6a electrochemistry - answer

WAJA F4 Chemistry 2009 Chapter 6: Electrochemistry

CHAPTER 6: ELECTROCHEMISTRYANSWERActivity 1(a) electricity, (b) molten, (c) aqueous, (d) solution, (e) chemical, (f) changes, (g) electricity, (h) molten (i) aqueous (j) moving ions (k) molecules, (l) moving ions, (m) moving ions.

Electrolyte Non-electrolytemolten aluminium oxide Solid lead(II) chloridelead(II) nitrate solution solid sodium chloridesodium chloride solution magnesiummolten lead(II) chloride glucose solutiondilute ethanoic acid glacial ethanoic acidsodium hydroxide solution molten naphthaleneaqueous ammonia ethanol

silvertetrachloromethane

Activity 2(a) molten, (b) aqueous, (c) elements, (d) electricity, (e) positive, (f) negative, (g) battery, (h) switch, (i) carbon, (j) carbon, (k) molten lead(II) bromide

Activity 31. Solid sodium chloride contains sodium ions and chloride ions which are in fixed position and

not freely moving.2. In solid state, sodium ions and chloride ions are strongly attracted by electrostatic forces in a

lattice.3. Aqueous sodium chloride contains freely moving ions to conduct electricity.4. During electrolysis cations are attracted to the cathode and anions are attracted to the

anode.5. Electric circuit is complete due to the flow of electrons along the connecting wires and

movement of ions in the solution.6. If the electrodes are placed further apart, the ammeter reading will decrease because there

will be an increase in internal resistance.

Activity 41Electrolyte (Molten) Cation Anion

Name Formula Name FormulaSodium chloride Sodium ion Na+ Chloride ion Cl–

Lead(II) oxide Lead(II) ion Pb2+ Oxide ion O2–

Potassium bromide Potassium ion K+ Bromide ion Br–

2.

Electrolyte(molten)

Substance discharged at the electrodes and the half equation

Anode Cathode (i) Aluminium oxide Oxygen gas

Half equation:2O2– O2 + 4e Aluminium Half equation: Al3+ + 3e Al

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Electrolyte(molten)

Substance discharged at the electrodes and the half equation

Anode Cathode(ii) Potassium iodide Iodine

Half equation : 2I– I2 + 2e Potassium Half equation: K+ + e K

(iii) Sodium chloride Chlorine gas Half equation: 2Cl– Cl2 + 2e

Sodium Half equation: Na+ + e Na

(iv) Zinc bromide Bromine gas Half equation: 2Br– Br2 + 2e

Zinc Half equation: Zn2+ + 2e Zn

Activity 51. Electrolysis of molten lead(II) bromide(a) Lead(II) ions and bromide ions (or Pb2+ and Br– ions), (b) Bromide ions (or Br– ions), (c) Lead(II) ions or (Pb2+ ions), (d) 2Br– Br2 + 2e, (e) Pb2+ + 2e Pb, (f) bromine gas, (g) lead metal.

2. Electrolysis of molten zinc chlorideNote: The flow chart is similar to that shown in question 1.Name of substance: Molten ZnCl2, (a) Zn2+ and Cl– ions, (b) Cl– ions, (c) Zn2+ ions, (d) 2Cl– Cl2 + 2e, (e) Zn2+ + 2e Zn, (f) chlorine gas, (g) zinc metal.

Activity 61 (a) Position of ions in the electrochemical series (b) Concentration of ions in the electrolysis (c) Types of electrodes used in the electrolysis2 hydrogen H+ ; hydroxide, OH—

3. Cation: K+, Na+, Ca2+, Mg2+, Al3+, Zn2+, Fe2+, Sn2+, Pb2+, H+, Cu2+, Ag+

Anion: F- , SO42- , NO3- , Cl- , Br - , I- , OH-

4. (a) lower, selectively (b) high, selectively discharged (c) solution, corrode, dissolved

Activity 7(a) Cations: copper(II) ion, Cu2+ and hydrogen ion, H+

Anions: sulphate ion, SO42– and hydroxide ion, OH–

(b) Anode: electrode K Cathode: electrode J(c) (i) hydroxide ion, OH–

(ii) hydroxide ion is lower than sulphate ion in the electrochemical series.(iii) Gas bubbles (iv) Place a glowing wooden splinter into the test tube containing the gas. If splinter

rekindles, then the gas is confirmed to be oxygen.(v) 4OH- H2O + O2 + 4e

(d) (i) copper(II) ion, Cu2+ (ii) The position of copper(II) ion is lower than hydrogen ion in the electrochemical series (or concentration of copper(II) ion is higher than the concentration of hydrogen ion)(iii) Brown solid deposited on the electrode(iv)Cu2+ + 2e Cu

(e) Blue colour of copper(II) sulphate solution become paler.

Activity 8

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3 Na+, Cl–, H+ , OH– Na+, Cl–, H+ , OH–

Cu2+, SO42–, H+, OH–

4 Na+, H + Na+, H + Cu 2+ , H+

5 2H+ + 2e H2 2H+ + 2e H2 Cu+ + 2e Cu

6 Gas bubbles are formed Gas bubbles are formed Brown solid deposited on the cathode

7 Cl – , OH– ions Cl–, OH – ions SO42–, OH –

8 2Cl– Cl2 + 2e 4OH– 2H2O + O2 + 4e Cu Cu2+ + 2e

9 Gas bubbles are formed Gas bubbles are formed Copper plate becomes thinner

Activity 9

1. (a) extraction of metals, (b) purification of metals, (c) electroplating of metals, (d) carbon, (e) cryolite, (f) cathode, (g) anode, (h) electroplating metal, (i) object/substance, (j) electroplated, (k) attractive, (l) resistant.

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: CarbonAnode: carbon

Cathode: Pure copperAnode: impure copper

Cathode: iron spoonAnode: silver

2. Electrolyte used Aluminium oxide and cryolite

Copper(II) sulphate solution

Silver nitrate solution

3. Half equation representing the process.

Cathode:Al3+ + 3e AlAnode:2O2– O2 + 4e

Cathode:Cu2+ + 2e CuAnode:Cu Cu2+ + 2e

Cathode:Ag+ + e AgAnode:Ag Ag+ + e

Activity 10

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1. different, electrolyte, wires2. chemical, electrical3.

Electrochemical series of metalsCation formed and number of electrons

released during the process

K K+ + e

Na Na+ + e

Ca Ca2+ + 2e

Mg Mg2+ + 2e

Al Al3+ + 3e

Zn Zn2+ + 2e

Fe Fe2+ + 2e

Sn Sn2+ + 2e

Pb Pb2+ + 2e

H H+ + e

Cu Cu2+ + 2e

Ag Ag+ + e

[ One way (mnemonic) to remember the ECS: Kalau Nak Cari Makan, Ali, Zainal, Fee-fee Sentiasa Pergi Hutan Cari Argentum]

4. (a) An example of a simple voltaic cell is a magnesium ribbon and a copper plate immersed in dilute sodium chloride solution.(b) Magnesium is placed higher than copper in the electrochemical series.(c) Hence magnesium atom releases electrons more easily than a copper atom and the

magnesium strip act as the negative terminal of the cell.(d) At the negative terminal, each magnesium atom releases two electrons and the Mg2+

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

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

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

Activity 11

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Tendency of metal atoms to donate electrons to form ions increases


1.(a)

(b) (i) Zinc(ii) Tendency of zinc to donate electrons is higher than copper. (Zinc is more

electropositive than copper)(iii) Zn Zn2+ + 2e

(c) Cu2+ +2e Cu

(d) (i) Zinc strip becomes thinner

(ii) Brown solids deposited (copper strip becomes thicker)

(e) 1. To allow ions to flow into the beakers and maintain electrical neutrality of the solutions

2. To complete the circuit.

2. Please refer to page 106 of the Form Four Chemistry Text Book for answer to question 2

Activity 12

DIFFERENCESElectrolytic cell Aspect Chemical cell

It requires a source of electric current Source of electric current

It does not require a source of electric current

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

Conversion of energy

The chemical reactions that occur at the electrodes produce an electric current. Chemical energy electrical energy

The electrodes may be of the same Type of The electrodes must be of two different

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DIFFERENCESElectrolytic cell Aspect Chemical cell

material such as carbon electrodes metalsElectrons flow from the positive electrode (anode) to the negative electrode (cathode).

Direction of flow of

electrons

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

Ions donate electrons at the positive electrode. (Oxidation)

Type of reaction at

positive terminal

Ions receive electrons at the positive electrode. (Reduction)

Ions receive electrons at the negative electrode.(Reduction)

Type of reaction at negative terminal

Atoms donate electrons at the negative electrode.(Oxidation)

Activity 13(a) Experiment I = 1.6 V Experiment II = 0.8 V Experiment III = 3.2V(b)

Note: Must have at least three headings consisting (i) Experiment, (ii) Negative terminal or positive and (iii) voltmeter reading, V

(c) Apparatus: Voltmeter, 100 cm3 beakers, copper wires with crocodile clipsMaterials: Sodium nitrate solution,1.0 mol dm-3, sandpaper, metal P, metal Q, metal R, metal

S, metal T

(d) 1. Manipulated variable: Pair of metals (or distance between two metals in the electrochemical series)

2. Responding variable: Voltmeter reading3. Controlled variable: Concentration of sodium nitrate solution (or 1.0 mol dm-3

sodium nitrate solution)

(e) The further the distance between two metals in the electrochemical series, the higher the voltmeter reading.

(f) Metal P is more electropositive than Q. (or metal P is placed higher than metal Q in the electrochemical series.)

(g) P P2+ + 2e(h) P, S, Q, T(i) 2.4 V(j) No, because metal X is less electropositive than S.(k) Cu + 2TNO3 Cu(NO3)2 + 2T(l) Please refer to page 109 of the Form Four Chemistry Text Book.

Experiment Negative terminal

Positive terminal

Voltmeter reading (V)

I P Q 1.6II S Q 0.8III S T 3.2

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6a electrochemistry - answer

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