12th Class Chemistry Chapter 5 Halogens and Noble Gases Short Question Answers

The term halogen is derived from the Greek halos which means salts and genes mean born.The most of the salts are the derivatives of halogens.

Astatine was discovered in 1940 much after other halogens.It is an unstable element and it is radioactive in nature.Its half life period is 8.3 hours.

The important minerals of fluorine and iodine are as follows.
Fluorine:      Fluorsper      CaF₂
Cryolite         Na₃AIF₆
Apatite          [CaF₂.3Ca₃(PO₄)₂]
Iodine           NaIO₃ and NalO₄ are the deposits in Chile brine wells.

When sodium iodide is added in a definite ratio with NaCI then it is called iodized salt.Iodine is good for health and this iodized salt is suitable in those areas where there is a deficiency of iodine in water.

F₂ and CI₂ arre gases Br₂ is a liquid while I₂ is a solid.Fluorine is the yellow gas chlorine is greenish yellow gas bromine is a reddish brown liquid while iodine is a deep violet solid.

The size of the halogen atoms go on increasing down the group.Their bond lengths go on increasing.Longer bonds are weaker so their dissociation energies go on decreasing down the group.

Halognes have tendencies to accept one electron and so they oxidize other species.Anyhow their oxidizing capabilities decrease down the group.Fluorine is the strongest oxidizing agent while iodine is the weakest.

There are many aspects the size of F^– and Fᵒ are very small.Fluorine has high intonation potential and electronegativity as compared to other halogens.Moreover its bond dissociation energy is very small as compared to CI₂.It has no available d.orbital to expand its oxidation state.

HF undergoes extensive hydrogen bonding and forms the zigzag structure.Due to this reason it is the weakest acid among the halogen acids.

The size of the halogens atoms and halogen ions increase down the group from F^-1 to I^-1.The reason is the increasing number of shells and increasing shielding effect.

These are the electrons affinity values which affect the oxidizing power halogens.The electrons affinities decrease down the group so their oxidizing powers decrease.But fluorine show an exceptional behavior.

The melting and boiling points of halogens increase down the group.Iodine is solid at room temperature.This is due to the presence of dispersion forces among the molecules of I₂.

They can oxidize dyes to colorless substances.Litmus and universal indicators can be decolorized.Chlorine is used in the bleaching powder which bleaches the substances due to oxidizing capabilities of chlorine.

Iodine is used in the form of tincture of iodine.This medicine is prepared by dissolving iodine in ethyl alcohol.It is antiseptic and kills the germs.

Chlorine is used in water treatment.Chlorine is an oxidizing agent and releases atomic oxygen by reacting with water.That atomic oxygen is very reactive and kills the bacteria present in water.

The reaction of H₂ and F₂ is very fast.HCI and HBr can be prepared form H₂ and halogens at high temperature.The reaction of H₂ and I₂ is very slow and is reversible.

Halogen acids break the H-X bonds in water.All the HX molecules are polar.Water is also polar.H⁺ ions after dissociation are converted into hydronium ions.X^– ions are solvated by water  molecules.In this way the products are stabilized as compared to the reactants.

HF…………………….H⁺+F^–
HCI…………………..H⁺+CI^–
HBr…………………..H⁺+Br^–
HI……………………..H⁺+I^–
Anyhow the tendency to be ionized increases from upper to the downward direction.

Bond dissociation energy of halogen acids go on decreasing from upper to the downward direction.This is due to increasing bond lengths and decreasing differences of electronegativities between hydrogen and halogen.

The heat of fusion and heat of vaporization do not show the regular trend from HF to HI but the gear of formation and bond dissociation energy change in a systematic way.

The reducing properties of halogen acids increase from HF to HI.HI is the best reducing agent.It can reduce S to H₂S.

HF has sufficient hydrogen bonding.H⁺ ions are entrapped between two strongly electrcnegative atoms of fluorine.So it is difficult for H⁺ ions to be dispersed in water and to make HF to act as a good acid.The acid strength goes on increasing from HF to HI.HI has least bond dissociation energy.

Important oxides of chlorine and bromine are as follows:
Chlorine:     Dichlorine monoxide CI₂O
Chlorine dioxide CIO₂
Chlorine hexaoxide CI₂O₆
Chlorine heptaoxide CI₂O₇
Bromine:         Bromine monoxide Br₂O
Bromine dioxide BrO₂
Bromine  trioxide Br₂O₃

When  CI₂O₇  is reacted with water.It gives HCIO₄ which shows that CI₂O₇  is the anhydride of HCIC₄.

CI₂ reacts with NaOH is cold state to give NaCI and NaCIO.The oxidation number of chlorine changes from zero to -1 and zero to +1.This is self oxidation-reduction reaction.
2NaOH + CIᵒ₂………………. NaCI^-1+ NaCIO⁺+H₂O

When chlorine reacts with NaOH in the hot state.NaCI and NaCIO₃ are produced.It shows that chlorine atoms change their oxidation number from zero to -1 and +5.This is a self oxidation reduction reaction.

The general formulas for oxyacids of halogens are:
HXO,HXO₂,HXO₃ and HXO₄.
Fluorine does not have such compounds.Moreover bromine  and iodine also do not give HbrO₂ and HIO₂.

Oxyacids of fluorine do not exist.The oxyacids of other halogens are mostly unstable compounds and can not be isolated in pure form.They are stable in aqueous solution or in the form of their salts.

Greater the number of oxygen atoms in the oxyacids greater the polarity of the OH bonds and greater the dissociating powers in the solution of that acid.

Since the electronegativity of chlorine is greater than bromine which is greater than iodine so acid strengths decrease from HCIO₄ to HIO₄.

HCIO₄ is a powerful oxidizing agent when it is hot.When it is heated it decomposes with explosion.Pure anhydrous HCIO₄ can be prepared by distilling the mixture of KCIO₄ and concentrated H₂SO₄ under reduced pressure.
KCIO₄ + H₂SO₄……………………. KHSO₄+HCIO₄
conc.                                             Perchloric acid.
It is prepared at a pressure of 10-20 torr.But it is distilled at 40-60ᵒC under a pressure of 60 torr.

It is due to the presence  of OCI^– ion.This ion is generated when bleaching powder  is dissolved in water.OCI^– gives oxygen which oxidizes which oxidizes and bleaches the other substances.
CaCI(OCI)……..H₂O………Ca⁺² +CI^–+OCI^–

Amount of chlorine which is set free during the reaction of bleaching  powder with acid is called available chlorine.It is calculated  by reacting bleaching powder with H₂SO₄.The theoretical percentage of available chlorine is 49 but mostly its varies from 35 to 40%.

When bleaching powder is reacted with HCI HBr,and HI then CI₂ Br₂ and I₂ are produced.In other words halogen acids are oxidized.The oxidation number of halogens increase from-1to zero.

Bleaching powder can bleach cotton linen pulp and wood pulp.The colored matter is oxidized and becomes colorless.

Chlorofluorocarbons obtained from CCI₄ are called freons.It is a mixture of CCI₂F₂ and CCIF₃.They are used as solvents lubricants insulators and in refrigeration.

When C₂F₄ is polymerized.It give called telfon.It is used in the construction of chemical planets.It is used as insulating in cables.

Chlorine acts as a bleaching agent in the manufacture of antiseptics insecticides weed killers and herbicides.It is also used for the preparation of HCI,PVC.CHCI₃and CCI₄.

Bromine is used for the preparation of C₂H₄Br₂ which acts as germicide fungicide and as an oxidizing agent.AgBr is used in photography.Iodine is used in pharmaceutical industries and in the prepatation of tincher of iodine and as an analgesic.I₂ is used in the preparation of iodoform and dyes.

The elements of group VIII-A have complete outermost orbitals.So they do not show tendency to react with others.That is why they are chemically inert and are though to be noble in reactivity.

Elements of group VIII i.e.He Ne,Ar,Kr,Xe, and Rn are noble gases.They have fulfilled outer shell and are least reactive.Their least reactivity makes then inert.

The oxidation states of element develop due to incomplete shell.Noble gases have complete shells,so have zero oxidation states.

Radon is present is very small amounts in air.It is radioactive and it is the decay product of Ra²²⁶.

They are least soluble in water.Xe is slightly soluble in water that is 110 cm³ in 1 dm³ at 20ᵒC.Anyhow solubility increases down the group.

Helium is used in the treatment of asthama filling of weather balloons producing low temperature preservation of food creating inert atmosphere and in welding.It is used in signal lights and atomic reactors.Argon is used in filling electrical bulbs in Geiger counter producing inert atmosphere filling fluorscent tubes and mixing with neon in neon signs.

Radon is used in the radiotherapy of cancer earthquake predictions preparation of ointments for the treatment of cancer and photographing the interior of opaque materials.

When noble gases are maintained at low pressure in the discharge tube and electric current is passed through them spectra of different colors are produced.These spectra are utilized in advertising lamps.