ELECTROCHEMISTRY
Electrical Conductor
Electrical Conductor – A substance that conducts electricity but is not chemically
changed in the process.
• Mostly metals conduct electricity except not metal carbon in the form of
graphite also conduct electricity.
• Non- Metals do not conduct electricity.
• Metals conduct electricity due to the movement of free electrons (Delocalized
electrons).
• The Battery acts as an ‘electron pump’. Electrons are repelled (pushed) into the
circuit from the negative terminal of the battery. They are attracted to the
positive terminal.
• The battery ‘pumps’ all the electrons in one direction.
Note – Metallic alloys are held together by the same type of bonding as the metal
elements, so they can also conduct electricity.
INSULATORS
• Solid covalent non-metals are insulators, they do not conduct electricity.
Whether they are giant molecular or simple molecular structures.
• They don’t conduct electricity because they are not having free electrons.
Note – There is no chemical change when an electric current is passed through a
metal or graphite. e.g., the copper wire is still copper when the current is
switched off!
Conductivity in Liquids
• Electric Current by liquids is conducted by the movement of ions.
• The process by which an electric current flows through a liquid compound or
solution is called electrolysis.
• During electrolysis, the compound undergoes a chemical reaction and it
changes chemically.
• Liquids that conduct electricity by the movement of ions are called
electrolytes and liquids that do not conduct electricity in this way are called
non – electrolytes.
Electrolytes
Non – Electrolytes
Sulfuric Acid
Distilled Water
Molten Lead Bromide
Ethanol
Sodium Chloride Solution
Petrol
Hydrochloric acid
Parraffin
Copper(II) Chloride
Molten Sulfur
Sodium Hydroxide Solution
Sugar Solution
Electrolysis and the Movement of Ions
• Conductivity of electricity by compounds also depends on the type of bonding
in the compounds.
• Compounds having covalent bonds do not conduct electricity.
e.g, Petrol, Sugar Solution, pure water, etc.
• Compounds having ionic bonds conduct electricity in the molten or in dissolved
state.
e.g, sodium Chloride, Copper sulfate, Calcium Chloride, etc.
• Ionically bonded compounds conduct electricity in a molten or dissolved state
(liquid) by the movement of ions.
• Ionic compounds will not conduct electricity when they are solid because their
ions are fixed in position and can not move. As the solid particles present can
only vibrate about a fixed position.
• When electrolytes conduct electricity during electrolysis, a chemical change
takes place and the ionic compound is broken down to form ions and these ions
moves toward the opposite electrodes.
PbBr2
Pb
+
Br2
• During electrolysis positively charged Lead ions move towards the cathode
while negatively charged bromide ions move towards the anode.
Activity – Electrolysis
• Electrolysis of Copper(II)chromate(VI) [CuCrO3] dissolve in water to give a
green solution and placed in a lower part of a U – tube.
• A Colorless solution of dilute hydrochloric acid is then layered on top of the salt
solution in each arm of the tube and graphite rods are fitted.
• These graphite electrodes carry the current into and out of the solution.
• In electrolysis, the negative electrode is called the cathode and positive
electrode is called the anode.
• After passing the current , the solution around the cathode becomes blue due
to the movement of Cu+2 ions towards the cathode.
Around the anode solution becomes yellow due to the movement of
chromate ions (CrO4 -2) towards the cathode.
The use of Coloured ions in solution has shown the direction that positive and negative ions
move in an electric field.
Electrolytic Cells
• The apparatus in which electrolysis is carried out is known as an electrolytic
cell.
• Direct current (DC) is supplied by a battery or power pack.
• Graphite electrodes carry the current into and out of the electrolyte.
• Graphite is chosen because it is unreactive (inert) to the electrolyte and the
product of electrolysis.
• In the external circuit electrons flow from the negative terminal of the battery
to the cathode, and then from the anode back to the positive terminal.
Note – Sometimes Platinum electrodes are also used in place of graphite
electrodes.
• In the electrolyte it is the ions that move to carry the current.
• Positive ions (Cations) moves towards the cathode.
• Negative ions ( Anions) moves towards the anode.
Conductivity In Metals
Electrolytic Conductivity
Electric current is conducted by the flow of electrons.
Electric Current is conducted by the flow of ions.
It is a property of metals, alloys and carbon as graphite
A property of ionic solids
Takes place in solids and liquids
Take place in a liquid state ( Dissolved or Molten)
No Chemical change take place
Chemical decomposition takes place
Q. Why ionic compounds does not conduct electricity in the solid state?
or
Why Solid electrolytes do not conduct electricity?
Ans. A solid ionic compound will not conduct electricity because the ions are in
fixed position in a solid, they cannot move. The electrolyte must be melted or
dissolved in water for it to conduct.
Products of Electrolysis
Electrolysis of Molten Ionic Compounds Lead(II) Bromide
• An electrolytic cell is used to electrolyse molten compounds. Heat must be
supplied to keep the salt molten.
• The electrical energy from the cell has caused a chemical change
(decomposition). The cell decomposes the molten lead bromide because the
Pb+2 and Br- ions present move to opposite electrodes where they are
discharged.
• At Cathode –
Pb+2 + 2 ePb
Lead ions move towards the cathode where they discharged to form a bead of
liquid metal and get collected at the bottom of the cathode.
• At Anode 2BrOverall Equation –
Pb+2 + 2Br-
Br2 + 2 e-
PbBr2
During electrolysis, the flow of electrons continues through the circuit. For every
two electrons taken from the cathode by a lead ion, two electrons are set free at
the anode by two bromide ions.
So, overall, the electrons released at the anode flow through the circuit
towards the cathode.
Electrolysis of Lead Bromide
Examples of the electrolysis of Molten Salts
Electrolyte
Product at Cathode
Product at Anode
Lead Bromide [PbBr2]
Lead [Pb]
Bromine [Br2]
Sodium Chloride [NaCl]
Sodium [Na]
Chlorine [Cl2]
Potassium Iodide [KI]
Potassium [K]
Iodine [I2]
Copper(II)bromide (CuBr2)
Copper (Cu)
Bromine (Br2)
Zinc Chloride (ZnCl2)
Zinc (Zn)
Chlorine (Cl2)
Aluminium Oxide
Aluminium (Al)
Oxygen (O2)
Notes• The electrolysis of molten salts is easier if the melting point of salts is
important
Electrode Half-Equations
Electrolyte
Cathode Reaction
Anode Reaction
Lead Bromide, PbBr2
Pb+2 + 2e-
Pb
2Br-
Sodium Chloride, NaCl
Na+ + e-
Na
2Cl-
Cl2 + 2e-
Aluminium Oxide, Al2O3
Al+3 + 3e-
Al
2O-2
O2 + 4e-
Copper(II) Bromide, CuBr2
Cu+2 + 2e-
Cu
2Br-
Br2 + 2e-
Br2 + 2e-
Electrolysis of Solutions
• Ionic Compounds dissolve in water and produce ions, ions are free to move and
conduct electricity. The electrolysis of ionic solutions also produces chemical
changes.
• Product of salts on electrolysis are different for molten salt and dissolved salt.
• In the dissolved state water is also present with the electrolyte. Water is a
simple molecule which split into Hydrogen (H+) ion and Hydroxide (OH-) ion.
H2O
H+ + OHNote –1. Pure water do not conduct electricity because ions produced are not
enough to conduct the electricity.
2. Water is a weak electrolyte, during electrolysis hydrogen and hydroxide ions
compete with the ions from acid or salt to be discharged at the electrode.
Electrolysis of Dilute Sulfuric Acid
• Pure water is a poor conductor of electricity but it decomposes easily with a
few drops of sulfuric acid to give the ions – H+, OH- and SO4 -2.
• At each electrode only one type of ion gets discharged.
• At Cathode – only H+ ions move towards the cathode. So, only Hydrogen ions
get discharged at the anode.
2H+ + 2 eH2
• At Anode – Sulphate and Hydroxide ions move towards the anode but
hydroxide ions are preferentially discharged at the anode.
4 OH2 H2O + O2 + 4e-
Hoffman Voltameter
• The ratio of the volume of gas produced is approximately 2:1.
• Hydroxide (\(OH^{-}\)) ions are discharged preferentially over sulfate
(\(SO_{4}^{2-}\)) ions during the electrolysis of dilute sulfuric acid because
hydroxide ions have a lower electrode potential. This means that hydroxide
ions are more easily oxidized and discharged.
Electrolysis of Salt Solution
Example – Sodium Chloride
• A concentrated solution of sodium chloride is electrolyzed to produce Na+ , H+,
Cl- and OH- ions.
• Reaction at the Cathode – Both metal ion (Na+ ) and Hydrogen ion (H+) move
towards the cathode and compete with each other. The hydrogen ion is
preferentially discharged at the cathode.
At the cathode, it is an H+ ion that accepts electrons, as Sodium is more
reactive than Hydrogen.
2H+ + 2eH2
• Reaction At Anode – Cl- ions are more readily discharged than OH- ions. A pale
green colour chlorine gas is liberated at the anode.
2 ClCl2 + 2 e-
• The Na+ and OH- ions are left behind in the solution to form sodium Hydroxide,
which makes the solution alkaline.
• Overall Reaction –
2NaCl + 2H2O
2NaOH + H2 + Cl2
• The products of the reaction Hydrogen, Chlorine, and Sodium Hydroxide are
industrially very important and forms the basis for the Chlor – Alkali industry.
Effect of Concentration During Electrolysis
• Electrolysis of dilute solution of sodium chloride results in the formation of
oxygen at the anode rather than chlorine.
• When the concentration of chloride ions is low, it is the hydroxide (OH-) ions
that are discharged preferentially.
4 OH2H2O + O2 + 4 e• Hydrogen is still produced at the cathode.
Electrolytic Solution
Product at the Cathode
Product at the Anode
Concentrated NaCl
Hydrogen
Chlorine
Dilute NaCl
Hydrogen
Oxygen
Concentrated KBr
Hydrogen
Bromine
Dilute KBr
Hydrogen
Oxygen
Electrolysis of Copper Sulfate Solution
• Blue copper(II) Sulphate solution is electrolysed by using inert graphite
electrodes.
• At the Cathode – Red Brown copper is deposited at the cathode.
Copper is less reactive than hydrogen, therefore it is the copper ion that is
preferentially discharged at the cathode.
Cu + 2 + 2eCu
• At the Anode - Oxygen gas is produced at the anode as the hydroxide ions
from water are discharged rather than the sulfate ions.
4OH2H2O + O2 + 4e-
• As the electrolysis proceeds, the blue colour of the solution will fade as the
copper ions causing the colour are discharged.
• The electrolytic solution will also become more acidic as OH- ions are
discharged.
Rules For Ion Discharge at the Electrode
At the Cathode • The more reactive a metal, the more it tends to stay as ions and not discharged. The
H+ ion will accept electrons instead to form a Hydrogen molecule while reactive
metal stays in the solution.
• In Contrast, the ions of less reactive metal will accept the electron readily and
discharged as metal atom. In this case metal ion is discharged at the electrode while
Hydrogen ion remains in the solution.
Na+ Mg+2
Al+3
Zn+2
Reactivity decreases
More likely to be discharged
H+
Cu+2
Ag+
At the Anode –
• If the ions of halogen (Cl-, Br- or I-) are present in a high enough concentration,
they will give up electrons more readily than OH- ions will. Molecules of
Chlorine, Bromine or Iodine will form, and OH- remain in the solution.
• If no halogen ion is present or the solution is too dilute, the OH- ions will give
up the electrons. When OH- ions are discharged oxygen is formed.
• Sulfate and nitrate ions are not discharged in preference to OH- ions.
SO4 2-
NO3 -
OH -
More likely to discharge
Cl- Br- I-
Electrode Half – Equations in Electrolysis
• Reaction at the electrodes involves the loss and gain of electrons.
• Negative ions move toward the anode and lose electrons while positive ions
move toward the cathode and gain electrons.
• Oxidation can be defined as the loss of electrons and reduction can be defined
as the gain of electrons.
Example – Overall Reaction : 2Cu2+ + 4OH2Cu + 2H2O + O2
At the Cathode – the reduction of metal or hydrogen ions always takes place at
the cathode
Cu+2 + 2eCu
At the Anode – the oxidation of non-metal ions always takes place at the anode
4OH2H2O + O2 + 4e-
Electrolysis of Copper(II) sulfate with copper
Electrodes
• A cell is set up having copper electrodes and copper sulfate electrolytes.
• As electrolysis takes place, the cathode gains mass as copper is deposited on
the electrode.
At the cathode: Cu+2 + 2eCu
• The anode, loses mass as copper dissolves from the electrode
At the Anode : Cu
Cu+2 + 2eNote – The colour of the copper sulfate solution does not change because the
concentration of the Cu+2 ions in the solution remains the same.
Electroplating
• Electroplating can be used to coat one metal with another. Mostly unreactive
metal is coated over the other metal.
• The most commonly used metals for electroplating are copper, chromium,
silver and tin.
• Electroplating gives a protective coating to the metal underneath.
• Tin plating of steel cans prevent them from rusting.
• Chromium plating of articles such as door knobs, bath taps, motorcycle parts,
etc., resist corrosion. It is a hard metal that resist scratching and wear and it
can also polished to give an attractive finish.
• Nickel silver is electroplated to achieve attractive appearance of silver. Most
used in cutlery electroplating. Nickel silver is an alloy of Copper, Zinc and
Nickel.
Basic Rule of Electroplating
• Electroplating is carried out in an electrolytic cell.
• The object to be plated must be made the cathode.
• Electrolyte must be a solution of a salt of metal to be plated on the object.
Usually it is the salt of the same metal used for the anode.
• The anode is made of a strip of metal to be plated on the object.
• As the process proceeds anode dissolve in to the solution and metal ions move
towards the cathode, and get deposited on the object.
• As the metal is continuously moving into the solution from the anode, colour of
the solution remain the same throughout the electrolysis.
Hydrogen – Oxygen Fuel Cell
• In the electrochemical cells and batteries chemical reaction produce electrical
energy.
• In a hydrogen – oxygen fuel cell, chemical energy of reaction between
hydrogen and oxygen are converted in to electrical energy.
• Hydrogen fuel cell operates continuously, with no need of recharging as long as
fuels are continuously supplied to the electrodes.
• 2 H2 + O2
2H2O
This reaction releases the large amount of energy. Water is the only product.
• Hydrogen can be regarded as the non – polluting fuel.
Advantage and Disadvantage of Hydrogen as a Fuel
for motor vehicles
Advantages
Disadvantages
Renewable if produced using solar or wind energy
Non – renewable if generated using nuclear energy or
energy from fossil fuels
Lower flammability than petrol
Large fuel tanks are required
Virtually emission free
Only few filling station are available where car can be
topped up with hydrogen
Zero emissions of carbon dioxide
Currently expensive
Non – toxic
Hydrogen – Powered Vehicles
• Despite many difficulties some prototypes of hydrogen powered vehicles are
made.
• Some prototypes burn hydrogen in a modified combustion engine while
majority use the hydrogen fuel cell. Electricity from the hydrogen fuel cell
powers the electric motor of the vehicle.
• Using a fuel cell operating an electric motor, hydrogen has an efficiency of 60%
compare with 35% for petrol engine.