TOPIC 6 : ELECTROCHEMISTRY

ELECTROLYSIS OF MOLTEN COMPOUND

Worksheet 1 Complete the table below

Electrolyte(molten)

Substance formed and the half equation

Anode Cathode

(i) Aluminium oxide Oxygen gas Half equation :

……………………………

Aluminium Half equation :

……………………………

(ii) Potassium iodide Iodine Half equation :

……………………………

Potassium Half equation :

……………………………

(iii) Sodium chloride Chlorine gas Half equation :

……………………………

Sodium Half equation :

……………………………

(iv) Zinc bromide Bromine gas Half equation :

……………………………

Zinc Half equation :

……………………………

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Electrolysis is a process whereby a compound is decompose into its constituent elements when an electric current passes through an electrolyte Electrolyte - Substances that can conduct electricity when they are in molten state or aqueous solution and undergo chemical changes Non-electrolyte - Substances that cannot conduct electricity either in molten or aqueous solution. Anode - Electrode which is connected to the positive terminal of a battery Cathode - Electrode which is connected to the negative terminal of a battery

ELECTROLYSIS OF AQUEOUS SOLUTIONWorksheet 2

1) Electrolysis of dilute sulphuric acid solution and dilute sodium chloride solution have been carried out by using the sets of apparatus below with carbons as the electrodes.

(i) (ii)

2) Fill in the blanks

The application of electrolysis in industries are ………... .., ……………… and ………………

In the extraction of aluminium, from its ore, ………………electrodes are used and ………….. is

added to aluminium oxide to lower its melting point.

In purification of metals, the pure method is made the ………………. and the impure metal is

made the ……………. The electrolyte used is an aqueous salt solution of the metal ions.

In electroplating of metals, the …………….is made the anode and the ………………….is made

the cathode. The electrolyte used is an aqueous salt solution of the electroplating metal.

DiluteSulphuric acid

X Y Y P Q

DiluteSodium chloridesolution

2

a) State the ions present in the dilute sulphuric acid solution

b) State the ions that move towards X

c) State the ion discharges at X

d) State the ion discharges at Y

e) Name the product at X and Y

a) Name the ions present in dilute sodium chloride solution.

b) Name the ions that move towards P

c) Name the ions that move towards Q

c) Name the ion discharges at P

d) Name the ion discharges at Q

e) Name the product at P and Q

The purposes for electroplating metals are to make the electroplated object more

………………….. and ……………… to corrosion.

Worksheet 3

An electric current is passed through aqueous copper(II)sulphate solution in the apparatus shown below.

a. Classify the cations and the anions present in the aqueous solution cations anions:

(i) ………………………. ……………………….

(ii) ………………………. ………………………..

b Identify the anode and cathode : ………………………………….

c Give colour of (i) copper(II)sulphate solution : …………………

(ii) carbon electrodes : …………………..

d What can you observe at J?

…………………………………………………………………………..

e What can you observe at K ?

…………………………………………………………………………..

f Give one test to confirm the gas released at K

…………………………………………………………………………..

g If carbon rod is replaced with zinc plate, what can you observe at K ?

……………………………………………………………………………

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carbon electrodes Aqueous copper(II) sulphatesolution

J K

SIMPLE VOLTAIC CELL

THE ELECTROCHEMICAL SERIES – is an arrangement of elements according to their tendencies to form ions

Magnesium ribbon

G

Copper plate

4

1 Magnesium is higher than copper in the electrochemical series

2 Magnesium atom releases electrons more easily than a copper atom.

Mg(s) Mg2+ (aq) + 2e

3 Mg2+ ions are released into the sodium chloride solution

4 the electrons that are released flow through the external circuit

5 At the copper plate the electrons are accepted by the H+ ions in sodium chloride solution.

2 H+ + 2e H2 (g)

6 The flow of electrons from the magnesium ribbon to the copper plate results in the flow of electrical current.

7 The magnesium ribbon acts as the terminal

negative while the copper plate acts as positive terminal.Dilute

hydrochloricacid

K NaCaMgAlZnFeSnPbHCuAg

Ag

Tendency of metal atoms to donate electrons to form ions increases

K+ + eNa+ + eCa2+ + 2eMg2+ + 2eAl3+ + 3eZn2+ + 2eFe2+ + 2eSn2+ + 2ePb2+ + 2eH+ + eCu2+ + 2eAg+ + e

Tendency of cations to accept electrons to form metals increases

VVoltmeter

Zinccopper

Copper(II) sulphate solution

Zinc sulphate solution

Porous pot

e- e-

- e-

- e-

e-

e- + e- +

DANIELL CELL Zinc is more electropositive than copper, Zinc becomes thenegative terminal

Porous pot is used to,a. separate copper(II)sulphate from zinc sulphate solutionb. to allow the electrons and ions to flow through it, thereby connecting the circuit.

Salt bridge is use toa. connect the two separate solutionsb complete the circuit by allowing the ions to move from one solution to another

VVoltmeter

copper

Copper(II) sulphate solution Zinc sulphate

solution

Zinc_

_

-__-

__-__-

Salt bridge

5

1 Zinc atoms donate electrons to 5. The overall chemical reaction : produce zinc ions, Zn2+

2. Zinc ions go into zinc sulphate Zn(s) + Cu2+ (aq) Zn2+(aq) + Cu(aq) solution Zn(s) Zn2+ (aq) + 2e 6. Daniell Cell can represented by the 3 The electrons travel through the following equation wire in the external circuit to the Zn(s) / Zn2+(aq) Cu2+ (aq) /Cu(aq) copper electrode 7. The more electropositive metal and its4. At the copper electrode, the product are written on the left, while the electrons are taken by Cu2+ ions in less electropositive ion and its product copper(II)sulphate solution. Copper are written on the right. metal is then deposited on the electrod Cu2+(aq) + 2e Cu(s)

Worksheet 4

You are given a magnesium strip, a zinc strip, zinc nitrate solution , wires, a beaker and a voltmeter.

a) Draw a diagram of voltaic cell by using the apparatus and materials given.

b) Which metal is the (i) negative terminal? …………………………………………(ii) Give reason.

………………………………………………………………………

(ii) write the half equation 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) (i) If zinc is replaced with copper, predict any change of the voltmeter reading.

…………………………………………………………………………………….. (ii) Give reason to f(i)

……………………………………………………………………………………….

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Worksheet 5

Three experiments were conducted to determine the potential differences between three pairs of metal : P and Q, Q and S and S and T and carbons are used as electrodes in different voltaic cells50 cm3 of 1 mol dm-3 copper(II) nitrate solution is poured into a beaker as the electrolyte.

Experiment IThe electrodes P and Q are immersed into the solution. The two electrodes are connected to a voltmeter using copper wire . Electrode Q is the positive terminal. After a few minute the voltmeter reading is recorded.

Experiment IIThe 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. After a few minute the voltmeter reading is recorded.

Experiment IIIThe 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. After a few minute the voltmeter reading is recorded.

Based on Experiment 1, II and III, answer the questions below.

Experiment 1 Experiment II Experiment III

Reading Of Reading Of Reading Of Voltmeter : ...................... Voltmeter : ...................... Voltmeter : ......................

a) Construct a table to record the data from the above experiments

b) Arrange the metals P, Q, S and T in descending order of their reactivity

c) Based on Experiments I and II, state the inference for the experiment.

d) Another voltaic cell is set-up using metals T and Q as electrodes. Predict the potential difference produce in the cell.

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01 2 3 4

56

V

01 2 3 4

56

V

01 2 3 4

56

V