4 Electrochemistry Flashcards
Electrolysis
The breakdown of an ionic compound, molten or in aqueous solution, by the passage of electricity.
Electrolytic cell
An electrochemical cell in which electrical energy is transferred into chemical energy.
Components:
A battery
Two electrodes, one of which is connected to the Negative terminal of the battery (cathode) as the Negative electrode
and the other is connected to the positive terminal (anode) as the positive electrode
an electrolyte (aqueous or molten substance that conducts electricity).
The main stages in electrolysis are:
- The electrolyte is made molten (melted through heating) or dissolved in water to give an aqueous solution.
- An electric current is passed through the cell.
- Cations (metal ions or hydrogen ions) move towards the negatively charged cathode.
- Anions (non-metal ions) move towards the positively charged anode.
- The cations come from the metal ions. For example, in the electrolysis of molten sodium chloride, NaCl, the cation is the sodium ion which will gain an electron to form solid sodium metal.
- The anions come from the non-metal ions, in the case of molten sodium chloride, the chloride ion loses an electron to form chlorine gas.
Redox electrolysis
The electrolyte must be molten or in aqueous solution so that the ions can move to their oppositely charged electrodes.
Electrons flow from the negative terminal of the battery and charge up the cathode with negative charge.
This will attract the cations (positive ions) and make them move towards the cathode (negative electrode) to gain electrons (reduction).
The anions (negative ions) move towards the anode with positive charge and lose electrons (oxidation).
Electrons flow from the anode (positive electrode) to the cathode (negative electrode).
Ionic half-equations can be written to show the reactions occurring at both the cathode and the anode.
At the cathode: cations/metal ions/hydrogen ions gain electrons and reduction happens:
Mn + + n e– → M reduction
At the anode: non-metal ions/anions lose electrons and oxidation happens:
2X– → X2 + 2e– oxidation
Electrodes are made of and why
platinum/graphite/carbon due to their inertness (non-reactivity). They are also good conductors of electricity.
Products formed at the electrodes in molten electrolysis
the general principle is that metals and hydrogen are formed at the negative electrode (cathode) and non-metals (other than hydrogen) are formed at the positive electrode (anode).
Products formed at the electrodes in aqueous electrolysis: Cathode
Cathode:
Potassium: Form cations readily, hard to reduce
Sodium
Lithium: Hydrogen at the Cathode
Calcium
Magnesium
Aluminium
Water: is reduced
Zinc
Iron
Lead
Hydrogen: Metal at the Cathode
Copper
Silver
Gold: Easy to reduce
Products formed at the electrodes in aqueous electrolysis: Anode
Anode:
Fluorine: Form anions readily, hard to oxidise
Chlorine: Oxygen
Water: is oxidised
Bromine: Non-metal
Iodine: Easily to oxidise
The effect of dilution on electrolysis
If a dilute solution of aqueous sodium chloride solution is electrolysed, the products can change.
At the cathode (–), hydrogen is still produced:
2H+(aq) + 2e– → H2(g)
At the anode, due to an excess of OH– ions present from the dilute solution, O2 is discharged from the following reaction:
2H2O(l) + O2(g) + 4e – → 4OH–(aq)
The Na+ and Cl– ions stay in solution, so effectively water has been decomposed. The reason is that in a dilute solution, water molecules are present in excess, so both hydrogen and hydroxide ions are discharged preferentially.
There are three main applications of electrolysis:
Electroplating
Purification of copper/refining copper
Electrolysis of concentrated sodium chloride (brine).
Electroplating
Coating one metal with a thin layer of another metal either to improve the appearance of an object or to prevent corrosion.
The object to be electroplated is made the cathode (by connecting to the negative terminal of the power supply) and the anode is made from the metal that will provide the coating layer.
The electrolyte is a solution containing metal ions (a salt) of the metal providing the electroplating layer
Copper refining and purification
The electrolyte is copper sulfate solution.
1. At the anode (+), copper atoms lose electrons and go into solution as copper(II) ions:
Cu(s) → Cu2+(aq) + 2e–
- The Cu2+ ions are attracted towards the cathode (–) where they gain electrons forming copper atoms:
Cu2+(aq) + 2e– → Cu(s)
A layer of pure copper builds up on the cathode.
- Precious metals such as platinum, silver and gold, that might have been present in the impure copper anode drop to the bottom and this mixture is known as anode slime.
- Anode loses mass and cathode gains mass.
Electrolysis of concentrated sodium chloride solution
2NaCl (aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
brine → sodium hydroxide + chlorine + hydrogen
At the cathode (–):
2H+ + 2e– → H2
At the anode (+):
2Cl –(aq) → CI2(g) + 2e–
Since H+ and Cl – ions leave the solution, Na+ and OH– remain in it. Eventually, when there are no more Cl – ions, water is allowed to evaporate and NaOH is obtained as a product.
The transfer of charge during electrolysis to include:
the movement of electrons in the external circuit
the loss or gain of electrons at the electrodes
the movement of ions in the electrolyte
