Work suitable for OCR, Edexcel and AQA.
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Electrolysis of molten sodium chloride. The ions present in molten sodium chloride are, Na +(l) and Cl - (l).
The anode reaction [Positive electrode]
Chloride ions go to the anode. {Note some equations cannot put clearly here but they come clearly in CD}
Cl - (l) ------------ à Cl + e
Cl + Cl -------- à Cl 2 (g)
A chloride ion loses an electron to become a chlorine atom then two atoms of chlorine combine to a molecule of chlorine. Non-metals such as chlorine and oxygen always get liberated at the anode. To remember we write it as anode, where O for Oxygen.
The cathode [Negative electrode]
Na +(l) + e --------------- à Na(s)
A sodium ion accepts an electron to become a sodium atom then sodium metals get deposited at the cathode. As a rule all metals and hydrogen always liberate or deposit at the cathode. A gas like hydrogen gets liberated while a metal like sodium gets deposited. It’s easy to remember when we wrote catHode instead of cathode, capital H for Hydrogen.
The complete reaction
2Na + (l) + 2Cl - (l) ------------ à 2Na(s) + Cl 2(g)
Question : During the electrolysis of molten sodium chloride, if 1.06g of sodium chloride got deposited at the cathode, what volume of chlorine gas would be given out at the anode, show the working?
Answer : According to the equation, 2x23g of sodium are deposited when 2x35.5g of chlorine gas or one mole is liberated at RTP or 24000cm 3 of chlorine gas is liberated. That means 71g of chlorine occupy a volume of 24000cm 3 . In other words 71g or 24000cm 3 is one mole of chlorine.
According to the equation, 2x23g of sodium chloride gives out 71g of chlorine gas by weight then need to find out the weight of chlorine gas given out by 1.06g of sodium chloride. It works out to be 1.636g of chlorine. We know 71g or one mole of chlorine occupy a volume of 24000cm3. Now need to find out what volume is occupied by the gas liberated by 1.06g of chlorine gas. The working is put into following form as follows:
Mass of chlorine = (71 x 1.06 )/ (23 x 2)
Mass of chlorine = 1.636g
Volume of chlorine = 24000 x 1.636/71
,, ,, = 553.01cm 3
Note: These are two different things; molten sodium chloride and aqueous sodium chloride are two different things.
Electrolysis of aqueous sodium chloride . This is also called the electrolysis of brine or salt solution. The ions present are, Na + (aq), Cl-(aq), H + (aq), OH - (aq) . All the positive ions go to cathode [negative electrode] and all the negative ions go to the anode [positive] electrode.
Although Na + (aq) ions and H + (aq) ions travel to the cathode [negative electrode], only H + (aq) ions are liberated as H 2 (g) because Na + (aq) ions are more reactive hence it will not be liberated. Na + (aq) ions remain in solution.
Cathode [negative electrode]
2H + (aq) + 2e - H 2 (g)
Both Cl - (aq) ions and OH - (aq) ions travel to the anode [positive electrode] but only Cl - (aq) get liberated as Cl 2 (g) while OH - (aq) ions remain in solution. The reason is Cl - (aq) ion will give the electron to become a Cl atom more readily than OH - (aq) ion. Eventually two Cl atoms combine and liberate as chlorine gas.
2Cl - (aq) Cl 2 (g) + 2e -
The remaining Na + (aq) ions and OH - (aq) ions combine to form NaOH.
The complete reaction for electrolysis of brine can be given as
Brine ------ à Sodium hydroxide + chlorine + hydrogen
2NaCl(aq) + 2H 2 O(l) ----- à 2NaOH + Cl 2 (g) + H 2 (g)
Electrolysis of copper (2) chloride.
All metals and Hydrogen liberates at the Negative electrode. That means metals like copper and magnesium deposit at the cathode and also Hydrogen gas is given out at the cathode.
During electrolysis, the ions like Cu 2+( aq) and Cl - (aq) ions travel through the solution that is through the electrolyte and electrons travel through the wires in the circuit. The wires and the battery are called the external circuit and the electrolyte is called the internal circuit. You have to explain your work in the exam like this and must make it clear.
Chloride ion means it has eight electrons in the external shell of the ion. It cannot come out as chloride ions; it is expected to come out as chlorine gas. Hence when a chloride ion travel to the Anode (+) [positive electrode], it gives up an electron to the Anode and become a chlorine atom, when two such chlorine atoms covalently bonded, then a molecule of chlorine gas is formed and it comes out at the ANODE [positive electrode].
The electron that is taken from the chloride ion travel through the external wires to the other electrode, Cathode. [Negative electrode] and electrons get collected at the Cathode. Then Cu 2+(aq) ions come to the Cathode [Negative electrode] looking for electrons as it is short of electrons. The electrons combine with each copper two ion and deposits at the Cathode as copper metal. You must get these things like a picture in the mind, then it is easy to understand them. In some books it does not stress very much about the external and internal circuits.