“Electrolysis” experiment

Elec­trol­y­sis is of­ten used for the in­dus­tri­al pro­duc­tion of many el­e­ments. In this sim­ple ex­per­i­ment we as­sem­ble a home de­vice for elec­trol­y­sis and break down wa­ter!

Safe­ty pre­cau­tions

Wear pro­tec­tive gloves and glass­es. Ob­serve safe­ty rules with sharp ob­jects and elec­tric­i­ty.

Warn­ing! Only un­der adults su­per­vi­sion.

Reagents and equip­ment:

• 20% sodi­um sul­fate so­lu­tion;
• 1% lit­mus so­lu­tion;
• 3 beakers;
• 2 clamps;
• source of di­rect cur­rent (for ex­am­ple a bat­tery);
• pen­cil;
• sta­tionery knife;
• piece of pa­per to fit in the beaker.

Step-by-step in­struc­tions

Pre­pare the elec­trode: With the sta­tion­ary knife, cut the pen­cil in half and re­move the graphite rod. Cut it into two pieces and con­nect to the clamps.

Pour the sodi­um sul­fate so­lu­tion and lit­mus so­lu­tion into a beaker. The col­or of lit­mus in the sodi­um sul­fate so­lu­tion is pur­ple. Low­er a piece of pa­per into the beaker with the so­lu­tion to sep­a­rate the space be­tween elec­trodes. Then low­er the pre­pared elec­trodes into the beaker, which are con­nect­ed to a source of di­rect cur­rent, for ex­am­ple a bat­tery. Ob­serve the re­lease of gas­es on the elec­trodes and the change of col­or in the elec­trode spa­ces.

Pro­cess­es de­scrip­tion

Elec­trol­y­sis is an ox­i­da­tion-re­duc­tion process which takes place on elec­trodes when a di­rect cur­rent pass­es through an al­loy or so­lu­tion of elec­trolyte.

An elec­trol­yser is a con­tain­er in which elec­trol­y­sis takes place. It con­sists of two elec­trodes – pos­i­tive (an­ode) and neg­a­tive (cath­ode). They are sep­a­rat­ed by a mem­brane. An­ions (neg­a­tive par­ti­cles) will move to­wards the an­ode dur­ing elec­trol­y­sis, and cations (pos­i­tive par­ti­cles) to the cath­ode. In the so­lu­tions of elec­trolytes, sub­stances that con­duct an elec­tric cur­rent, the com­po­nent parts of the elec­trolyte may be sub­ject­ed to elec­trol­y­sis, i.e. the ions of which it con­sists, and so may wa­ter. How­ev­er, there are cer­tain rules which should be re­mem­bered.

For ex­am­ple, ions of ac­tive met­als such as sodi­um, potas­si­um, cal­ci­um and oth­ers will not be sub­ject­ed to elec­trol­y­sis in aque­ous so­lu­tions. Wa­ter will be sub­ject­ed to elec­trol­y­sis, with the re­lease of hy­dro­gen and hy­drox­ide ions. These met­als may only be ob­tained in the elec­trol­y­sis of an al­loy.

In the elec­trol­y­sis of so­lu­tions of salts with ions of met­als of medi­um re­ac­tiv­i­ty – man­ganese, chromi­um, zinc and oth­ers – the met­al and hy­dro­gen will be de­posit­ed on the an­ode. In the elec­trol­y­sis of so­lu­tions of salts of non-re­ac­tive met­als, for ex­am­ple plat­inum, mer­cury and gold, only the met­al will be de­posit­ed on the an­ode.

To un­der­stand which met­al is re­ac­tive and which is not, you sim­ply have to look at the re­ac­tiv­i­ty se­ries of met­als: on the left are re­ac­tive met­als (from ce­sium to alu­minum), in the mid­dle are met­als of medi­um re­ac­tiv­i­ty (from alu­minum to hy­dro­gen), and af­ter hy­dro­gen are non-re­ac­tive met­als. In aque­ous so­lu­tions, only an­ions of oxy­gen-free acids will be sub­ject­ed to elec­trol­y­sis (sul­fide, bro­mide and io­dide ions) and hy­drox­ide ions. An­ions of acids con­tain­ing oxy­gen (sul­fates, phos­phates and oth­ers) and flu­o­ride ions will not be sub­ject­ed to elec­trol­y­sis in aque­ous so­lu­tions, only from al­loys.

The elec­trol­y­sis of a so­lu­tion of sodi­um sul­fate takes place ac­cord­ing to the fol­low­ing scheme

The dis­so­ci­a­tion of sodi­um sul­fate takes place in wa­ter, i.e. it breaks down into ions When an elec­tric cur­rent is passed through the wa­ter, the ions start to move. Cations to the cath­ode, and an­ions to the an­ode.

As sodi­um sul­fate is a salt of a re­ac­tive met­al and an acid con­tain­ing oxy­gen, wa­ter is sub­ject­ed to elec­trol­y­sis with the re­lease of gas – hy­dro­gen on the cath­ode, and oxy­gen on the an­ode. Be­fore elec­trol­y­sis be­gins, the lit­mus in­di­ca­tor has a pur­ple col­or, as the sodi­um sul­fate so­lu­tion has a neu­tral medi­um.

In the course of elec­trol­y­sis, the medi­um of the elec­trodes changes: it be­comes al­ka­line by the cath­ode and the lit­mus turns blue, and by the an­ode it be­comes acidic and the lit­mus turns red.

Na₂­SO₄↔2Na⁺+SO₄²⁻

Cath­ode(-): 2H₂O + 2e = H₂ + 2OH⁻

An­ode(+): 2H₂O — 4e = O₂ + 4H⁺

Thus, we may con­clude that the elec­trol­y­sis of wa­ter is tak­ing place:

2H₂O (elec­trol­y­sis) → 2H₂ + O₂

Elec­trol­y­sis has wide ap­pli­ca­tion:

In in­dus­try for ob­tain­ing many el­e­ments such as sodi­um, alu­minum, lithi­um, chlo­rine, flu­o­rine and oth­ers, and also im­por­tant sub­stances, for ex­am­ple sodi­um hy­drox­ide and potas­si­um chlo­rate.

It is used for dec­o­ra­tive coat­ings of items, for ex­am­ple with gold or chromi­um.

Elec­trol­y­sis is used to clean an­cient arte­facts of non-metal­lic im­pu­ri­ties, for ex­am­ple coins and swords.