ELECTROLYSIS of HYDROCHLORIC ACID

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

7. The electrolysis of hydrochloric acid

Methods of investigating the electrolysis of dilute hydrochloric acid solution are described. The formation of the products of electrolysing aqueous hydrogen chloride are fully explained with the appropriate electrode equations. What are the products of the electrolysis of hydrochloric acid solution?

Reminders: Electrolysis (of hydrochloric acid) is a way of splitting up (decomposition) of the compound (hydrogen chloride in water) using electrical energy. The electrical energy comes from a d.c. (direct current) battery or power pack supply. A conducting liquid, containing ions, called the electrolyte (hydrochloric acid), must contain the compound (hydrogen chloride) that is being broken down. The electricity must flow through electrodes dipped into the electrolyte to complete the electrical circuit with the battery. Electrolysis can only happen when the circuit is complete, and an electrical current (electricity) is flowing, then the products of electrolysing dil. hydrochloric acid are released on the electrode surfaces where they can be collected. Electrolysis always involves a flow of electrons in the external wires and electrodes and a flow of ions in the electrolyte and there is always a reduction at the negative cathode electrode (which attracts positive ions, cations) and an oxidation at the positive anode electrode (which attracts negative ions, anions) and it is the ions which are discharged to give the products. These revision notes on the electrolysis of hydrochloric acid solution should prove useful for the new AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses.

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7. The electrolysis of hydrochloric acid

The products of electrolysing hydrochloric acid are hydrogen gas and chlorine gas

You can demonstrate the electrolysis of hydrochloric acid using the Hoffman voltameter with platinum electrodes (right diagram).  The Hofmann voltameter is filled with the electrolyte (hydrochloric acid) by opening the taps at the top of the outer tubes to allow any gas to escape. The gases formed on the electrolysis of the dilute hydrochloric acid can be collected via the same taps.

In the simple electrolysis cell (left diagram), the graphite (carbon) electrodes are, through a large rubber bung, 'upwardly' dipped into an solution of dilute hydrochloric acid. In this cheap and simple apparatus the gaseous products (hydrogen and chlorine) are collected in small test tubes inverted over the carbon electrodes. You have to fill the little test tubes with the electrolyte (dil. hydrochloric acid), hold the liquid in with your finger and carefully invert them over the nearly full electrolysis cell. The electrolysis will only take place when electricity is passed through the dilute hydrochloric acid solution.

In both experiments above the platinum or carbon electrodes are inert. You can also use the simple apparatus illustrated on the right, using two inert metal wire electrodes.

The electrolysis will only take place when electricity is passed through the hydrochloric solution

The electrolyte hydrochloric acid, provides a high concentration of hydrogen ions H+ and chloride ions Cl– to carry the current during the electrolysis process.

The electrode reactions and products of the electrolysis of dil. hydrochloric acid are illustrated by the theory diagram above

The electrode half-equations for the electrolysis of dilute hydrochloric acid HCl(aq)

(a) The negative cathode electrode reaction for the electrolysis of hydrochloric solution

The positive hydrogen ions H+ (from hydrochloric acid) are attracted to the negative cathode electrode, and are reduced by electron gain to form hydrogen gas at the negative electrode.

2H+(aq) + 2e– ==> H2(g)

positive ion reduction by electron gain

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

All acids give hydrogen at the negative cathode.

 

(b) The positive anode electrode reaction for the electrolysis of hydrochloric solution

The (+) anode attracts both the OH– (from water) and Cl– ions (from hydrochloric acid). Only the chloride ion is discharged in appreciable quantities.

Chloride ions are oxidised by electron loss to form chlorine gas at the positive electrode.

2Cl–(aq) – 2e– ==> Cl2(g)

 or  2Cl–(aq) ==> Cl2(g) + 2e– 

negative ion oxidation by electron loss

 

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

Also, if most of the chloride ions have been discharged as chlorine molecules, you can then get some oxygen gas formed at the anode i.e. like in the electrolysis of water.

2H2O(l) – 4e– ==> 4H+(aq) + O2(g)

or

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

 

Overall equation for the electrolysis of hydrochloric acid: 2HCl(aq) ==> H2(g) + Cl2(g)

This could be written more accurately as an ionic equation: 2H+(aq) + 2Cl–(aq) ==> H2(g) + Cl2(g)

 

Extra COMMENTS on the electrolysis of hydrochloric chloride solution

1. Tests for the gases formed in the electrolysis of sodium chloride solution

The (–) cathode gas - colourless gas gives a squeaky pop with a lit splint – hydrogen

The (+) anode gas - pale green gas turns damp blue litmus red and then bleaches it white – chlorine

Chlorine forms a weakly acid solution in water, which is why it turns blue litmus pink-red, but it is NOT the crucial observation, that's the bleaching action of chlorine.

2. You can collect samples of gases through the taps on the Hofmann voltameter or from the little test tubes in the simple school electrolysis cell.

3. Theoretically, in the electrolysis of hydrochloric acid solution, the gas volume ratio for H2 : Cl2 is 1 : 1, BUT chlorine is slightly soluble in water and  therefore the volume of chlorine gas observed is less than predicted.

Why 1 : 1 ratio? It takes two electrons to reduce two hydrogen ions to a hydrogen molecule. It takes the removal of two electrons, one from each chloride ion, to form a chlorine molecule. So, for the same quantity of current passing (electron flow), you should expect to form equal numbers of hydrogen and chlorine molecules.

SUMMARY OF PRODUCTS FROM THE ELECTROLYSIS OF HYDROCHLORIC ACID

with inert carbon (graphite) electrodes or inert platinum electrodes

Electrolyte negative cathode product negative electrode

cathode half-equation

positive anode product positive electrode

anode half-equation

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– 

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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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