CHAPTER 6 – ELECTROCHEMISTRY

A ELECTROLYTES AND NON-ELECTROLYTES

Activity 1

1. State the meaning of electrolyte: An electrolyte is a substance that can conduct (e)……………………in ………………

state or (a) ……………………. (s)……………………. and undergo (c)……………………

(c)…......................….

2. A non-electrolyte is a substance that cannot conduct (e)……………………. either in

(m)……………………. state or (a)……………………….. solution.

3. Ionic compounds in molten state or in aqueous solution are electrolytes

because these substances contain freely (m) …………………. (i) …………………….

4. Covalent compounds are non-electrolytes and these substances contain neutral

(m) ................... and no freely (m) ……………………… (i)…………… However,

certain covalent compounds such as hydrogen chloride, ammonia and ethanoic

acid when dissolved in water are electrolytes. This is because these compounds react

with water to produce freely ………………… …………..

5. Ionic compounds in molten state or in aqueous solution are electrolytes while

covalent compounds and ionic compounds in solid state are non-electrolytes. Metals

are non-electrolytes but are good conductors of electricity.

Learning Outcomes:

You should be able to:

state the meaning of electrolyte, classify substances into electrolytes and non-electrolytes. relate the presence of freely moving ions to electrical conductivity.

Activity 2

Classify the substances below into electrolyte and non-electrolyte

Electrolyte Non-electrolyte

Solid lead(II) chloride, molten aluminium oxide, lead(II) nitrate solution, solid sodium chloride, sodium chloride solution, magnesium, molten lead(II) chloride, glucose solution, glacial ethanoic acid, dilute ethanoic acid, molten naphthalene, ethanol, silver, tetrachloromethane, sodium hydroxide solution, aqueous ammonia

B ELECTROLYSIS OF MOLTEN COMPOUNDS

1. What is meant by electrolysis?Electrolysis is a process whereby compounds in (a) ......................solution or (m) ..

…………… states are broken down (or decomposed) into its constituent element when (e)

…………………..(c) ……………………passes through it.

2. Anode is electrode which is connected to the (p) …………………. terminal of a battery.

3. Cathode is electrode which is connected to the (n) ……………………… terminal of a battery.

4. Carbon or platinum or graphite is chosen as electrodes as they are chemically inert or unreactive.

Learning Outcomes:

You should be able to:

describe electrolysis, describe electrolytic cell, identify cations and anions in a molten compound, describe evidence for the existence of ions held in a lattice in solid state but move freely in

molten state, describe electrolysis of a molten compound, write half-equations for the discharge of ions at anode and cathode, predict products of the electrolysis of molten compounds.

Activity 3

Set-up of apparatus of electrolysis of molten lead(II) iodide. Name the main apparatus and materials in the diagram.

What happen during electrolysis?

- Electrolyte contains freely moving ions- cations are attracted to the cathode and anions are attracted to the anode.

- Discharge of ions at cathode and anode

- Product produce at cathode and anode

- Electric circuit is complete due to the flow of electrons along the connecting wires and movement of ions in the solution.

Activity 4

Write ionic formula of anion and cation in each electrolyte (molten) and write it’s half equation

Electrolyte

(Molten)

Anion Cation

Ionic formula

Half equation Ionic formula

Half equation

Sodium chloride

NaCl

Lead(II) oxide

PbO

Potassium iodide

KI

Zinc bromide

ZnBr2

Lead(II) iodide

PbI2

Magnesium oxide

MgO

Molten lead(II) iodide

Consists of

(Ions that are present)

( Movement of ions)

To cathode

(Half equation)

At Cathode

(Products formed)

At Cathode

Activity 5

The flow chart below is used to predict the products formed at the electrodes during the electrolysis of molten lead(II) iodide.

Molten Zinc oxide

Consists of

(Ions that are present)

( Movement of ions)

To cathode

(Half equation)

At Cathode

(Products formed)

At Cathode

Activity 6

The flow chart below is used to predict the products formed at the electrodes during the electrolysis of molten Zinc oxide

C ELECTROLYSIS OF AQUEOUS SOLUTION

Learning Outcomes:

You should be able to:

identify cations and anions in an aqueous solution, describe the electrolysis of an aqueous solution, explain using examples factors affecting electrolysis of an aqueous solution, write half equations for the discharge of ions at the anode and the cathode, predict the products of electrolysis of aqueous solutions.

1. Molten electrolyte contains only solute. So that means 2 ions exists. But electrolyte of aqueous solution contains solute and solution (water). So 4 ions present. 2 ions from solute and another 2 ions from water

2. 3 factors that affecting electrolysis of aqueous solution are:

(a) …………………………………………………………………………………………………

(b) ………………………………………………………………………………………………….

(c) ………………………………………………………………………………………………….

3. List the electrochemistry series (cations and anions) in order of increasing ease of discharge.

4. The following statements refer to the factors that affect the electrolysis of an aqueous solution. Fill in the blanks.

(a) When electrolysis is conducted on (d)………………….. (s)………………. and (i)………………… electrodes, the cation and anion at the lower position in the electrochemical series will be selected to be discharged

(b) If the concentration of ion in electrolyte is high, that ion will be more likely to be discharged in electrolysis.

(c) In the electrolysis of copper(II) sulphate, CuSO4 using copper electrodes, no ions are discharged at the anode. Instead, the copper anode (d)………………. to form Cu2+

ion in the electrolyte.

Activity 7

The diagram below shows the set-up of apparatus of an electrolytic cell containing concentrated copper(II) sulphate solution. Two test tubes filled with copper(II) sulphate solution were placed over the electrodes J and K to collect any gas evolved. The switch is then turned on so that electrolysis of copper(II) sulphate solution can occur.

(a) Identify the cations and the anions present in the aqueous solution.

Cations: …………………………………….. Anions: …………………..………………………

(b) Identify which electrode ( J or K ) is the anode and the cathode:

Anode ……………………………….. Cathode ……………………………………….

(c) (i) Which ion is selectively discharge at the anode? ………………………………………

(ii) Give a reason for your answer in (c) (i). …………………………………………………..

…..…………………………………………………………………………………………….

(iii) What do you observe at the anode? ………………………………………………………

(iv) Give one test to confirm the gas released at K. …………………………………………

………………………………………………………………………………………………. (v) Write a half equation to represent the discharge of ions at anode.

…….……………………………………………………………………………………………

(d) (i) Which ion is selectively discharge at the cathode? …………………………………………

(ii) Give a reason for your answer in (d) (i) ………………………………………………….

…………………………………………………………………………………………………

(iii) Which do you observe at the cathode? …………………………………………………

(iv) Write a half equation to represent the discharge of ions at the cathode.

…………………………………. ………………………………………………………….

(e) What do you observe about the copper(II) sulphate solution? …………………………………………………………………………………………………..

Activity 8

(a) The table below shows two electrolytic cells with electrolytes of different

concentration. You are required to answer each section by writing your answer in

the spaces provided.

1. In the diagrams, label the cathode with the symbol “” and the anode with the symbol “+”.

2. Show the direction of the flow of the electrons with arrowheads, “ > “

3. Write the formula of all ions in the electrolyte.

4. a. Write the formula of ions which are attracted to the cathode.

b. Underline the formula of ion which

is selectively discharged.

c. State the factor that affect the

selective discharged of ion

5. Write the half equation to represent

the reaction at the cathode.

6. What will you observe at the cathode?

7. a. Write the formula of ions which are attracted to the anode.

b. Underline the formula of ion which

is selectively discharged.

c. State the factor that affect the

selective discharged of ion

8. Write the half equation to represent the reaction at the anode.

9. What will you observe at the anode?

(b) The table below shows two electrolytic cells with the same electrolytes with

different electrodes. You are required to answer each section by writing your

answer in the spaces provided.

1. In the diagrams, label the cathode with the symbol “” and the anode with the symbol “+”.

2. Show the direction of the flow of the electrons with arrowheads, “ > “

3. Write the formula of all ions in the electrolyte.

4. a. Write the formula of ions which are attracted to the cathode.b. Underline the formula of ion

which is selectively discharged.

c. State the factor that affect the

selective discharged of ion

5. Write the half equation to represent the reaction at the cathode.

6. What will you observe at the cathode?

7. a. Write the formula of ions which are attracted to the anode.b. Underline the formula of ion

which is selectively discharged.

c. State the factor that affect the

selective discharged of ion

8. Write the half equation to represent the reaction at the anode.

9. What will you observe at the anode?

10 What do you observe about the

copper(II) sulphate solution?

Explain

D ELECTROLYSIS IN INDUSTRIES

Learning Outcomes:

You should be able to:

state uses of electrolysis in industries, explain the extraction, purification and electroplating of metals involving electrolysis in industries, write chemical equations to represent the electrolysis process in industries, justify uses of electrolysis in industries, describe the problem of pollution from electrolysis in industry.

Activity 9

1. Fill in the blanks.

The application of electrolysis in industries are (a) ……………………………………….

(b) ……………………………………………… and (c) ……………………………………

In the extraction of aluminium from its ore, (d) ….……………… electrodes are used and

(e) ……..………………. is added to aluminium oxide to lower its melting point.

In purification of metals, the pure metal is made the (f) ….………………. and the impure

metal is made the (g) ……….……………. The electrolyte used is an aqueous salt

solution of the metal ions.

In electroplating of metals, the (h) ………..…………….is made the anode and the (i)

……………… to be (j) ..…..………………….. is made the cathode. The electrolyte

used is an aqueous salt solution of the electroplating metal.

The purposes of electroplating metals are to make the electroplated object more (k)

………………………………….. and (l) …………………..……………… to corrosion.

2. Below are shown the three uses of electrolysis in industries. Fill in the blanks.

Extraction of aluminium from bauxites

Purification of copper from impure mined copper

Electroplating of iron spoon with silver

1. Substance used as cathode and anode

Cathode:

Anode:

Cathode:

Anode:

Cathode:

Anode

2. Electrolyte used

3. Half equation representing the process.

Cathode:

Anode:

Cathode:

Anode:

Cathode:

Anode:

E VOLTAIC CELLS

Learning outcomes:

You should be able to:

describe the structure of a simple voltaic cell and Daniell cell, explain the production of electricity from a simple voltaic cell, explain the reactions in a simple voltaic cell and Daniell cell, compare and contrast the advantages and disadvantages of various voltaic cells,

Activity 10 1. A simple voltaic cell can be constructed by immersing two ……………………….

metals in an ………………………. connected by …………………

2. In a voltaic cell, ……………………… energy is converted to ……………………. energy.

3. More electropositive metal will donate electron and serve as negative terminal (anode)

4. Less electropositive metal will receive electron and serve as positive terminal (cathode)

5. Electron will flow from negative terminal to positive terminal through connecting wire (external circuit)

6. The diagram below shows an example of a simple voltaic cell.

In the text box below are sentences explaining the production of electricity from a simple voltaic cell. The sentences are listed in random order. You are required to arrange these sentences in the best possible order so as to give a clear description of the reactions occurring in a simple voltaic cell.

Answer:

(a) ………………………………………………………………………………………………………

………………………………………………………………………………………………………

(b) ………………………………………………………………………………………………………

………………………………………………………………………………………………………

(c) ……………………………………………………………………………………………………..

………………………………………………………………………………………………………

(d) ………………………………………………………………………………………………………

………………………………………………………………………………………………………

(e) ………………………………………………………………………………………………………

………………………………………………………………………………………………………

(f) …………………………………………………………………………………………………………

…………………………………………………………………………………………………………

(g) …………………………………………………………………………………………………………

………………………………………………………………………………………………………….

An example of a simple voltaic cell is a magnesium strip and a copper strip immersed in dilute sodium chloride solution.

The electrons then flows from the magnesium ribbon to the copper plate through the wire and this results in the flow of electrical current.

Hence magnesium atom releases electrons more easily than a copper atom and the magnesium act as the negative terminal of the cell.

The overall equation for the reaction is given as follows. Mg(s) + 2H+(aq) Mg2+(aq) + H2(g)

Magnesium is placed higher than copper in the electrochemical series. At the negative terminal, each magnesium atom releases two electrons and the Mg2+

formed moved into the solution. Mg(s) Mg2+ (aq) + 2e

At the positive terminal which is the copper plate, the electrons are accepted by the H+ ions in sodium chloride solution.

2H+ + 2e H2 (g)

Activity 111. (a) Draw and label the set-up of apparatus of a Daniell cell consisting of a salt bridge.

(b) (i) Which metal in the Daniell cell is the negative terminal?

…………………………………………………………………………………………………(ii) Give reason for your answer in (b)(i).

………………………………………………………………………………………………… (iii) Write a half equation to represent the reaction occurring at the negative terminal.

…………………………………………………………………………………………………. (c) Write a half equation to represent reaction at positive terminal.

…………………………………………………………………………………………………… (d) What do you observe at

(i) negative terminal

………………………………………………………………………………………………….. (ii) positive terminal

………………………………………………………………………………………………….. (e) State two functions of the salt bridge.

1. ……………..……………………………………………………………………………… 2. ………………………………………………………………………………………………

Activity 12

What are the differences between an electrolytic cell and a voltaic cell?. Complete table below.

It does not require a source of electric current It requires a source of electric current

The electrical energy causes chemical reactions to occur at the electrodes.

The chemical reaction that occur at the electrodes produces electric current.

Electrical energy chemical energy Chemical energy electrical energy

The electrodes must be of two different metals

The electrodes may be of the same material such as carbon

Electrons flow from the positive electrode (anode) to the negative electrode (cathode).

Electrons flow from more electropositive metal (negative terminal) to the less electropositive metal (positive terminal).

Ions receive electrons at the positive terminal. Ions donate electrons at the positive terminal.

Ions receive electrons at the negative terminal.

Ions donate electrons at the negative terminal.

F THE ELECTROCHEMICAL SERIES

Learning Outcomes:

You should be able to:

describe the principles used in constructing the electrochemical series, construct the electrochemical series, explain the importance of electrochemical series, predict the ability of a metal to displace another metal from its salt solution, write the chemical equations for metal displacement reactions.

1. Electrochemical series is an arrangement of elements based on their tendency to (d)………………… electron to form ions

2. A metal that has high tendency to donate electron to form positive ion is placed at a higher position in the series

3. Electrochemical series can be constructed by 2 methods:(a) Potential difference between 2 metals(b) The ability of a metal to displace another metal

Activity 13

Three experiments were conducted to investigate the potential differences between three pairs of metals in a voltaic cell. An electrochemical series for four metals P, Q, S and T is then constructed based on the potential difference obtained. Three pair of metals used as electrodes in different voltaic cells are: P and Q, Q and S and S and T.

01 2 3 4

56

V0

1 2 3 45

6

V

01 2 3 4

56

V

All the metals are cleaned with sandpaper before used.

50 cm3 of 1.0 mol dm-3 sodium nitrate solution is poured into a beaker as electrolyte.

Experiment I

The electrodes P and Q are immersed into the solution. The two electrodes are connected to a voltmeter using copper wires. Electrode Q is the positive terminal. The voltmeter reading is recorded.

Experiment II

The electrodes Q and S are immersed into the solution. The two electrodes are connected to a voltmeter using copper wire. Electrode Q is the positive terminal. The voltmeter reading is recorded.

Experiment III

The electrodes S and T are immersed into the solution. The two electrodes are connected to a voltmeter using copper wire. Electrode T is the positive terminal. The voltmeter reading is recorded.

Based on Experiment I, II and III, answer the questions below.

(a) Record the voltmeter reading of each experiment in the spaces provided.

Experiment 1 Experiment II Experiment III

Voltmeter reading: …………. Voltmeter reading: …………… Voltmeter reading: ….……..

(b) Construct a table to record the data from the above experiments.

(c) List the apparatus and materials that you will need to carry out this experiment.

Apparatus: ……………………………………………………………………………………….

……………………………………………………………………………………………………..

Materials: ………………………………………………………………………………………..

……………………………………………………………………………………………………(d) State all the variables:

1. Manipulated variable: pair of metal

2. Responding variable: voltage value

3. Controlled variable: type and concentration of electrolyte

(e) State the hypothesis: 1. The metal that act as positive terminal is placed at a higher position in

electrochemical series2. The higher the potential difference, the further the position of 2 metals in the

electrochemical series

(f) Based on the information obtained in Experiment I, what can you infer about metal P and Q?

……………………………………………………………………………………………………(g) Write a half equation for the reaction occurring in negative of Experiment I, assuming the

cation has a +2 charge.

……………………………………………………………………………………………………….(h) Arrange the metals P, Q, S and T in descending order of their tendency to donate electrons.

...……………………………………………………………………………………………………(i) Another voltaic cell is set-up using metals T and Q as electrodes. Predict the potential

difference produced in the cell.

……………………………………………………………………………………………………..

(j) Given that metal X is placed between metal S and metal Q in the electrochemical series, can metal X displace metal S from its salt solution? Give an explanation for your answer

……………………………………………………………………………………………………….

……………………………………………………………………………………………………….(k) Given that copper is more electropositive than metal T, a displacement reaction will occur

when copper is immersed into a salt solution of metal T, TNO3. Write the chemical equation for this reaction.

……………………………………………………………………………………………………….(l) State three important uses of the electrochemical series

…………………………………………………………………..…………………………………

…………………………………………………………………………………………………….

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