Lesson Notes By Weeks and Term - Senior Secondary School 2

SUBJECT: CHEMISTRY

CLASS:  SS 2

DATE:

TERM: 1st TERM

REFERENCE MATERIALS

• New School Chemistry for Senior Secondary Schools by O.Y Ababio
• New System Chemistry for Senior Secondary Schools by T. Y. Toon et al
• S.S.C.E  Past Questions and Answers on Chemistry 
• U.T.M.E Past Questions and Answers on Chemistry

 
WEEK NINE

TOPIC: ELECTROCHEMICAL CELLS

CONTENT

• Electrolytic and Electrochemical Cells with their differences.
• Drawing and Writing of Cell Diagrams.
• Standard Electrode Potential.
• Calculation of e.m.f of a Cell.

ELECTROLYTIC AND ELECTROCHEMICAL CELL

Electrolytic cell is a device in which electrical energy is converted to chemical energy. An electrochemical cell is a device in which chemical energy is converted to electrical energy.

DIFFERENCES BETWEEN ELECTROLYTIC CELL AND ELECTROCHEMICAL CELL

ELECTROLYTIC CELL                    ELECTROCHEMICAL CELL 

1.   Electrical energy is converted to                Chemical energy is converted energy

chemicalenergy.        electrical energy.

2.   Cathode is negative electrode while anode Cathode is positive electrode while

is positive electrode.            anode is negative electrode.

3.   No salt bridge is required Salt bridge or porous partition required.                            is required.
4. Electrolyte may not contain ions of     Electrolyte must contain metal ions of metal electrode used.electrode used           
5. Electrons are pushed by an external source   Electrons are generated by oxidation  such as battery. at the anode.

Electrolytic cell is also known as galvanic cell it consists of two half cells

1. an oxidation half cell
2. a complementary reduction half-cell.

DRAWING AND WRITING OF CELL DIAGRAMS

A typical electrochemical cell consists of two electrodes (cathode and anode) and their electrolytes. The electrodes are connected by means of a salt bridge. A typical electrode or half-cell is a device in which an element (metal) is in contact with its own ions. A typical metallic electrode is symbolically represented as:

M(s)/Mn+ (aq) or Mn+ (aq)/M(s)for metals dipped into solutions of their own ions.

Conventional notation is used to describe an electrochemical cell without drawing. Example:

X/Xn+ //Yn+/Y

The single vertical lines (/) represents metal and metal ion interphase while the double lines (//) represents the salt bridge or a porous partition.  

For a simple electrochemical cell, there is flow of electron from the anode (negative electrode) to the cathode (positive electrode). Also, for an electrochemical to produce electricity, the anode element must be more reactive than the cathode element. That is, the anode element must be higher in the electrochemical series than the cathode.

For a given net reaction as shown below

Zn(s)   +   Cu 2+(aq)          Zn2+(aq)  +   Cu(s)

The Cell notation for the system is 

Zn/Zn2+ // Cu2+/ Cu

Since Zn is higher up in the electrochemical series, it will serve as the anode while copper will serve as the cathode. The anodic half-cell is always written at the left hand side while the cathodic half-cell is always the right hand side.

The diagram below shows the drawing of the above electrochemical cell:

Salt Bridge

In an electrochemical cell, instead of using the porous partition to maintain the electrical neutrality of the half-cell electrolytes, the half-cells can be linked by a SALT BRIDGE. Salt bridge is a filter paper soaked in sodium chloride solution or KCl or NH4NO3.

Functions of a salt bridge

1. It helps in completion of electric circuit
2. It enables the migration of ions from one half cell to the other in order to maintain electrical neutrality in the solution when current flows in the set up

EVALUATION

1. State two differences between electrolytic and electrochemical cells.

2a.    Write a cell notation for the equation below

Zn(s)   +  2Ag+(aq)          Zn2+(aq)  +   Ag(s)

1.  Draw the cell diagram for the cell above.

ELECTRODE POTENTIAL

This is the potential difference that is set up between metallic electrode and its electrolyte. Example

1. Copper ion/copper system:

If a copper plate is dipped into a CuSO4 solution, the net reaction is 

Cu2+(aq)    +    2e-            Cu(s)

That is: Cu2+(aq)/Cu(s)

1. Zinc ion/zinc system:

If a zinc plate is dipped into a ZnSO4 solution; the net reaction is

Zn2+(aq)    +    2e-                         Zn(s)

That is: Zn2+(aq)/Zn(s)

Standard Electrode Potential

The electrode potential of a given system depends on

1. the overall energy change
2. the concentration of ions in the solution
3. the temperature

Standard electrode potential (E0) is used to compare the electrode potential values of different metal ions/metal system.

In measuring standard electrode potential of metal ion/metal system difference (in volts) which exists between the metallic electrode and the standard hydrogen electrode, the standard hydrogen electrode potential is considered to have an arbitrary value of zero.

The standard electrode potentials of a metal ions/metal system is the potential difference set up between the metal and its ion compared to electrode potential of H+(aq)and H2 gas  system. 

Metal ion/Metal                            E0

K+(aq)/K(s)                                -2.92V

Ca2+(aq)/Ca(s)                                -2.87V

Na+(aq)/Na(s)                                -2.71V

Zn2+(aq)/Zn(s)                                -0.76V

2H+(aq)/H2(g)                                0.00

Cu2+(aq)/Cu(s)                                +0.36V

Ag+(aq)/Ag(s)                                +0.80V

Electrode potential of Cu =  +0.34V as electron flows from the hydrogen electrode to copper electrode.  At platinum hydrogen electrode

    H2(g)            2H+    +    2e-(oxidation)

At the copper electrode

Cu2+(aq)    +    2e-        Cu(s)        (reduction)

Overall reaction:  Cu2+(aq)+    H2(g)        Cu(s)    +    2H+(aq)

NOTE:  EÇ¿total is negative if electron flows from the metal electrode and positive when it flows from hydrogen electrode to the metal electrode

EVALUATION

1. Define electrode potential
2. List two functions of a salt bridge.

E.M.F OF A CELL

When two half-cells are joined together through a salt-bridge, thee.m.f of the cell thus formed is the algebraic difference between the two electrode potentials.

The e.m.f. of the cell formed by the system Zn(s)/Zn2+(aq)//Cu2+(aq)/Cu(s) is defined as the standard electrode potential of the right hand electrode minus the standard electrode  potential of the left-hand electrode.

EÇ¿total = EÇ¿R- EÇ¿L

    = +0.34 – (-0.76)

        =+1.10V

A positive e.m.f. indicates that the left- hand electrode(zinc) is partially capable of reducing copper (ii) ions to copper

But for the system Cu(s)/Cu2+(aq)//Zn2+(aq)/Zn(s)

EÇ¿total = EÇ¿R - EÇ¿L= -0.76-0.34= -1.10V

A negative e.m.findicates that the left-hand electrode (copper) cannot reduce zinc ions to zinc atom.

In general, when this convention is used, then a positive e.m.f indicates that the reaction is thermodynamically feasible as written down from left to right.

A negative e.m.f implies that the reaction is thermodynamically impossible as written down.

GENERAL EVALUATION/REVISION

1. Calculate the e.m.f of a cell when zinc half cell is connected with a salt bridge to a copper half cell if the EÓ© of Zn2+(aq)/Zn(s) =  -0.76V and Ag+(aq)/Ag(s)  =  +0.80V
2. Give two examples of primary cells
3. Define electrode potential and list two functions of a salt bridge.
4. What is standard electrode potential?
5. State Aufbau principle and Hund’s rule of maximum multiplicity.

READING ASSIGNMENT

New School Chemistry for Senior Secondary Schools by O. Y. Ababio, pages 215-216

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY

1. A Porous materials used to maintain electrical neutrality in solution of an electrochemical cell is called A. Pot B. Salt bridge C. Paper D. none of the above
2. The element with the electronic configuration 1s22s22p63s23p64s2 is a/an
3. group two element B. transition metal C. alkaline earth metal D. none of the above
4. Oxidation takes place at the anode during electrolysis because anode
5. is deficient in electrons B. is different in protons C. has excess electrons D. none of the above
6. The potential difference set up between metallic electrode and the electrolyte solution is known as A.  Lead accumulator B. electrode potential C. Voltammeter
7. none of the above
8. In an electrochemical cell, the cathode is made A. negative B. positive c. neutral
9. none of the above

SECTION B

1a.    State THREE differences between an electrolytic cell and an electrochemical cell

1.   Mention TWO function of a salt bridge
2.   What is the voltage of the cell represented as Ca(s)/Ca2+(aq)//Cu2+(aq)/Cu(s) given that electrode potential of Ca2+(aq)/Ca(s) is -2.87V and   that of Cu2+(aq)/Cu(s) is +0.36V