1-5 Bonding In Compounds (part 5)

V. Bonding in compounds

Part 5

Bonding in compounds

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Topic

V.

Reference Reading

Integrated Chemistry Today (2nd Ed.), L.H.M Chung, Book 1A, 74–78

Objectives

1.5.2.1 – 1.5.2.5 – deduce the existence of charged particles called ions in solutions of compounds made of metals and non-metal (electrolytes) from electrolysis experiments – recognise that some ions are coloured – deduce the colour of an ion by observation of the colours of a series of compounds – explain the migration of ions towards oppositely charged electrodes based on the existence of ions – recognise that the colours of gemstones are due to the presence of traces of coloured ions

Note

1.

Evidence of existence of ion

a) Conductivity of electrolyte Electrolysis of molten electrolyte (molten lead(II) bromide)

When electricity passes through molten lead(II) bromide, lead(II) bromide decomposes. A silvery metal forms at the negative pole and a brown gas evolves at the positive pole. N.B.

Lead(II) bromide (m.p. 373ºC) is used in this experiment because it has a lower melting point than the other electrolytes e.g. sodium chloride (m.p. 801ºC). It can be melted into liquid easily by a bunsen flame. Furthermore, the brown bromine gas evolved at the positive pole is toxic. The experiment must be carried out in the fume cupboard.

Molten lead(II) bromide conducts electricity because it contains mobile positive ion and negative ion. Electricity conducts through the molten electrolyte by the migration of the ions. Positive ion is called cation and negative ion is called anion.

Cation The naming of the electrodes refers to the directions of the movement of the ions. The positive electrode in electrolysis is called anode because the anions move towards it. While the negative electrode is called cathode because the cations move towards it.

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V. Bonding in compounds

Part 5

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Microscopic interpretation of electrolysis of molten lead(II) bromide

At positive electrode (anode)

At negative electrode (cathode)

1 bromide ion → 1 bromine atom + 1 electron 2 bromine atoms → 1 bromine molecule many bromine molecule → brown bromine gas

1 lead ion + 2 electrons → 1 lead atom many lead atoms → liquid lead metal

or

or

Br-(l) → Br(l) + e- (the electrons flow to the battery) 2Br (l) → Br2(g)

Pb2+(l) + 2e- → Pb(l) (the electrons flow from the battery)

Both bromide ions (Br -) and lead(II) ions (Pb2+) discharge during electrolysis and become bromine (Br) and lead (Pb) atoms. The continuous discharges of ion, movements of ions and movements of electrons in the external circuit keep the current flow in the circuit. N.B.

1. 2.

Bromide ion (Br -) is not the same as bromine atom (Br), bromide ion (Br -) carries a negative charge while bromine atom (Br) is electrically neutral. There is no connection between the polarity of an electrode and the name of the electrode, the name of an electrode is solely depending on the direction of movement of cations and anions. i.e. a positive electrode may be known as anode or cathode in different situations. e.g. The positive electrode of an electrolytic cell is known as an anode. The positive electrode of an electrochemical cell is known as a cathode.

V. Bonding in compounds

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Use of equation

The major function of an equation is to represent a reaction. The reactants are put on the left hand side of the arrow and the products are put on the right hand side of the arrow. Reactants → Products There are several different types of equations. e.g. word equation, unbalanced equation, balanced equation, half ionic equation and ionic equation. They are used in different situation depending on the usage. For example, burning of hydrogen in oxygen to form water can be represented by word equation (Represent the macroscopic meaning only)

hydrogen gas + oxygen gas → water

This can only be obtained from the direct observation of experiment.

Unbalanced equation (Represent the macroscopic meaning and a little microscopic meaning)

H2(g) + O2(g) → H2O(l)

A formula is used to represent individual particle. (hydrogen gas consists of H2 molecules, oxygen gas consists of O2 molecules and water consists of H2O molecules. Writing of this equation requires a little chemical knowledge.)

balanced equation (Represent the macroscopic meaning and more microscopic meaning)

2H2(g) + O2(g) → 2H2O(l)

The number of atoms on the right hand side equals to the number of atoms on the left hand side. i.e. the no. of atoms is unchanged. There are exactly 4 H atoms and 2 O atoms on both sides. It shows that 2 hydrogen molecules react with 1 oxygen molecule to produce 2 water molecules.

N.B.

Formula is a short hand in chemistry, it is used to represent a substance. e.g. hydrogen gas consists of hydrogen molecules, therefore, the formula of hydrogen gas is H2. Just the symbol H is not correct in this case, because in hydrogen gas, no isolated H atom can be found.

The decomposition of liquid lead(II) bromide by electrolysis into lead metal and bromine gas can be represented by word equation balanced equation

liquid lead(II) bromide → lead metal + bromine gas PbBr2(l) → Pb(l) + Br 2(g) (bromine gas consists of Br 2 molecule)

The use of half-ionic equation and ionic equation will be discussed in the latter part of the course.

V. Bonding in compounds

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Electrolysis of aqueous solution of electrolyte (aqueous copper(II) chloride solution) In molten electrolyte, ions are free to move in the liquid. In aqueous solution of electrolyte, ions are also free to move. The only difference is that the ions in an aqueous solution is surrounded by water molecules and not really free. They are usually known as mobile ions only.

An aqueous copper(II) ion, Cu2+(aq) In the electrolysis of aqueous solution of copper(II) chloride, reddish brown copper metal deposits on the cathode (negative pole) and yellowish green chlorine gas evolves at the anode (positive electrode).

At positive electrode (anode)

At negative electrode (cathode)

1 chloride ion → 1 chlorine atom + 1 electron 2 chlorine atoms → 1 chlorine molecule many chlorine molecules → yellowish green chlorine gas

1 copper ion + 2 electrons → 1 copper atom many copper atoms → reddish brown copper metal

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

2Cl → Cl2(g)

Cu2+(aq) + 2e- → Cu(s)

Overall equation Cu2+(aq) + 2e→ Cu(s) + 2Cl-(aq) → Cl2(g) + 2e-----------------------------------------------------------------Cu2+(aq) + 2e- + 2Cl-(aq) → Cu(s) + Cl2(g) + 2eCu2+(aq) + 2Cl-(aq)

→ Cu(s) + Cl2(g)

The net effect of electrolysis of copper(II) chloride solution is the same as decomposition of copper(II) chloride into copper metal and chlorine gas. It is also observed that the colour of the solution turns paler and paler during electrolysis. N.B.

The subscript (s), (l), (g) and (aq) are used to denote the physical state of a substance. They means solid, liquid, gaseous and aqueous (water solution) respectively. The arrow sign '→' in an equation is similar to the equal sign '=' in mathematics. If the same species appears on both sides of the equation, they can be canceled out.

V. Bonding in compounds

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b) Coloured ions In the above experiment, what makes the colour of the blue copper(II) solution turn paler ? It is found that almost all solution containing copper(II) ions are blue in colour e.g. copper(II) sulphate and copper(II) nitrate. Therefore, it is believed that the blue colour of copper(II) chloride solution is from the copper(II) ion. As the concentration of the copper(II) ion decreases in the electrolysis, the solution gets paler. By comparing the colour of different solutions, it is found that some ions are coloured in aqueous medium. The colours of the ions are summarized as follows : Copper(II) ion, Cu2+(aq) Iron(II) ion, Fe2+(aq) Iron(III) ion, Fe3+(aq) Cobalt(II) ion, Co2+(aq) Nickel(II) ion, Ni2+(aq) N.B.

blue green yellow pink deep green

Chromium(III) ion, Cr3+(aq) Chromate ion, CrO42-(aq) Dichromate ion, Cr2O72-(aq) Manganese(II) ion, Mn2+(aq) Permanganate ion, MnO4-(aq)

deep green yellow orange very pale pink / colourless deep purple

Usually, only the metal ion containing transition metal has colour. e.g. iron(III) ion (Fe3+(aq)), containing transition metal iron, is yellow in colour. e.g. permanganate ion (MnO4-(aq)) ,containing transition metal manganese, is deep purple in colour. e.g. nitrate ion (NO3-(aq)), containing no transition metal, is colourless.

Besides the substance itself, the colour of a substance also depends on the environment. e.g. I2 is a black solid if it is pure, it forms a brown solution in alcohol but forms a purple solution in organic solvent. Similarly, the colour of an ion also depends on the environment. The colour of the ions listed above are only correct in dilute aqueous solution. e.g. Aqueous solution of copper(II) sulphate is blue in colour but anhydrous (absolutely dry) copper(II) sulphate is white in colour. Concentrated copper(II) chloride solution is green but dilute copper(II) chloride solution is blue. Aqueous solution of cobalt(II) chloride is pink but it is blue in colour when absolutely dry.

V. Bonding in compounds c)

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Migration of ions

The presence of coloured ion is further supported by the experiment of migration of ions. Electrolysis of solid potassium permanganate (K+MnO4-(s))

A filter paper is saturated with (soaked in) sodium sulphate solution. Sodium sulphate is an electrolyte which makes the paper conductive. The solid potassium permanganate dissolves slowly and the ions becomes mobile. Under the influence of the electricity, the position of the purple spot move towards the positive pole (anode). If the purple spot doesn’t carry any charge, the spot should spread evenly on the filter paper instead of moving towards the positive electrode only. This proves the existence of negatively charged purple permanganate ion (MnO4-). There is no observable change at the negative pole (cathode) because potassium ion (K+) is colourless. The migration of colourless potassium ion is not visible. d) Colour of gem stone A high grade quartz or diamond is colourless but most gemstones are coloured.Trace amount of coloured ion is embedded in the colourless quartz to give the gemstone colour. Depending on which kind of ion is present, different gemstones have different colours. Gemstone Jade Emerald Amethyst Peridot Turquoise Topaz Sapphire

Glossary

Colour Green Green Purple Light green Greenish blue Yellow Blue

Ions embedded Cr3+ Cr3+ Mn3+ Fe2+ Cu2+ Fe3+ (titanium oxide)

electrolysis mobile cation anion electrode cathode anode polarity equation word equation unbalanced equation balanced equation half-ionic equation ionic equation formula decomposition mobile ion gem stone embedded

migration

V. Bonding in compounds

Past Paper Questions

94 I 7 b iii 96 I 9 b I 97 I 6 a i ii iii iv

Part 5

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97 I 8 b i

94 I 7 b iii 7b The table below lists some physical properties of lead, bromine and lead(II) bromide. Lead Bromine Lead(II) bromide Melting point 328 ºC -7 ºC 370 ºC Electrical conductivity in the solid state Conducting Non-conducting Non-conducting Electrical conductivity in the liquid state Conducting Non-conducting — iii Will lead(II) bromide conduct electricity in the liquid state ? Explain your answer. Yes, when molten / in liquid state, the Pb2+ and Br - ions can move freely / are mobile ∴ it conducts electricity. 1 mark C Some candidates confused liquid state with aqueous state in their answers.

1

96 I 9 b i 9b A student carried out a copper-plating experiment in the laboratory using the set-up shown below :

i C

Explain why copper(II) sulphate solution can conduct electricity. The solution contains mobile ions Many candidates wrongly explained the electrical conductivity of copper(II) sulphate solution by the presence of 'free ions' instead of by the presence of 'mobile ions'.

1

97 I 6 a i ii iii iv 6a A student used the following experimental set-up to study the migration of ions.

C

The student placed a drop of potassium dichromate solution at A and a drop of a deep blue solution at C. It is known that the two solutions do not react and the deep blue colour of the solution at C is due to the cation present. i Write the formula of the ion responsible for the orange colour of potassium dichromate. 1 Cr2O721 mark ii Why was the filter paper moistened with sodium sulphate solution ? 1 The sodium sulphate solution provides ions for the conduction of electricity / acts as an electrolyte / completes the circuit. 1 mark Parts (i) and (ii) were well answered, but the performance of candidates in parts (iii) and (iv) was poor. iii An electric current was passed through the circuit for some time. 4 (1) What would be the colour change at A ? (2) What would be the colour change at B ? Explain your answer. (1) The orange colour becomes paler / colourless / fades. 1 mark (2) Green / brown / purple colour was observed. 1 mark

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Explanation: Under the influence of the electric field, cations in the deep blue solution are attracted to the negative pole (move to the left) and negative / Cr2O72- ions are attracted to the positive pole (move to the right). 1 mark Or, Under the influence of the electric field, the cations and anions are respectively attracted towards the negative and positive poles. (1 mark)

C

iv

C

1 mark The orange negative ions and the blue positive ions mix / meet at B to give the green colour. Many candidates did not realize that ions migrate under the influence of an electric field. Some candidates simply omitted part (iii). Of those who attempted this part, many wrongly considered the changes to be the result of a redox reaction. Using the same apparatus and materials, suggest how you could show that the colour changes in (iii) are NOT due to diffusion. reverse the polarity of the d.c. supply. 1 mark or, connect the left hand electrode to the positive pole and the right hand electrode to the negative pole. Observation: orange colour will appear at the left of A and blue colour will appear at the right of C. 1 mark In part (iv), many candidates suggested cutting off the d.c. supply. But they did not present their arguments properly in terms of the direction of movement of the colour spots and the time required for such movement to occur.

97 I 8 b i 8b Read the following paragraph concerning chromium and answer the questions that follow: The Greek word "chroma" means colour. Many chromium-containing compounds and chromium containing gemstones are beautifully coloured. The oxidation number of chromium in its compounds can be +2, +3 and +6. i Jade. a green-coloured gemstone, is chromium-containing. Suggest what chromium ion present in jade is responsible for its green colour. chromium(III) / Cr3+ 1 mark C Parts (i) were reasonably well answered. 90 26 C

92 33 D

26 Dry zinc chloride solid is a non-conductor of electricity because A. it is a non-electrolyte. B. it exists as molecules. C. its ions are not mobile. D. metallic bonding is not present.

33 Which of the following ions is/are coloured ? (1) Pb2+(aq) (2) Cr3+(aq) (3) MnO4-(aq) A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only

2

1

V. Bonding in compounds Part 5 98 5 Directions: Questions 4 and 5 refer to the following experiment: The circuit shown below is set up and the solid lead(II) bromide is heated until it becomes molten.

C

99 13 B

5

Which of the following can be deduced from the experimental results ? A. Solid lead(II) bromide contains mobile ions. B. Molten lead(II) bromide contains delocalized electrons. C. Molten lead(lI) bromide can be decomposed by electricity. D. Solid lead(II) bromide is a covalent compound but molten lead(II) bromide is an ionic compound.

13 Which of the following ions is responsible for the yellow colour of topaz ? A. Ni2+ B. Fe3+ C. Cr3+ D. Mn3+

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