Selective Discharge and the purification of Copper

Molten sodium  chloride only has the presence of two lions, the Sodium and Chloride ions. However, in aqueous solutions, there are ions of the compound (in this case: Sodium and Chloride) and ions of water (Hydrogen and Hydroxide). Therefore in aqueous solutions, there is competition between ions of similar polarity over which would be discharged.  


The rule governing which ion would be discharged is the reactivity series. This is because the more reactive an atom is, the less electronegative it is. As such, it has a lower tendency to gain electrons. In the case of Sodium and Hydrogen ions, since sodium ions  are more reactive than hydrogen ions, the hydrogen ions are discharged instead of the sodium ions. Thus we say hydrogen ions are reduced to form hydrogen atoms. 


Equation: H⁺ + e^- ---> H 

Hydrogen atoms join to form diatomic molecule: 
H + H ---> H₂

_{Test for hydrogen gas : Insert a lighted splint above aqueous Sodium Chloride solution near the cathode, lighted splint extinguishes with a "pop" sound.} 

Both Chloride and hydroxide ions are attracted to the positively charged anode. However, only the chloride ions are discharged, to produce chlorine atoms, Thus, we say chloride ions are oxidised to form chlorine atoms. 

Equation: Cl^- ---> Cl + e^-

Chlorine atoms join to form chlorine gas:
Cl + Cl ---> Cl<sub>2


Test for Chlorine gas: Chlorine gas turns damp blue litmus paper red then bleaches it.</sub> 

In the case of anions, halide ions like chloride, bromide and iodide will be discharged. Sulfate ions are not discharged, hydroxide ions are discharged. However, this only applies to concentrated solutions.

4OH^- (aq) -->  O₂(g) + 2H₂O (l) + 4e^-


Test for Oxygen gas: Insert a glowing splint above solution near the anode end. If splint rekindles, oxygen gas is present. 

In dilute solutions, for instance when the concentration is 1.0mol/dm^3, or less, hydroxide ions are discharged except in the cases of copper and silver. This is also known as the concentration effect, another factor affecting the selective discharging of ions during electrolysis. However, concentration only affects the anions and not the cations. 

The kind of electrodes used also affect the selective discharge of ions. In a usual electrolytic cell, inert electrodes like carbon or graphite would be used. However, in other instances, like the electrolysis of Copper (II) Sulfate solution, Copper is used as the electrode.

In Copper(II) Sulfate solution the ions present are Copper(ii) cation, Sulfate anion, Hydrogen cation and Hydroxide anion. 

If we are using inert electrodes, like carbon,  Copper(ii) and hydroxide will be discharged to give Copper metal and Oxygen gas. Hydrogen cation and Sulfate  will be left behind, so the pH of the solution decreases and becomes more acidic as electrolysis progresses, and the intensity of the blue color fades.

If however,  copper strips are used as the electrodes, instead of the Copper(II) ions in the solution being discharged, the copper strip at the anode begins eroding and the Copper metal is  oxidised to form Copper(II) ions.a

Meanwhile, Copper(II) ions will be travelling to the copper cathode to become Copper metal again. Hence a pink deposit at the anode is observed as newly formed copper metal has a pink appearance. Thus we say copper(II) ions are reduced to form copper metal.

The blue  copper sulfate solution will remain  blue, since the concentration of Copper(II) ions in the solution doesn’t change. Any increase from the erosion of the anode is balanced by the discharge of ions at the cathode.

Therefore, another factor influencing selective discharge of ions is the type of electrodes used in the electrolytic cell!

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Obtaining Copper
Copper is an important element in our daily lives. For instance, it is often manufacturing into wires for copper is a good conductor of electricity and is an abundant element, making it cheap. They are commonly used as domestic plumbing as they do not react with water and are malleable. They can also be used as boilers or heat exchangers as they do not react with water and are good conductors of heat Copper is commonly extracted from copper ores by electrolysis using inert electrodes, at the cathode end, where reduction occurs. Since this copper extracted is not pure enough, it is purified through another process.
Copper Purification process:
The aforementioned setup, an electrolytic cell consisting aqueous copper(II) sulfate with copper electrodes is commonly used to refine copper.  Pure copper is used as the cathode, conversely, impure copper is used as the anode.
At the cathode, copper(II) ions are deposited as pure copper.

At the anode, copper goes into solution as Copper(II) ions.



The concentration of the solution remains unchanged as for every copper ion deposited at the cathode to be reduced, another copper ion goes into solution after it is oxidised at the anode. Therefore, there is only a transfer of copper  from the anode to the cathode, and the concentration of copper(II) ions in the solution remains the same. 


Since the impure copper is purified, there will definitely be impurities. Metals in the impure anode below copper in the reactivity series will not be ionized, remaining as metals.These metals falls to the bottom of the anode as forming "anode sludge" with unreactive materials left from the ore. Metals above copper in the reactivity series will be ionized at the anode by oxidation but will not be discharged at the cathode if their concentration does not become too high. When their concentration becomes too high and the concentration of copper(II) ions fall, the rate of purification decreases as some of the electric current is used to discharge the zinc ions. 


Copper produced through this process is 99% pure.