Electricity and Chemistry
Electricity and chemistry
Electrolysis-breakdown of an ionic compound in molten or aqueous solution by the passage of electricity.
Electrolyte- is the substance that is decomposed, it is a substance that conducts electricity when in molten form or in solution.
The electricity is carried through the electrolyte by ions
Why should the electrolyte be in a molten or aqueous solution?
In the molten state and in solution the ions are free to move to the appropriate electrodes due to weakened forces of attraction between them.
- Substances that do not conduct electricity when in the molten state or aqueous are called non-electrolytes.
- Substances that conduct electricity to a small extent are called weak electrolytes.
The electric current enters and leaves the electrolyte through electrodes, which are usually made up of carbon or platinum.
Why use Platinum/graphite as electrodes?
They are unreactive metals, which are called inert electrodes
because they do not react with the products of electrolysis.
There are 2 electrodes:
- Cathode-negative electrode(attracts cations)
- Anode-positive electrode(attracts anions)
Cations are positively charged ions
Anions are negatively charged ions
NB-Do remember unlike charges attract
General Principles of Electrolysis
How do you find out what product is being formed?
Electrolysis of molten compounds:
Positive ions move to the cathode (negative electrode)and gain electrons to form atoms, and negative ions move to the anode(positive electrode)and lose electrons to form atoms.
Non metals are produced at the anode whereas metals and hydrogen gas are produced at the cathode.
Eg:Electrolysis of molten lead(||)bromide(PbBr2)
- The bulb is there just to ensure electricity is passing through.
- When the electrodes are first connected,the bulb does not light,because the solid compound does not allow electricity to pass through it.However,when the compound is heated until it is molten,the bulb does light up.
- The lead(||)bromide is acting as an electrolyte now. When this happens an orange-red vapour is seen at the anode and lead metal is produced at the cathode.
- The break-up(decomposition) of lead(||)bromide into its constituent elements by the passage of an electric current=electrolysis
molten lead(||)bromidebromine+lead
PbBr2(l) –> Br2(g)+Pb(l)
Lead formation
For lead metal to be formed or deposited, at the cathode, the lead ions must be attracted towards the electrode.To produce lead
metal atoms these lead ions must collect 2 electrons at the cathode(metal ions are positive):
lead ion + electrons lead atom
Pb2+ (l) + 2e– –> Pb(l)
This is a reduction process as it is gaining electrons.
Bromine formation
To form bromine molecules each bromide ion must lose all of its negative charges at the anode to form a neutral bromine atom(nonmetal ions are negative):
bromide ionbromine atom+electron
Br–(l) –> Br + e–
Two bromine atoms then combine to form a bromine molecule:
bromine atoms bromine molecule
2Br –> Br2(g)
Electrolysis of aqueous compounds:
Soluble compounds such as NaCl are dissolved in water. Now realize that there are more ions involved in his electrolyte. The ions from the water and the compound itself.
At the cathode, the metal ion and hydrogen ion are positive, but only one can be discharged.
Rule=The ion of the least reactive element will be the one that will be discharged. Hence if the metal is above hydrogen in the reactivity series, hydrogen will be discharged, and if below hydrogen, the metal will be discharged.
Reactivity series
| Potassium |
| Sodium |
| Lithium |
| Calcium |
| Magnesium |
| Carbon |
| Zinc |
| Iron |
| Hydrogen |
| Copper |
At the Anode,the negative ion from the compound and hydroxide ion from the water are present.
Rule=If a halide ion is present it will always be discharged(Chloride, Bromide etc).
And if a halide is not present in the electrolyte then always the hydroxide is discharged.
Eg: Electrolysing Copper(II)sulfate solution using carbon electrodes
Ions present in solution
From compound-Cu2+(copper ions)
-SO42-(hydrogen ion)
From water (as it is aqueous)-H +(hydrogen ion)
-OH–(hydroxide ion)
Reaction in Anode(positive terminal):
OH– and SO42 are present as unlike charges attract.
As there is no halide ion, hydroxide ions are discharged.
What happens is the hydroxide decomposes:
hydroxide ions water molecules+oxygen molecules+electrons
OH–(aq) –> 2H2O(l) + O2 (g) +4e–
Reaction in Cathode(negative terminal):
Cu2+ and H + are present in this terminal. Copper is lower than hydrogen in the reactivity series, hence copper is displaced and discharged.
How is copper formed:
Copper ions + electrons copper atoms
Cu2+(aq) + 2e– –> Cu(s)
The copper accumulates around the cathode as the reaction is
happening under normal temperature hence the copper is solid.
Purification of Copper
As copper is a very good conductor of electricity it is used for electrical wiring and cable. However, even small amounts of impurities cut down his conductivity quite noticeably whether in fine wires or larger cables. The metal must be 99.9% pure to be used in this way, to ensure this level of purity, the newly extracted copper has to be purified by electrolysis.
Hence copper electrodes are used
During the process the impure anode loses mass because the copper atoms lose electrons and become copper ions,Cu2+(aq)
copper atoms Copper ions + electrons
Cu(s) Cu2+(aq) + 2e–
The electrons released a the anode travel around the external circuit to the cathode. There the electrons are passed on to the copper ions, Cu2+(aq), from the copper(ll)sulfate solution and the copper is deposited or copper plated onto the cathode.
Copper ions + electronscopper atoms
Cu2+(aq) + 2e– Cu(s)
Electroplating
Electroplating is the process involving electrolysis to the plate, or coat, one metal with another or plastic with metal. In simpler terms,it is the process of depositing metals from solution in the form of a layer on other surfaces such as metals or plastics.
Often the purpose of electroplating is to give a protective coating to the metal beneath.
For example, Bath taps are chromium-plated to prevent corrosion, and at the same time are given a shiny, more attractive finish.
Used in jewelry for a better look, to prevent corrosion, and have better electrical conductivity.
eg: Electroplating a spoon with silver
NB: The product to be electroplated is placed as the cathode
The Ag+ ions are attracted to the cathode, where they gain electrons, causing a coating of silver on the spoon at the cathode.
NB-How are electrons transferred?
The electron transfer occurs at the external circuit from the anode to the cathode.
How are the ions transferred?
The ion transfer occurs at the electrolyte itself.
Metals such as copper and aluminum (a light metal which also requires electrolysis to extract from bauxite using cryolite, more mentioned in metal extraction) are used as they are good conductors of electricity, with plastics acting as insulators as they do not conduct electricity due to no free electrons.