CHAP_6_E

CHAPTER 6 ELECTROCHEMISTRY

(E) VOLTAIC CELLS (GALVANIC CELL)

VOLTAIC CELL • CHEMICAL ENERGY

ELECTRICAL ENERGY

• KNOWN AS A GALVANIC CELL • TWO DIFFERENT METAL RODS (ELECTRODES) ARE IMMERSED IN AN ELECTROLYTE

• THE REACTIONS BETWEEN THE ELECTRODES AND THE ELECTROLYTE PRODUCE A POTENSIAL DIFFERENCE BETWEEN THE TWO EELCTRODES

STRUCTURE OF A VOLTAIC CELL

MAGNESIUM RIBBON

SODIUM CHLORIDE, NaCl

COPPER PLATE

- The more electropositive metal will ionise by losing electrons and becomes negative terminal [anode] - The electrons are taken by the less electropositive metal and becomes positive terminal [cathode] - Electrons pass from the anode to cathode through external circuit

Investigating the simple voltaic cell

COPPER PLATE

MAGNESIUM RIBBON SODIUM CHLORIDE,NaCl Type of  metal

Voltmeter reading (V)

Observation

Magnesium / copper 

Magnesium electrode dissolvesBubbles of gas produced at copper cathode

Copper / copper 

No changes at the electrode

How does a voltaic cell work? Voltmeter 

V Electron flow

Copper plate

Magnesium ribbon Mg

Mg

2+

2H+ + 2e

+ 2e

H2

Sodium chloride, NaCl solution Overall equation : Mg + 2H+

Mg2+ + H2

DANIELL CELL • ZINC and COPPER as electrodes • ZINC (anode) & COPPER (cathode) • Each metal immersed into a solution of its ion • Two solutions are connected through a salt bridge or  porous pot

The purposes of salt bridge or  porous pot

1. Complete the circuit by allowing the ions move from one solution to another 2. Separate the two electrolytes 3. Prevent the zinc from reacting directly with copper (II) sulphate solution

What is a salt bridge? A salt bridge is actually a filter paper soaked in a concentrated solution that will not react with the electrolyte. The common solutions used are potassium nitrate and potassium chloride

• The porous pot is more commonly used because the distance of the ions have to travel Porous pot is less

Salt bridge

Zinc

Zinc sulphate solution

Zinc sulphate solution

Porous pot

Copper (II) sulphate solution

Copper (II) sulphate solution

Copper

ELECTROCHEMISTRY REACTIVE METAL

LESS REACTIVE METAL

ANODE

CATHODE

DONATE ELECTRON

ACCEPT  ELECTRON

Zn

Cu

R  e  a  c  t   i   v i    t    y  d   e  c r   e  a  s  e

K Na Ca Mg Al Zn Fe Sn Pb H Cu Ag

Reactions in the Daniell cell 1. Zinc atoms lose electrons to produce zinc ions 2. Zinc ions go into zinc sulphate solution

3. The electrons travel through the wire in the external circuit to the copper  electrode

Reactions in the Daniell cell 4. At the copper electrode, the electrons are taken by Cu2+ ions in the copper (II) sulphate solution 5. Copper metal is deposited on the electrode 6. The overall chemical equation:

ELECTROCHEMISTRY REACTIVE METAL, Zn ANODE DONATE ELECTRON Z

Zn2+ + 2e

Zinc electrode dissolve

LESS REACTIVE METAL, Cu CATHODE ACCEPT  ELECTRON

Cu2+ + 2e

Cu

Copper become bigger 

Zn Cu

R  e  a  c  t   i   v i    t    y  d   e  c r   e  a  s  e

K Na Ca Mg Al Zn Fe Sn Pb H Cu Ag

ELECTROCHEMISTRY

REACTIVE METAL

LESS REACTIVE METAL

ANODE

CATHODE

DONATE ELECTRON

ACCEPT ELECTRON

Varous types of  voltaic cell

Lead acid Accumulator

Dry cell

Advantages Rechargeable Can be made to have higher voltage

Advantages Portable Cheap Stable voltage of 1.5 v

Disadvantages Heavy Expensive Spilled easily

Mercury cell

Alkaline cell

Nickel-cadmium cell

Advantage Portable

Advantage Portable Voltage of 1.5 V More long lasting

Advantages Portable Rechargeable up to hundreds of times

Disadvantages Disadvantages Disadvantages Produce low Not rechargeable Disadvantages Not voltage of 1.25 Produces voltage Not rechargeable rechargeable Expensive May leak of 1.3 V only Heavy Poisonous

Differences between electrolytic cells and voltaic cells Electrical energy  Chemical energy

Energy change

From positive terminal Electron flow to negative terminal

Chemical energy  electrical energy From negative terminal to positive terminal

Cathode: Ions take in Negative terminal Anode: Electrons electrons released, forming ions node : Ions release Positive terminal Cathode, electrons electrons taken in Two similar (usually carbon) or different metals

Types of  electrodes

Two different metals

Thank you.