SUMMARY of the products of electrolysis

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ELECTROCHEMISTRY revision notes on electrolysis, cells, experimental methods, apparatus, batteries, fuel cells and industrial applications of electrolysis

9. Summary of the products of the electrolysis of various electrolytes

The products and electrode equations for a variety of electrolytes is summarised below. These revision notes on electrolysis products should prove useful for the new AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (91, 9-5 & 5-1) science courses.

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9. Summary of the products of the electrolysis of various electrolytes

What are the products of the electrolysis of molten aluminium oxide, aqueous copper sulphate solution, aqueous sodium chloride solution (brine), hydrochloric acid, sulphuric acid, molten lead(II) bromide, molten calcium chloride? add overall equations?

Electrolyte

solution or melt

negative cathode product negative electrode

cathode half-equation

positive anode product positive electrode

anode half-equation

comments
molten aluminium oxide

Al2O3(l)

molten aluminium Al3+(l) + 3e ==> Al(l) oxygen gas

2O2(l) 4e ==> O2(g)

or  2O2(l)  ==> O2(g) + 4e

The industrial method for the extraction of aluminium its ore
aqueous copper(II) sulfate

CuSO4(aq)

copper deposit any conducting electrode e.g. carbon rod, any metal including copper itself

Cu2+(aq) + 2e ==> Cu(s)

oxygen gas inert electrode like carbon (graphite rod) or platinum

(i) 4OH(aq) 4e ==> 2H2O(l) + O2(g)

or  4OH(aq) ==> 2H2O(l) + O2(g) + 4e

(ii) 2H2O(l) 4e ==> 4H+(aq) + O2(g)

or 2H2O(l) ==> 4H+(aq) + O2(g) + 4e

The blue colour of the copper ion will fade as the copper ions are converted to the copper deposit on the cathode
aqueous copper (II) sulphate

CuSO4(aq)

copper deposit any conducting electrode e.g. carbon rod, any metal including copper itself

Cu2+(aq) + 2e ==> Cu(s)

copper(II) ions the copper anode dissolves copper anode

Cu(s) 2e ==> Cu2+(aq)

or  Cu(s) ==> Cu(s) + 2e

This is the basis of the method of electroplating any conducting solid with a layer of copper. When using both copper cathode and anode, the blue colour of the copper ion does not decrease because copper deposited at the () cathode = the copper dissolving at the (+) anode.
Copper(II) chloride

CuCl2(aq)

copper deposit Cu2+(aq) + 2e ==> Cu(s) chlorine gas

2Cl(aq) 2e ==> Cl2(g)

 or  2Cl(aq) ==> Cl2(g) + 2e 

molten sodium chloride

NaCl(l)

molten sodium Na+(l) + e ==> Na(l) chlorine gas

2Cl(l) 2e ==> Cl2(g)

 or  2Cl(l) ==> Cl2(g) + 2e

This a method used to manufacture sodium and chlorine.
aqueous sodium chloride solution (brine)

NaCl(aq)

hydrogen

2H+(aq) + 2e ==> H2(g)

or 2H3O+(aq) + 2e ==> H2(g) + 2H2O(l)

or 2H2O(l) + 2e ==> H2(g) + 2OH(aq)

chlorine gas

2Cl(aq) 2e ==> Cl2(g)

 or  2Cl(aq) ==> Cl2(g) + 2e

This is the process by which hydrogen, chlorine and sodium hydroxide are manufactured
hydrochloric acid

HCl(aq)

hydrogen gas 2H+(aq) + 2e ==> H2(g)

or 2H3O+(aq) + 2e ==> H2(g) + 2H2O(l)

chlorine gas

2Cl(aq) 2e ==> Cl2(g)

 or  2Cl(aq) ==> Cl2(g) + 2e 

All acids give hydrogen at the cathode.

Theoretically the gas volume ratio is H2:Cl2 is 1:1, BUT, chlorine is slightly so there seems less chlorine formed than actually was.

sulphuric acid

sulfuric acid

H2SO4(aq)

hydrogen gas 2H+(aq) + 2e ==> H2(g)

or 2H3O+(aq) + 2e ==> H2(g) + 2H2O(l)

oxygen gas

(i) 4OH(aq) 4e ==> 2H2O(l) + O2(g)

or  4OH(aq) ==> 2H2O(l) + O2(g) + 4e

(ii) (+) 2H2O(l) 4e ==> 4H+(aq) + O2(g)

or 2H2O(l) ==> 4H+(aq) + O2(g) + 4e 

All acids give hydrogen at the cathode. Whereas hydrochloric acid gives chlorine at the anode, the sulfate ion does nothing and instead oxygen is formed. This is the classic 'electrolysis of water'.

Theoretically the gas volume ratio is H2:O2 is 2:1 which you see with the Hofmann Voltameter

molten lead(II) bromide

PbBr2(l)

molten lead Pb2+(l) + 2e ==> Pb(l) bromine vapour

2Br(l) 2e ==> Br2(g)

 or  2Br(l) ==> Br2(g) + 2e

A good demonstration in the school laboratory brown vapour and silvery lump provide good evidence of what's happened
molten calcium chloride

CaCl2(l)

solid or molten calcium Ca2+(l) + 2e ==> Ca(s) chlorine gas

2Cl(aq) 2e ==> Cl2(g)

 or  2Cl(aq) ==> Cl2(g) + 2e

The basis of the industrial method for the manufacture of calcium metal
Molten anhydrous zinc chloride

ZnCl2(l)

solid zinc Zn2+(l) + 2e ==> Zn(s)

 

chlorine gas

2Cl(aq) 2e ==> Cl2(g)

 

A good demonstration in the school laboratory - safer than using lead bromide
Silver nitrate

AgNO3(aq)

solid silver

Ag+(aq) + 2e ==> Ag(s)

oxygen gas

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

electroplating experiment
Sodium bromide

 NaBr(aq)

hydrogen gas 2H+(aq) + 2e ==> H2(g)  

2Br(aq) 2e ==> Br2(aq)

School experiment
Potassium iodide

KI(aq)

hydrogen gas 2H+(aq) + 2e ==> H2(g)  

2I(aq) 2e ==> I2(aq/s)

School experiment
Sulfate salts of reactive metals > hydrogen theoretically hydrogen gas 2H+(aq) + 2e ==> H2(g)  

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

School experiment

Similar results with most nitrate salts of reactive metals

           
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ELECTROCHEMISTRY INDEX:  1. INTRODUCTION to electrolysis - electrolytes, non-electrolytes, electrode equations, apparatus 2. Electrolysis of acidified water (dilute sulfuric acid) and some sulfate salts and alkalis 3. Electrolysis of sodium chloride solution (brine) and bromides and iodides 4. Electrolysis of copper(II) sulfate solution and electroplating with other metals e.g. silver 5. Electrolysis of molten lead(II) bromide (and other molten ionic compounds) 6. Electrolysis of copper(II) chloride solution 7. Electrolysis of hydrochloric acid 8. Summary of electrode equations and products 9. Summary of electrolysis products from various electrolytes 10. Simple cells (batteries) 11. Fuel Cells e.g. the hydrogen - oxygen fuel cell 12. The electrolysis of molten aluminium oxide - extraction of aluminium from bauxite ore & anodising aluminium to thicken and strengthen the protective oxide layer 13. The extraction of sodium from molten sodium chloride using the 'Down's Cell' 14. The purification of copper by electrolysis 15. The purification of zinc by electrolysis 16. Electroplating coating conducting surfaces with a metal layer 17. Electrolysis of brine (NaCl) for the production of chlorine, hydrogen & sodium hydroxide AND 18. Electrolysis calculations


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