Chapter - 3 Noble Gases

CHAPTER - 3: GROUP - 18 NOBLE GASES

CHAPTER – 3 GROUP – 18 NOBLE GASES: MEMORY CARD  GROUP – 18 Elements: Helium, Neon, Argon, Krypton, Xenon and Radon.  Noble gases or rare gases: These are so called because they are available in very trace quantity.  Inert Gases: These are so called because they are chemically inert due to their stable octate state electronic configuration.  Aerogens: These are so called because they are available in atmospheric air except Radon.  Radon is obtained by the radioactive disintegration of Radium – 226 226 222 + 2He4 88Ra 86Rn  Discovery of Noble Gases DISCOVERED BY

% BY VOLUME IN AIR

ELEMENT

YEAR

FOUND IN

Helium

1868

Lockyer

Spectrum during total solar eclipse

0.00052

Neon

1898

Ramsay & Travers

Fractional distillation of liquid Argon

0.0015

Argon

1894

Ramsay & Rayleighs

Air by removing O2 and N2

Krypton

1898

Ramsay & Travers

Fractional distillation of liquid Argon

0.00011

Xenon

1898

Ramsay & Travers

Fractional distillation of liquid Argon

0.0000087

Radon

1900

Fredrich Dorn

Radioactive disintegration of Radium

-

0.93

 Rare Gases are obtained from atmospheric air in two stages o Step – 1: Isolation of Rare Gases from Air by Ramsay & Rayleighs Method  In step – 1, an electric discharge is passed into a mixture of air and oxygen in the ratio 9:11 by volume. NO and NO2 are formed during electric discharge. These gases are absorbed by NaOH. The residual oxygen gas is absorbed by alkaline pyrogallol solution. A mixture of noble gas is left behind. o Step – 2: Separation of Noble Gases from the mixture by Dewar’s Charcoal Adsorption Method  Dewar’s Charcoal Adsorption Method is based on the selective adsorption or differential adsorption of noble gases on activated charcoal. The adsorption capacity depends upon atomic mass of the noble gas and the temperature of adsorption. The amount of adsorption increases with the increase in atomic mass and decrease in temperature. MIXTURE NOBLE GASES He, Ne, Ar, Kr and XE Charcoal -1 at – 100oC He and Ne Unadsorbed brought in Contact with Charcoal – 3 at – 180oC

He Unadsorbed

Ne Adsorbed in Charcoal -3 Warm Ne Evolved is collected

Ar, Kr and Xe Adsorbed in Charcoal -1 connected to Charcoal -2 at liquid air temperature – 196oC

Ar diffuses into Ch -2 Warm Ar Evolved

Kr & Xe remain adsorbed in Ch -1 warmed to – 90oC

Kr is Evolved

Xe remains in Ch -1 is warmed to to get Xe

 In 1962, Neil Bartlett prepared the first noble gas compound of Xenon, viz Xe[PtF6] xenon hexafluoroplatinate(V).  He obtained a complex compound O2+PtF6 by oxidising oxygen gas to O2+ with PtF6. So he thought that PtF6 should also oxidise Xe to Xe+. Since first ionisation energy of Xenon (1170 kJm ol–1) is quite close to that of O2 (1175 kJm ol–1)  Neil Bartlett treated deep red vapours of PtF6 with equal volume of xenon gas at room temperature and got a yellow crystalline solid of xenon hexafluoroplatinate(V). Xe (g) + PtF6 (g) Xe+[PtF6]– xenon hexafluoroplatinate(V)  Several compounds of noble gases have been prepared after this preparation with oxygen and fluorine.  Eg: XeF2, XeF4, XeF6, XeO3, XeOF4, XeO2F2, KrF2, KrO2 etc. However the compounds of Helium, Neon and Argon are not known so far because of their small size & hence very high ionisation energy.

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CHAPTER - 3: GROUP - 18 NOBLE GASES

ONE MARK EACH 1. Rare gases are chemically inert. Why? OR Inert gases show no tendency to lose or gain electrons. Why? A.

Because they have stable octate state electronic configuration in their valence shells.

2. Why noble gases are called rare gases? A.

Because they are available only in minute quantity in atmospheric air (except radon)

3. What is the source of noble gases? A.

Atmospheric air. (Except radon which is obtained by radioactive disintegration of Radium)

4. How is radon produced? A.

226 88Ra

By radioactive disintegration of Radium:

5. Which noble gas is most abundant in atmosphere? A.

Argon, approx. 1% by volume in air

6. Which noble gas is not a constituent of air? A.

Radon

7. Name the compound used to absorb oxygen from rare gas mixture. A.

Alkaline pyrogallol

8. Which noble gas is not adsorbed by charcoal? A.

Helium

9. Helium is used in airships. Give reason. A.

Because of its light weight, inertness and non-flammable nature.

10. Neon is used in beacon lights. Why? A.

It passes through fog and mist.

11. Argon is used for filling electric filament bulbs. Why? A.

Because of its inert nature and low thermal conductivity.

12. He and Ne cannot form compounds. Why? A.

Because of their small size they have very high I. E

13. Helium is used as a coolant in gas cooled nuclear reactors. Give reason. A.

Due to its high thermal conductivity & inert nature

14. Neon is used in signboards. Why? A.

Because orange red glow is given by Neon.

15. Which noble gas is most reactive? A.

Xenon

16. Which is the first noble gas compound prepared? A.

Xe[PtF6], xenon hexafluoroplatinate(V)

17. Who prepared the first noble gas compound? A.

Neil Bartlett.

18. Helium is not adsorbed in Dewar’s Charcoal Adsorption method. Why? A.

Because it is lighter than air.

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222 86Rn

+ 2He4

CHAPTER - 3: GROUP - 18 NOBLE GASES

TWO MARKS EACH 1. What is the principle used in the Dewar’s adsorption method? A. Dewar’s Charcoal Adsorption Method is based on the selective adsorption or differential adsorption of noble gases on activated charcoal. The adsorption capacity depends upon atomic mass of the noble gas and the temperature of adsorption. The amount of adsorption increases with the increase in atomic mass and decrease in temperature. 2. Write any two uses of Helium. A. Helium is used 1. in filling air ships and weather observation balloons. 2. a mixture of helium & oxygen is used by deep sea divers for respiration. 3. Write any two uses of Neon. A. Neon is used 1. in neon signboards 2. in beacon lights by pilots as it penetrates through fog & mist. 4. Write any two uses of Argon. A. Argon is used 1. for filling incandescent lamps. 2. to provide inert atmosphere for welding and in many metallurgical processes. 5. Write any two uses of Krypton. A. Krypton is used 1. in filling discharge tubes & electric bulbs 2. in miner’s cap lamps 6. Write any two uses of Xenon. A. Xenon is used 1. liquid xenon is used in research labs for detecting mesons & gamma photons 2. a mixture xenon & krypton is used in examining respiratory organs 7. Write any two uses of Radon. A. Radon is used 1. in radiotherapy treatment of cancer 2. in X-ray photography for detecting the irregularities in solids. 8. A mixture of helium & oxygen is used by deep sea divers instead of air for respiration. Give reason. A. If air is used for respiration, nitrogen of air dissolves in blood under high pressure of water. When the man comes up, hydrostatic pressures decreases & nitrogen is suddenly released from the blood causing intolerable pain. This is called bends. But a mixture of helium & oxygen is insoluble in blood & prevents from bends. 9. Xenon forms compounds with oxygen & fluorine. Why? A. Xe is large atom & can donate electron pairs. Fluorine & oxygen are highly electronegative atoms; hence most of the compounds of rare gases are those of xenon with oxygen & fluorine. 10. Explain the separation of He & Ne from their mixture. A. The mixture of unadsorbed Helium & Neon is brought into contact with a charcoal at – 180oC. Neon gets adsorbed and Helium remains unadsorbed and it is pumped out. The charcoal is warmed to collect the Neon. 11. Explain the preparation of first noble gas compound by Neil Bartlett. A. In 1962, Neil Bartlett prepared the first noble gas compound of Xenon. He obtained a complex compound O2+PtF6 by oxidising oxygen gas to O2+ with PtF6. So he thought that PtF6 should also oxidise Xe to Xe+. Since first ionisation energy of Xenon (1170 kJm ol–1) is quite close to that of O2 (1175 kJm ol–1) He treated deep red vapours of PtF6 with equal volume of xenon gas at room temperature and got a yellow crystalline solid of xenon hexafluoroplatinate(V). Xe (g) + PtF6 (g) Xe+[PtF6]– xenon hexafluoroplatinate(V)

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CHAPTER - 3: GROUP - 18 NOBLE GASES

THREE MARKS EACH 1. Explain the isolation of noble gases from air by Ramsay & Rayleigh’s method. PRINCIPLE: An electric discharge is passed into a mixture of air and oxygen in the ratio 9:11 by volume. NO and NO2 are formed during electric discharge. These gases are absorbed by NaOH. The residual oxygen gas is absorbed by alkaline pyrogallol solution. A mixture of noble gas is left behind. Pt Electrode

Air + O2 (9:11)

NaOH

NaOH

PROCESS: Dry and CO2 free air is mixed with oxygen in the ratio 9: 11 by volume and passed continuously into a glass flask of 50 dm3 capacity. An electric discharge of 6000 – 8000 volts from an induction coil is passed between two platinum electrodes. Sodium hydroxide is passed into the flask through a side tube. Nitrogen combines with oxygen to form nitric oxide which is oxidised to nitrogen dioxide.

6000 V – 8000 V INDUCTION COIL

N2

+ O2

2NO2 + O2

2NO 2NO2

The NO2 gas dissolves in sodium hydroxide solution forming a mixture sodium nitrite and sodium nitrate. 2NO2

+

2NaOH

NaNO2 + NaNO3 + H2O

The supply of air and electric discharge is stopped after some time. An alkaline pyrogallol solution is passed to absorb the oxygen gas completely. A mixture of rare gases is left behind. 2. How are noble gases separated by Dewar’s charcoal adsorption method? PRINCIPLE: Dewar’s Charcoal Adsorption Method is based Noble Gases To Vacuum pump on the selective adsorption or differential adsorption of noble gases on activated charcoal. The adsorption capacity depends upon atomic mass of the noble gas Dewar’s Flask and the temperature of adsorption. The amount of Dewar’s adsorption increases with the increase in atomic mass Bulb and decrease in temperature. Coconut PROCESS: A Dewar’s bulb is filled with coconut charcoal - 1 Charcoal and cooled in liquid air to – 100oC in a Dewar’s flask. The mixture of noble gases is passed into bulb and Cold Bath allowed to remain for about an hour. Heavier argon, krypton and xenon are adsorbed in charcoal - 1. The unadsorbed lighter helium and neon gas mixture is pumped out. The charcoal - 1 containing Ar, Kr and Xe is brought into contact with another charcoal - 2 cooled to liquid air temperature of –196oC. Argon diffuses into charcoal - 2 and evolved on heating this charcoal. The temperature of charcoal - 1 is raised to – 90oC, at this temperature, Krypton gets desorbed & it is collected. The charcoal - 1 containing Xenon is further warmed to 0oC to get Xenon. The mixture of unadsorbed Helium & Neon is brought into contact with charcoal - 3 at – 180oC. Neon gets adsorbed and Helium remains unadsorbed and it is pumped out. The charcoal - 3 is warmed to collect the Neon.

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