Chapter 18 Electrolysis

[Pages:10]Form 4 Chemistry Notes Chapter 18 : Electrolysis

Chapter 18 Electrolysis

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18.1 Electrolysis is one type of Redox

What is Electrolysis?

Electrolysis means decomposition by electricity. It is the process by which a redox reaction is forced to take place by the passage of a direct electric current through an electrolyte in the molten or in aqueous solution.

If alternating current (a.c.) is used during electrolysis, there will be no decomposition of the electrolyte.

A Basic Set-up for Electrolysis

The following set-up is used to carry out electrolysis.

Functions of the important parts: (1) Direct current (d.c.) supply

It is used to supply electricity to decompose the electrolyte. A.C. source cannot be used. (2) Ammeter

A current recorded indicates a complete circuit is formed. (3) Electrolyte

It is a compound (usually ionic compound), which when molten or in aqueous solution, conducts electricity and is decomposed to form the constituent elements. (4) Electrodes Conductors are immersed into the electrolyte to allow current to enter and leave. Inert metals (Platinum) and graphite are common electrodes used in electrolysis. (a) Cathode: It is the electrode where reduction takes place. In electrolysis, the cathode is the

negative electrode. (b) Anode: It is the electrode where oxidation takes place. In electrolysis, the anode is the

positive electrode.

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Chapter 18 : Electrolysis

Comparing Chemical Cell and Electrolytic Cell

Chemical Cell

A device for generating

Function

electricity by chemical

changes

Type of reaction involved

A spontaneous (occurs on its own) reaction takes place

Polarity of electrode cathode + (reduction)

(and change)

anode - (oxidation)

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Electrolytic Cell A device to bring about chemical changes by passage of an electric current Oxidation and reduction are forced to occur at the anode and cathode respectively - (reduction) + (oxidation)

18.2 Electrolysis of a Molten Salt The following set-up is used to electrolyse a molten salt of lead(II) bromide.

(1) D.C. power supply It acts a pump of electrons. It forces electrons from its negative pole to the cathode of the electrolytic cell.

(2) The graphite anode and cathode A graphite rod is an inert electrode which does not decompose during electrolysis.

(3) Molten lead(II) bromide Solid lead(II) bromide does not conduct electricity as there is no mobile ions. When it is molten, there are mobile lead and bromide ions. (a) At the anode: Anions migrate to the anode and undergo oxidation, 2Br-(l) Br2(g) + 2e(b) At the cathode Cations migrate to the cathode and undergo reduction, Pb2+(l) + 2e- Pb(l) The overall reaction is a redox reaction: Pb2+(l) + 2Br-(l) Pb(l) + Br2(g)

Form 4 Chemistry Notes Chapter 18 : Electrolysis

Question 18.1 Predict the products formed at each graphite electrode when a direct current is passed into (a) molten magnesium chloride

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(b) solid magnesium chloride

18.3 Predicting Preferential discharge of ions

Molten electrolyte When lead(II) bromide is molten, there are only lead(II) ions and bromide ions, no other cations and anions compete with the molten salt for discharge.

Preferential discharge of ions in aqueous electrolytes

Basic concepts related to aqueous electrolyte

Ionization of an acid in water.

When hydrogen chloride dissolves in water, the following process occurs:

ionization H Cl(g) + water

H+(aq) + Cl-(aq)

mobile ions of aqueous hydrochloric acid

Ionization is a process where ions are formed from covalent molecules.

Ionization of water An aqueous solution of a substance, e.g. NaCl, consists of the following ions: NaCl(aq) Na+(aq) + Cl-(aq) H2O(l) H+(aq) + OH-(aq) The sign indicates that the ionization of water occurs only very slightly.

Indicators to detect whether a solution is acidic or alkaline (1) An acidic solution is formed when there are more H+ ions than OH- ions.

Indicators like methyl orange and litmus could be used to detect the presence of excess H+ (and excess OH-) (2) An alkaline solution is formed when there are more OH- ions than H+ ions.

Concentration of solution The concentration of a solution is usually expressed in molar concentration (or molarity, unit: mol dm-3, short form M) A 10 M sodium chloride solution is very concentrated in terms of a huge amount of sodium chloride in water. A 0.01 M sodium chloride solution is very dilute in sodium chloride.

Form 4 Chemistry Notes Chapter 18 : Electrolysis

Question 18.2 (a) Give the formulae of the ions present in an aqueous solution of copper(II) sulphate.

(b) Is dilute nitric acid acidic, neutral or alkaline?

(c) Which is more concentrated, 0.1 M sulphuric acid or 1 M sulphuric acid?

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Electrolysing an aqueous sodium chloride solution

(1) Make a list of the cations and anions present in sodium chloride solution

NaCl(aq) Na+(aq) + Cl-(aq)

H2O(l) H+(aq) + OH-(aq) (a) Cations: Na+ and H+

(b) Anions: Cl- and OH-

(2) Which cation will be discharged depends on the preferential discharge.

Factors affecting preferential discharge of ions

There are three factors affecting preferential discharge (1) Position of ions in the Electrochemical Series (E.C.S.)

The position of an ion in the E.C.S. often determines its order of preference in discharge. The following diagram shows the order of preferential discharge of Cations and Anions.

Take the example of electrolysis of an aqueous solution of NaCl. Cations: Na+ and H+ compete with each other for discharge. The E.C.S. predicts the discharge of

H+. Anions: Cl- and OH- compete for discharge. The E.C.S. predicts the discharge of OH-.

(2) Concentration of ions in solution. When the concentration of one ion is much higher than the other, the more concentrated ion is discharged even though it is against the prediction of E.C.S. e.g. When a concentrated 3M NaCl solution is electrolysed, Cl- ions are discharged even though OH- ions are predicted to be discharged according to the E.C.S. Here, the concentration factor is more important than the E.C.S. factor.

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However, H+ ions are discharged even though a high concentration of Na+ as the difference in

E.C.S. between H+ and Na+ is so great that the concentration factor becomes not as important

than the E.C.S. factor.

(3) Material of electrodes When inert electrodes (graphite, platinum, gold) are used in electrolysis, the electrolytic reactions will be independent of the nature of the inert electrodes. However, when copper electrodes are used in the electrolysis of aqueous copper(II) sulphate solution, the copper electrode (anode) itself will take part in one of the electrolytic reactions (To be discussed later).

Questions 18.3 Refer to the diagram of position of the E.C.S. (p. 4 of notes), use colour markers to highlight the positions of H+(aq) and OH-(aq). In fact, their positions have special importance in the preferential discharge of ions in any aqueous solution. Fill in the blanks: In any aqueous solution, ___________ ions and ___________ ions are present. Preferential discharge of a cations at a cathode ? If a cation is below H+(aq) in the E.C.S., the ____________ should be discharged, forming the

_____________ . ? If a cation is above H+(aq), the ______________ should be discharged, forming ____________ . Preferential discharge of anions at an inert anode ? Basically, _________________ should be discharged, forming _____________ .

But in a concentrated halide solution (chloride, bromide or iodide), _______________ may be discharged instead (due to ________________ effect).

18.4 Electrolysis of Water

Electrolysis of Distilled Water

Hofmann voltameter could be used to electrolyse water. However there is no observable change

due to the following process: H2O(l) H+(aq) + OH-(aq)

occurs to a very slight extent. Without sufficient H+ and OH- ions distilled water is a poor conductor of electricity.

Electrolysis of Acidified Water

When sulphuric acid (H2SO4) is added to distilled water, the conducting power of water is greatly improved due to the following process

H2SO4(aq) 2H+(aq) + SO42-(aq) occurs with 100% efficiency.

A table is used to summarise the ions present in dilute sulphuric acid.

From H2SO4 From H2O

Ions attracted to the cathode 2H+(aq) H+(aq)

Ions attracted to the anode

SO42-(aq) OH-(aq)

(1) At the cathode

Only one type of cation is present. The following discharge reaction occurs: 2H+(aq) + 2e- H2(g) ........................................... (1)

(2) At the anode There are SO42- and OH- ions competing for discharge. SO42- ions are so inert that they will not be discharged normally. The OH- ions discharged to give oxygen and water accordingly. 4OH-(aq) O2(g) + 2H2O(l) + 4e-..............................(2)

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Overall reaction

When equation (1) is multiplied by a factor of 2 and added to equation (2), the overall reaction of

the electrolysis becomes

4H+(aq) + 4OH-(aq) 2H2(g) + 2H2O(l) + O2(g)

4H2O(l) 2H2(g) + 2H2O(l) + O2(g)

OR 2H2O(l) 2H2(g) + O2(g)

The electrolysis of dilute sulphuric acid is the same as the electrolysis of water, and the acid added

is only to increase the electrical conductivity of water.

Question 18.4 Add a little dilute sodium hydroxide solution to water. Then add the resulting solution to a Hofmann voltameter and electrolyse it, using platinum electrodes. (a) State the formulae of the cations and anions present in the solution.

(b) Name the product liberated at the (i) cathode

(ii) anode

(c) What is the volume ratio of the products in (b)?

18.5 Electrolysis of Sodium Chloride Solution

Electrolysis of Very Dilute sodium chloride solution Make a list of the cations and anions in a very dilute sodium chloride (NaCl) solution.

Ions attracted to the cathode Ions attracted to the anode From sodium chloride (NaCl) From water (H2O)

(1) At the cathode H+(aq) is preferentially discharged. 2H+(aq) + 2e- H2(g) As H+ ions are removed, there is an excess of OH- ions and the cathodic region is alkaline.

(2) At the anode OH-(aq) is preferentially discharged. 4OH-(aq) O2(g) + 2H2O(l) + 4eAs OH- ions are removed, there is an excess of H+ ions and the anodic region is acidic.

Overall reaction 2H2O(l) O2(g) + 2H2(g)

Electrolysis of Concentrated Sodium chloride solution Make a list of the cations and anions in a concentrated sodium chloride (NaCl) solution.

Ions attracted to the cathode Ions attracted to the anode From sodium chloride (NaCl) From water (H2O)

(1) At the cathode H+(aq) is preferentially discharged even though there are far more Na+ ions. Here, the E.C.S. factor is more important than the concentration factor. 2H+(aq) + 2e- H2(g) As H+ ions are removed, there is an excess of OH- ions and the cathodic region is alkaline.

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(2) At the anode The E.C.S. factor predicts OH- ions to be discharged. However as there are far more Cl- ions and since the difference in E.C.S. position between OH- and Cl- is very small. Here the

concentration factor is more important than the E.C.S. factor.

2Cl-(aq) Cl2(g) + 2e-

Overall reaction 2H+(aq) + 2Cl-(aq) H2(g) + Cl2(g)

Changes in the solution As H+ and Cl- ions are discharged, Na+ and OH- ions are left behind. Gradually the electrolyte

turned into a solution of sodium hydroxide (NaOH).

18.6 Electrolysis of Copper(II) sulphate solution with copper electrode

Make a list of the cations and anions in a copper(II) sulphate solution.

From copper(II) sulphate, (CuSO4) From water (H2O)

Ions attracted to the cathode Ions attracted to the anode

Cu2+(aq)

SO42-(aq)

H+(aq)

OH-(aq)

(1) At the cathode Cu2+(aq) is preferentially discharged according to the E.C. S. factor.

Cu2+(aq) + 2e- Cu(s)

(2) At the anode Neither SO42- nor OH- is discharged.

Here the preferred anodic reaction is the oxidation of the copper anode to form copper(II) ions.

This is called the electrode factor. Cu(s) Cu2+(aq) + 2e-

Changes at the electrodes The copper cathode becomes thicker due to the deposition of copper. The copper anode becomes thinner as it dissolves to form copper(II) ions.

Changes in the solution Copper(II) ions migrated to the cathode are replenished (constantly supplied) by the anode and there is no change in the copper(II) sulphate solution concentration. The electrolytic process can be considered as a transfer of copper from the anode to the cathode.

Question 18.5 An aqueous solution of copper(II) sulphate is electrolysed for some time using graphite electrodes. What colour change would you expect to observe in the solution?

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18.7 Products of Electrolysis of some aqueous solution of some aqueous solution ? a summary

The following table shows the results of electrolysis of some aqueous solutions.

Question 18.7 Predict (if possible) the product at each electrode in each of the following cases: (a) Electrolysis of lead(II) nitrate solution with a copper cathode and a platinum anode.

(b) Electrolysis of sodium bromide solution.

18.9 Industrial Applications of Electrolysis

Electroplating It is a process to plate one metal with a thin, even layer of another metal. A metal is electroplated to protect against corrosion and/or to improve appearance. Refer to the following diagram to plate an iron spoon with silver:

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