentration cells without transference.
Solved Problems Concentration Cell
Let us look at a problem related to concentration cells, this will give us a better understanding of the topic.
1. The Concentration Cell Below Determines the Flow of Electrons.
Zn|Zn²⁺(0.01 M)||Zn²⁺(0.1 M)|Zn
Ans. The question indicates the presence of a higher concentration of ions in the anode than the cathode. Hence, according to the concept, we learned today, Zn²⁺ ions from the anode would move towards the cathode until both the half cells have 0.55 M concentration.
2. Calculate the Cell Potential for a Concentration Cell with Two Iron Electrodes with Concentrations 0.2 M and 3.0 M.
Ans:
Reaction:
Fe²⁺+2e− ⟶ Fe(s)
Cell Representation :
Fe(s)|Fe²⁺(0.2 M)||Fe²⁺(3.0 M)|Fe(s)(8)
Nernst Equation:
E=E⁰−0.05922log0.023.0(9)
**E⁰= 0 for concentration cells
E = 0.0644 V
FAQs on Concentration Cell in Chemistry: Concepts, Examples & Applications
1. What is a concentration cell in Chemistry?
A concentration cell is a specific type of galvanic cell where both half-cells are composed of the same electrode and electrolyte materials. The generation of an electric current is not due to a chemical reaction but is driven solely by the difference in the concentration of the electrolyte solutions or the pressure of the electrodes. The cell operates spontaneously to equalise the concentrations, producing a small but measurable electromotive force (EMF).
2. What are the two main types of concentration cells?
The two primary types of concentration cells are:
Electrode Concentration Cell: In this type, identical electrodes are immersed in the same electrolyte solution, but the electrodes themselves have different 'effective' concentrations. A common example is using two hydrogen electrodes at different gas pressures.
Electrolyte Concentration Cell: This is the more common type where identical electrodes are placed in two solutions of the same electrolyte, but the solutions have different molar concentrations. The potential is generated due to the flow of ions from the more concentrated solution to the more dilute one.
3. Can you provide a simple example of an electrolyte concentration cell?
A classic example is a copper concentration cell. It is set up with two copper electrodes, each submerged in a copper sulfate (CuSO₄) solution. However, one solution might be 1.0 M while the other is 0.01 M. The half-cell with the lower concentration (0.01 M) acts as the anode (where oxidation occurs), and the half-cell with the higher concentration (1.0 M) acts as the cathode (where reduction occurs), connected by a salt bridge.
4. What is the formula used to calculate the EMF of a concentration cell?
The electromotive force (EMF or E_cell) of a concentration cell is calculated using the Nernst equation. Since the standard cell potential (E°_cell) is zero, the formula simplifies to:
E_cell = (2.303RT / nF) log₁₀(C₂ / C₁)
Where:
- R is the universal gas constant (8.314 J/K·mol).
- T is the temperature in Kelvin.
- n is the number of moles of electrons transferred in the reaction.
- F is the Faraday constant (approx. 96,500 C/mol).
- C₂ is the higher concentration (cathode) and C₁ is the lower concentration (anode).
5. What are some important applications of concentration cells?
Concentration cells are important for several scientific and biological applications. Key examples include:
- pH Meters: A pH meter works on the principle of a concentration cell, measuring the potential difference created by a difference in H⁺ ion concentration between a reference electrode and a sample solution.
- Determining Solubility Products: They can be used to find the K_sp of sparingly soluble salts.
- Nerve Impulse Transmission: In biology, the transmission of signals along nerve cells (neurons) is based on the potential difference created by concentration gradients of ions like Na⁺ and K⁺ across the cell membrane.
6. How is a concentration cell represented using standard cell notation?
Using the example of a zinc-based electrolyte concentration cell, the standard notation is written as:
Zn(s) | Zn²⁺(aq, C₁) || Zn²⁺(aq, C₂) | Zn(s)
In this notation:
- C₁ is the molar concentration of the anode (dilute solution).
- C₂ is the molar concentration of the cathode (concentrated solution).
- The single vertical line '|' denotes a phase boundary.
- The double vertical line '||' represents the salt bridge that connects the two half-cells.
7. Why is the standard cell potential (E°cell) of a concentration cell always zero?
The standard cell potential (E°_cell) is calculated by subtracting the standard electrode potential of the anode from that of the cathode (E°_cathode - E°_anode). In a concentration cell, both the anode and the cathode are made of the exact same material (e.g., both are copper electrodes). Therefore, their standard electrode potentials are identical. When you subtract a value from itself, the result is always zero. The potential in this type of cell arises exclusively from the concentration difference, not the chemical nature of the electrodes.
8. How does a difference in concentration actually create an electric potential?
A difference in concentration creates a natural thermodynamic drive to reach equilibrium. The system works to eliminate this difference. In the half-cell with the lower concentration, the equilibrium shifts to produce more ions, so the electrode undergoes oxidation (acting as the anode). In the half-cell with the higher concentration, the equilibrium shifts to consume ions, so reduction occurs (acting as the cathode). This spontaneous process forces electrons to flow from the anode to the cathode through an external wire, creating a measurable electric potential or EMF.
9. What is the difference between a concentration cell 'with transference' and 'without transference'?
The primary difference lies in how the two half-cells are connected and how ions move between them.
- A cell 'with transference' allows the electrolyte solutions to be in direct contact, often through a porous membrane. This permits ions from the solutions to migrate across the boundary, creating a 'liquid junction potential' which complicates the overall cell EMF.
- A cell 'without transference' uses a salt bridge to connect the two half-cells. The salt bridge completes the circuit by allowing its own ions (e.g., K⁺, Cl⁻) to migrate into the half-cells, which balances the charge without the two electrolyte solutions directly mixing. This design effectively eliminates the liquid junction potential, making the cell's behaviour more ideal and easier to calculate.
10. What happens to a concentration cell when it reaches equilibrium?
As a concentration cell operates, the concentration in the dilute half-cell (anode) increases due to oxidation, while the concentration in the concentrated half-cell (cathode) decreases due to reduction. This process continues until the concentrations in both half-cells become equal. At this point, the system has reached equilibrium. The driving force for the reaction disappears, the potential difference (EMF) drops to zero, and the cell stops producing an electric current.
