Give one example each of electrolytic concentration cells with and without transference. Also write the cell reactions and expressions for the emf of these cells
Answers
Explanation:
concentration cell is an electrolytic cell that is comprised of two half-cells with the same electrodes, but differing in concentrations. A concentration cell acts to dilute the more concentrated solution and concentrate the more dilute solution, creating a voltage as the cell reaches an equilibrium. This is achieved by transferring the electrons from the cell with the lower concentration to the cell with the higher concentration.
Introduction
The standard electrode potential, commonly written as Eocell, of a concentration cell is equal to zero because the electrodes are identical. But, because the ion concentrations are different, there is a potential difference between the two half-cells. One can find this potential difference via the Nernst Equation,
Ecell=E∘cell−0.0592nlogQ(1)
at 25oC. The E stands for the voltage that can be measured using a voltmeter (make sure if the voltmeter measures it in millivolts that you convert the number before using it in the equation). Note that the Nernst Equation indicates that cell potential is dependent on concentration, which results directly from the dependence of free energy on concentration. Remember that to find Q you use this equation:
aA+bB↽−−⇀cC+dD(2)
Q=[C]c[D]d[A]a[B]b(3)
When Q=1, meaning that the concentrations for the products and reactants are the same, then taking the log of this equals zero. When this occurs, the Ecell is equal to the Eocell.
Another way to use the Eocell , or to find it, is using the equation below.
E∘cell=E∘cathode−E∘anode
Zn(s)|Zn2+(1 M)||Cu2+(1 M)|Cu(5)
The double lines between the Zn2+(1 M) and the Cu2+(1 M) signify the salt bridge. The single lines, however, do not represent bridges; they represent the different phase changes, for example, from solid zinc to liquid zinc solution. If there is a comma where you would expect to see a single line, this is not incorrect. It simply means that no phase changes occurring
Problems
1.) For the concentration cell bellow determine the flow of electrons.
Fe|Fe2+(0.01 M)||Fe2+(0.1 M)|Fe(6)
SOLUTION:
The cells will reach equilibrium if electrons are transferred from the left side to the right side. As a result, Fe2+ will be formed in the left compartment and metal iron will be deposited on the right electrode.
2.) Calculate cell potential for a concentration cell with two silver electrodes with concentrations 0.2 M and 3.0 M.
SOLUTION:
Reaction:
Ag2++2e−⟶Ag(s)(7)
Cell Diagram:
Ag(s)|Ag2+(0.2 M)||Ag2+(3.0 M)|Ag(s)(8)
Nernst Equation:
E=E∘−0.05922log0.023.0(9)
**Eo= 0 for concentration cells
E = 0.0644 V