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Inorganic Chemistry - Halogen

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A Short Presentation of Inorganic Chemistry - Halogen
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Form 6 Inorganic Chemistry Group 17 : The Halogen Group Group Members: Alvin Lee Wen Xin Chong Kim Yeong Lai Kam Hoe Leong Wai Mun
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Form 6 Inorganic Chemistry

Group 17 : The Halogen Group

Group Members:Alvin Lee Wen XinChong Kim YeongLai Kam HoeLeong Wai Mun

Group 17 : F, Cl, Br, I, AtThe elements in Group 17 of the Periodic Table are called the halogens.

*Astatine not included in our syllabus.

Halogen – from the Greek, means “Produced in the form of sea salt” Example: Sea water has NaCl, MgCl2, MgBr2, NaIO3

Element Period Symbol Proton No. Electronic Configuration

Fluorine 2 F 9 [He]2s22p5 (or) 2, 7

Chlorine 3 Cl 17 [Ne]3s2 3p5 (or) 2,8,7

Bromine 4 Br 35 [Ar]3d104s24p5 (or) 2, 8, 18, 7

Iodine 5 I 53 [Kr] 4d10 5s2 5p5 (or) 2, 8, 18, 18, 7

Astatine 6 At 85 [Xe]4f145d106s2 6p5 (or) 2, 8,18,32,18,7

Cl2Etymology "halogen" – ἅλς (háls), "salt" or "sea", and γεν- (gen-), from γίγνομαι (gígnomai), "come to be"

General Properties Of HalogenElement Standard State mp (K) bp (K) Eo (V)

X2 + 2e– 2X–

F2 yellow gas 40 85 2.85

Cl2 Green gas 172 239 1.36

Br2 Red-brown liquid 266 332 1.06

I2 Violet Solid 387 458 0.62

cf. O2 + 4H+ + 4e– 2H2O Eo = +1.23 V

- All halogens are non-metal except for astatine, which is a radioactive metalloid.- All are too reactive to exist free in nature.- All exists as diatomic molecules: fluorine (F2), chlorine (Cl2), bromine (Br2), iodine (I2).- They usually found as the halide salts, almost all of which are soluble.- They are p-block elements, theirs’ electron configuration are ns2 np5 or 7 electrons in the outermost shell.- All require one electron to complete their octet electron configuration. - All are most electronegative elements compared to other elements.- All have high electron affinity and high ionisation energy.- Their standard electrode potentials, E0 values are large and positive.- They are strong oxidation agents.

Fluorine, F2• It is available in natural waters and also in sea water, soils, plants, bones and teeth.

Physical properties of F2 :• F2 is pale yellow gas with pungent smell• F2 is heavier than air and poisonous in nature.• F2 form yellow liquid and yellow crystals.• F2 is diamagnetic

Chemical properties of F2 :will discuss with other halogens later

Preparation of Fluorine• Its preparation is difficult because(i) It attacks all containers and all materials(ii) Anhydrous HF is a non – conductor(iii) HF is highly stable and can not be easily oxidised to F2, since F2 has the highest S.R.P.(iv) Aq.HF is a good conductor but on electrolysis it gives H2 and O2 because give back HF and O2.

Fluorine, F2

Whytlaw Gray’s method :• Electrolyte : Fused KHF2 (KF + HF in 1 : 12 ratio )• Electrolytic cell : Electrically heated copper vessel.• Cathode: Copper Vessel• Anode : Graphite surrounded by copper diaphragm perforated at the bottom.• Copper diaphragm prevents the mixing of H2 and F2

• Fluorspar stoppers are used. Various parts are coated with Teflon to prevent corrosion.• Products : At anode – F2 At cathode – H2

• Overall Equation: 2KHF2 H2 + F2 + 2KF

At anode: 2F– →F2 + 2e– At cathode: 2H+ + 2e– → H2

• F2 contains HF as impurity. HF is removed using NaF.NaF + HF → NaHF2

• F2 is almost pure with traces of HF

* Fluorine is so reactive that care must be taken to separate the H2 produced at the cathode from the F2 produced at the anode.

fused700 – 1000˚C

Chlorine, Cl2- It is available as chlorides in nature.- In sea water 2.5 % NaCl is present by weight.Minerals :1) Rock salt : NaCl2) Horn silver : AgCl3) Sylvine : KCl4) Carnallite : KCl.MgCl2·6H2OPhysical properties of Cl2 :• It is greenish yellow, pungent smelling gas• It is poisonous and affects mucous membrane• It causes headache and man prove fatal in large quantities• It condenses to yellow liquid and then to yellow solid.• It is about 2.5 times heavier than air.Chemical properties of Cl2 :will discuss with other halogens laterPreparation of Cl2: • It was prepared by scheele laboratory preparation : Oxidation of HCl with MnO2

• 4HCl + MnO2 → MnCl2 + Cl2 + 2H2O

Chlorine, Cl2

Industrial Preparation of Cl2

1. Down’s process : It involves electrolysis of fused NaCl (Electrolyte :fused NaCl)• electrolytic cell : Iron \ steel tank• Anode : Carbon rod• Cathode : Iron• Products at cathode and anode : Na and Cl2

• Addition of little amounts of CaCl2, KCl, KF : To decrease M.P. of NaCl• Possible impurity : Ca, To reduce fuel wastage , To reduce chances of burning Na• Iron wire gauze : prevents mixing of Na and Cl2 : To reduce dissolution of Na in electrolyte.

2. Nelson’s cell method : It involves electrolysis of brine solution (Electrolyte : aqueous NaCl)• Electrolytic cell : Iron tank• Anode : Graphite rod• Cathode : Iron tank• Asbestos lining : Separates anode from cathode• Product at anode : Cl2

• Products at cathode: H2, NaOH• Passage of steam : To keep the solution hot and clear the pores• Possible impurities in NaOH: NaCl, NaOCl,NaClO3

Bromine,Br2- the only non metallic element that is a liquid at normal room temperatures.- found in natural brines in wells- can be isolated from seawater by Cl2 oxidation

Cl2 + 2Br - → 2 Cl - + Br2

Physical properties of Br2 :• It is dark red liquid• It is poisonous• Bleaching agents, corrosive (do not get on the skin)• It has stronger ozone-depletion potential than Cl• It is about 2.5 times heavier than air.

Chemical properties of Br2 :will discuss with other halogens later

Preparation of Br2: • can be obtained by electrolysis from aqueous solution, but chemical means are more

frequently used.• 2Br –(aq) + Cl2(g) → Br2 (g/l) + 2Cl –(aq)

Bromine, Br2

Iodine,I2- found in seawater, concentrated by kelp, algae- can be isolated by Br2 or Cl2 oxidation, but in practice is done with Cl2

l2 + 2Br - → 2 l - + Br2

Physical properties of I2 :• It is bluish-black volatile solid• It is sublimes readily, release violet vapour• It has some metallic properties• It has radioactive isotope 131I , exposure to I-131 causes thyroid cancer

Chemical properties of Br2 :will discuss with other halogens later

Preparation of Br2: • can be obtained by electrolysis from aqueous solution, but chemical means are more

frequently used.• 2I –(aq) + Cl2(g) → I2 (g/l) + 2Cl –(aq)

Iodine, I2

THE COLOUR INTENSITY OF GROUP 17 ELEMENTS★ They are all coloured non-metallic elements and the colour gets darker down the group.

★ At room temperature,

★ Colour of halogens in gaseous state, in aqueous solution and in non-polar solvents such as

tetrachloromethane, CCl₄ are shown below:

Halogen Gaseous state Water CCl₄Chlorine, Cl Greenish-yellow Pale yellow-

greenPale yellow-

greenBromine, Br Reddish-brown Reddish-brown Reddish-brown

Iodine, I Purple Brownish-orange Violet

Halogen ColourChlorine, Cl Greenish-yellow gasBromine, Br Dark red liquid

Iodine, I Purple black solid

THE VOLATILITY OF GROUP 17 ELEMENTS

★ The higher the melting and boiling point, the stronger the intermolecular forces of attraction.

★ The van der Waals forces of attraction between the halogen molecules increase as the molecular size of the molecules increases.

★ Volatility is a measure the tendency of a substance to vaporize.★ The forces of attraction increases going down the group causing the particles more difficult

to escape from the substances.★ Volatility decreases.

The melting point and boiling point of chlorine, bromine and iodine increase on going down the group.

Thus, the volatility of halogens decreases on going down the group.

Melting point, boiling point and enthalpy of halogensElement Cl2 Br2 I2

Melting point/ ˚C -101.0 -7.2 114.0

Boiling point/ ˚C -34.7 58.8 184.0

∆H vaporisation/(kj/mol) +10.2 +15.0 +30.0

Physical state (298 K) Pale yellow gas

Reddish brown liquid

Black solid

•The halogens have a simple molecular structure consisting of a X2 molecule with strong covalent bond holding the molecules together.•The intermolecular of halogens are held together by Van der Waals forces.•Going down the group the boiling point, melting point, and heat of vaporisation increases due to the size of the halogens.•In iodine, the Van der Waals forces is strong enough to sustain iodine in the solid state.•Hence, the metallic properties increase down group 17.

THE RELATIVE REACTIVITY OF THE ELEMENTS AS OXIDISING AGENT1) The halogen molecule is the electron acceptor (the oxidizing agent) and is reduced by

electron gain to form a halide ion.X2 (aq) + 2e- → 2X- (aq)

2) Oxidizing power of halogen can be explaint:★ In term of atomic radii

The atomic radius of halogen increases in the order:

F₂ < Cl₂ < Br₂ < I₂

Fluorine has the smallest size, therefore it is the strongest oxidizing agent and has the greatest ability to gain an electron.

Chlorine has the smaller size than bromine and iodine, therefore it has greater ability to gain electron and it a stronger oxidizing agent compare to bromine and iodine.

★ In term of standard electrode potential, Eo

Cl2 (aq) + 2e- → 2Cl- (aq) ; Eo = +1.36V

Br2 (aq) + 2e- → 2Br- (aq) ; Eo = +1.07V

I2 (aq) + 2e- → 2I- (aq) ; Eo = +0.54V

The more positive the Eo value, the stronger the element acts as an oxidising agent.

3) The reactivity of halogens as oxidising agent are as follow:

★ Chlorine oxidises bromide ions to bromine and iodide ions to iodine.

The iodide ions have lost electrons to form iodine molecules. They have been oxidised.

The chlorine molecules have gained electrons to form chloride ions. They have been reduced.

This is obviously a redox reaction in which chlorine is acting as an oxidising agent.

★ Bromine oxidizes the iodide ions to iodine.

Bromine is a weaker oxidising agent than chlorine, hence is not strong enough to convert chloride ions into chlorine.The iodide ions have lost electrons to form iodine molecules. They have been oxidised.The bromine molecules have gained electrons to form bromide ions. They have been reduced.Bromine is the oxidising agent.

★ Iodine is the weakest oxidising agent compared to chlorine and bromine. Thus, it cannot oxidise chlorine ions or bromide ions.

Iodine does not oxidize iron (II) ions.

2Fe2+ (aq) + I2 (aq) → 2Fe3+ (aq) + 2I- (aq) ; Eo = -0.23V

The negative Eo value shows that the reaction is not feasible. In contrast, both chlorine and bromine can oxidise iron (II) ions (Fe2+) to iron (III) ions (Fe3+).

2Fe2+ (aq) + CI2 (aq) → 2Fe3+ (aq) + 2CI- (aq) ; Eo = +0.59V2Fe2+ (aq) + Br2 (aq) → 2Fe3+ (aq) + 2Br- (aq) ; Eo = +0.30V

Hence, the reactivity decreases going down the group.The reactivity of halogen decreases in the order :

F₂ > Cl₂ > Br₂ > I₂

Reactivity decreases

Reaction Of Chlorine, Bromine and Iodine with Hydrogen

Halogens reacts directly with hydrogen to form hydrogen halides under suitable condition. Hydrogen halides are also known as the hydrides of halogens.

Where X = F, Cl, Br, and I.

On descending the group, the order of reactivity between the halogens and hydrogen decreases.This is shown by the fact that the reactions become less vigorous and the reaction rates become slower.

H₂ (g) + X₂ (g) → 2HX (g)

H₂ (g) + F₂ (g) → 2HF (g)

A mixture of chlorine and hydrogen will explode of exposed to sunlight or ultraviolet at room temperature. Anyhow, a jet of hydrogen can burn safely in chlorine to form hydrogen chloride gas.

This method is used in the industry for the large scale production of hydrogen chloride.

Only at a high temperature of 200 ˚C, bromine reacts with hydrogen and in the presence of platinum catalyst to form hydrogen bromide.

However, iodine and hydrogen react slowly to form hydrogen iodide at 400 ˚C and in presence of platinum catalyst.

H₂ (g) + Cl₂ (g) → 2HCl (g)

H₂ (g) + Br₂ (g) 2HBr (g)Pt

200 ˚C

H₂ (g) + I₂ (g) 2HI (g)Pt

400 ˚C

Thermal Stability of Hydrogen Halides1. When heated, the H-X bond breaks and the hydrogen halides decompose to halogens and

hydrogen.

2. Hydrogen fluoride and hydrogen chloride are very stable toward heating. At 2000 ˚C, hydrogen chloride dissociates only slightly to form hydrogen and chlorine.

3. Hydrogen bromide decomposes at 600 ˚to produce hydrogen and brown vapour.

2H – X (g) H₂ (g) + X₂ (g)

2HCl (g) H₂ (g) + Cl₂ (g)2000 ˚C

2HBr (g) H₂ (g) + Br₂ (g)600˚C

4. Hydrogen iodide decomposes to produce hydrogen and violet fumes of iodine at low temperature (200 ˚C)

Conclusion:

The results show that the thermal stability of hydrogen halides decreases with increasing relative molecular mass.

The relative stability of hydrogen halides can be explained in terms of bond length and strength. On decesending Group 17, the atomic size of halogens increases, and hence the H-X bond length increases.

The decomposition of hydrogen halides involves breaking the H-X bonds. The longer the bond length, the weaker the bond and the more easily the bond can be broken.

2HI (g) H₂ (g) + I₂ (g)200˚C

HF > HCl > HBr > HI

H-F < H-Cl < H-Br < H-Ibond length increases

Reaction Of Halide IonsThe Reactions of the Halides Ions with Aqueous silver Ions

1. Chloride ions react with aqueous silver nitrate to produce a white precipitate of silver chloride.

The white precipitate of silver chloride dissolves readily in dilute ammonia solution to form a colourless solution of silver complex.

2. Bromide ions react with aqueous silver nitrate to form a cream precipitate of silver bromide.

The cream coloured silver bromide is insoluble in dilute ammonia solution but dissolves in concentrated ammonia solution

Ag⁺ (aq) + Cl⁻ (aq) → AgCl (s)

AgCl (s) + 2NH₃ (aq) → [Ag(NH₃)₂]⁺ (aq) + Cl⁻ (aq)

Ag⁺ (aq) + Br⁻ (aq) → AgBr (s)

AgBr (s) + 2NH₃ (aq) → [Ag(NH₃)₂]⁺ (aq) + Br⁻ (aq)

3. Iodide ions react with aqueous silver nitrate to form a yellow precipitate of silver iodide.

The Yellow precipitate of silver iodide is insoluble in dilute and concentrated ammonia solution.

4. The action of silver nitrate solution on halide ions followed by ammonia solution can be used as a test to confirm the presence of halides ions.

The Reaction of Halide Ions with Concentrated Sulphuric Acid

5. When concentrated sulphuric acid is added to solid halides and the mixture heated strongly, white fumes of hydrogen halides are produced initially.

Concentrated sulphuric acid also acts as an oxidising agent. The final products will depend on whether HX produced will be oxidised by concentrated sulphuric acid

Concentrated sulphuric acid is not powerful enough to oxidise hydrogen chloride to chlorine, but it can oxidise HBr (colourless) to Br₂ (reddish-brown vapour) and HI (colourless) to I₂ (violet vapour)

Ag⁺ (aq) + I⁻ (aq) → AgI (s)

X⁻ (s) + H₂SO₄ (l) → HX (g) + HSO₄⁻ (aq)

2. When solid sodium chloride is heated with concentrated sulphuric acid, hydrogen chloride gas is produced.

3. The reaction between solid sodium bromide and concentrated sulphuric acid produces not only hydrogen bromide, but also other gases such as bromine and sulphuric dioxide.

NaBr (s) + H₂SO₄ (l) → NaHSO₄ (s) HBr (g)

… initial reaction

2HBr (g) + H₂SO₄ (l) → Br₂ (g) + 2H₂O (l) + SO₂ (g)

… oxidation

Overall reaction:

In this reaction, Br⁻ ions are oxidised to Br₂ and H₂SO₄ is reduced to SO₂.

2Br⁻ (s) + 3H₂SO₂ (l) → 2H₂SO₄⁻ (s) + Br₂ (g) + SO₂ (g) + 2H₂O (l)

NaCl (s) + H₂SO₄ (l) → NaHSO₄ (g) + HCl (aq)

4. When solid sodium iodide is heated with concentrated sulphuric acid, more complicated reactions take place to produce a mixture of gases, which include HI, I₂, SO₂, H₂S (hydrogen sulphide).

NaI (s) + H₂SO₄ (l) → NaHSO₄ (s) HI (g)

… initial reaction

2HI (g) + H₂SO₄ (l) → I₂ (g) + 2H₂O (l) + SO₂ (g)

8HI (g) + H₂SO₄ (l) → 4I₂ (g) + 4H₂O (l) + H₂S (g)

… oxidation

Overall reaction:

In this reaction, I⁻ ions are oxidised to I₂ and H₂SO₄ is reduced to SO₂ and H₂S.

2I⁻ (s) + 3H₂SO₂ (l) → 2HSO₄⁻ (s) + I₂ (g) + SO₂ (g) + 2H₂O (l)8I⁻ (s) + 9H₂SO₂ (l) → 8HSO₄⁻ (s) + 4I₂ (g) + H₂S (g) + 4H₂O (l)

or

5. The reaction of concentrated sulphuric acid with solid ionic halides show that the ease of oxidation of halide ions increases in order:

6. As the ionic radius of the halide ion increases from Cl⁻ to I⁻, it becomes easier to remove an electron from the halides ion. Thus, the reducing power of the halide ion increases in the order:

Cl⁻ < Br⁻ < I⁻

Cl⁻ < Br⁻ < I⁻

1Ā = 0.1nm / 1X10-10m

Reaction of Chlorine with Cold Dilute Sodium Hydroxide

1. When chlorine gas is passed into cold dilute sodium hydroxide solution, the products obtained are sodium chloride, sodium chlorate (I) and water.

2. This is a disproportionation reaction in which chloride is simultaneously oxidised to chlorate (I) ion (ClO -) and reduced to chloride ion (Cl-).

Cl2 (g) + 6NaOH (aq) NaCl (aq) + NaClO (aq) + H2O (l)15˚C

Cl2 (g) + 2OH- (aq) Cl- (aq) + ClO- (aq) + H2O (l)

oxidised

0 +1

-1reduced

sodium chlorate (I)

Reaction of Chlorine with Hot Concentrated Sodium Hydroxide

1. When chlorine gas is passed into hot, concentrated sodium hydroxide solution, the products obtained are sodium chloride, sodium chlorate (V) and water.

2. Sodium chlorate (V) can also be obtained by heating sodium chlorate (I) solution.

3Cl2 (g) + 2NaOH (aq) 5NaCl (aq) + NaClO3 (aq) + 3H2O (l)70˚C

3NaClO (g) 2NaCl (aq) + NaClO3 (aq)reduced

+1 -1

+5oxidised

sodium chlorate (V)

…reaction (1)

0 -1 +5

…reaction (2)

3. Reactions (1) and (2) are also disproportionation reactions. In reaction (1), Cl2 is oxidised to NaClO3 and is simultaneously reduced to NaCl and in reaction (2), NaClO is oxidised to NaClO3 and simultantaneously reduced to NaCl.

Important Uses of Halogens and Halogen CompoundsAntiseptics

1. Many antiseptics are chlorine compounds. For example, TCP (trichlorophenol) and Dettol are used as antiseptics and disinfectants.

TCP (trichlorophenol)Dettol (4-chloro-3,5-dimethylphenol)

2. A tincture of iodine is used as antiseptics for wounds. A tincture is medicine dissolved in alcohol.

Iodine Tincture

Bleaching Agents

1. Bleaching agents are oxidising agents. They oxidise coloured chemical compounds to form colourleess compounds.

2. Chlorine is used as bleaching agent in the paper and textile industry. Solutions of sodium chlorate(I) and bleaching powder (calcium chlorate(I), Ca(OCl)2) are used as household bleach.

calcium chlorate(I)sodium chlorate(I)

One of the examples of bleaching agents

Purifying Water

1. Chlorine kills harmful bacteria. It is used to sterilise drinking water and water in swimming water.

One of the examplesChlorine tablets

Insecticide

1. Halogen compounds such as DDT (dichlorodiphenyltrichloroethane) and sodium floride (NF) are used as insecticide.

DDT (dichlorodiphenyltrichloroethane)

Examples of insecticides

Black-and-White Photography

1. Photographic film consists of tiny crystals of silver bromide, AgBr, suspended in a thin layer of gelatin coated on the surface of the photographic paper.

2. On exposure to light, the silver bromide crystals start to deposit silver.These black silver crystals form the latent image which is not yet visible to the eye.

This is a redox reaction in which Ag+ is reduced to Ag and Br- oxidised to Br2.Ag+ + e- → Ag … reduction2Br - → Br2 + 2e- … oxidation

Since the reaction is catalysed by light, it is also known as photochemical reaction.

3. A developing process enhances the image by reducing more silver bromide crystals to metallic silver using a developer, called hydroquinone, C6H4(OH)2. Hydroquinone is a mild reducing agent. Silver bromide that are not exposed to light are not reduced by the developer.

AgBr + e- → Ag + Br - … reductionC6H4(OH)2

→ C6H4O2 + 2H+ + 2e- … oxidation

2AgBr 2Ag + Br2light

hydroquinone quinone

hydroquinone ( benzene - 1,4 - diol)

4. In this way, a negative is produced. The remaining silver bromide is removed by using sodium thiosulphate solution. Ag+ ions react with thiosulphate ions, S2O3

2-, to form the soluble complex ion, [Ag(S2O3)2]3- (aq). This process is called the fixing process.

5. A final photo print is produced by passing light through the negative onto photographic paper.

AgBr (s) + 2S2O32- (aq) [Ag(S2O3)2]3- (aq) + Br- (aq)

Figure: A Simplified Schematic Representation of the Silver Halide Process

STPM 2005 Pass Year Question 6 (P2)A solid halide reacts with concentrated sulphuric acid to form HX gas which is then oxidised to X2 gas. The solid plays an important role in black-and-white photography. Identify the solid halide. Explain your answer.………………………………………………………………………………………………………………………………………………………

The solid halide is silver bromide, AgBr, which is used in black-and-white photography. HX is therefore HBr and X2 is Br2.

Concentrated sulphuric acid acts as a strong acid and displaces the weaker acid, HBr, from its salt, silver bromide.

2AgBr + H2SO4 → Ag2SO4 + 2HBr

Concentrated sulphuric acid also acts as an oxidising agent and oxidises hydrogen bromide to bromine. 2HBr + H2SO4 → Br2 + SO2 + 2H2O

In black-and-white photography, the photographic film is coated with a thin layer of silver bromide. When exposed to light, the bromide ions are oxidised to bromine gas and the silver ions are reduced to form a layer of silver on the surface of the photographic film.

2AgBr → 2Ag + Br2


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