CHEM 1004 | Interfaces William Ramsay and the Discovery of Noble Gases

Scientific Method

Problem: different N2 densities

Hypothesis: a heavy impurity

Testing: Ramsay and Rayleigh’s experiments

Results: Argon isolated

Hypothesis accepted

Problem

Rayleigh pointed out that nitrogen isolated from the air had a density slightly higher than that of nitrogen prepared from chemical sources. A litre of pure nitrogen gas generated from a chemical reaction weighed 1.2505 g.

On the other hand, a litre of nitrogen gas generated from air by removing oxygen, carbon dioxide, and water vapour weighed 1.2572 g (at the same temperature and pressure). Rayleigh thought that this might be due to the presence of a light impurity in the former 

But Ramsay thought that it might be due to the presence of a heavy impurity in the 'atmospheric' nitrogen. 

In large-scale experiments he passed atmospheric nitrogen repeatedly over hot magnesium, which reacted to form solid magnesium nitride (Mg3N2) and left behind a small amount (about 1/80) of an unreactive gas. When he analyzed the gas spectroscopically, he observed, in addition to the lines of nitrogen, lines of a unknown gas.

Rayleigh’s apparatus with 50 cc Ar gave 0.3 cc gas whereas Ramsay’s apparatus with 23 L nitrogen gave 200 cc argon

The pair later announced their discovery of a new element in the atmosphere—the first inert gas—called “argon” from the Greek, meaning “the lazy one,” because of its unreactivity.

Ramsay suggested that it be placed in a new group of zerovalent elements in the periodic table between chlorine and potassium and later discovered helium, neon, krypton and xenon

Measurements

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Ramsay observed that certain minerals, notably those containing uranium, gave off a gas (later discovered to be helium) when heated with dilute sulphuric acid. Spectroscopy of the gas gave a brilliant yellow line, not coincident with, but very close to the sodium yellow line

A machine capable of liquefying air and oxygen and nitrogen were removed (liquefaction and fractional distillation), leaving a gas with a bright yellow and a bright green line (krypton)

Whytlaw-Gray’s balance was used to determine the AW of Rn (222) and typically involved 0.0658 x 10-5 cm3 of gas weighing 6.55 x 10-7 g

References

Questions

Give a brief account of the chemical methods which were used to remove oxygen and nitrogen from air in order to allow the discovery of the noble gases. [5]

There are three methods . Ramsay - Rayleigh's first method Ramsay - Rayleigh's second method Ramsay - Rayleigh's first method: In this method, pure and dry air, passed over soda - lime and potash solution to remove carbon dioxide is then passed through a long tube containing red hot copper to remove oxygen. Copper reacts with oxygen to form copper oxide , thus , removing oxygen from the air .

2 Cu  +  O2  2 CuO

Thereafter, the air is passed over heated magnesium ribbon, when nitrogen is removed by the following reaction.

3 Mg  +  N2  Mg3N2  ( magnesium nitride )

N2 and O2 are removed be repeating the above process, leaving behind the mixture of noble gases.

Ramsay - Rayleigh's second method:

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Nitrogen and oxygen present in the air are first converted into oxides of nitrogen by sending an electric discharge. The details of the process are given below.

A large flask of 50-60 litre capacity is fitted with a five holed rubber cork. Two stout platinum electrodes are introduced through two holes. Two long tubes for the circulation of sodium hydroxide solution are introduced through two other holes. In the fifth hole a long tube is inserts for sending a mixture of dry air and oxygen in the ratio of 9:11; An electric discharge of 6000 - 8000 volts is then passed.

As a result nitrogen of the air combines with oxygen to form oxides of nitrogen. They are absorbed by sodium hydroxide circulating in the flask to form sodium nitrite and nitrate.

N2   +  O2    2 NO2 NO   +   O2  2 NO2

2 NO2   +  2 NaOH   NaNO2  +  NaNO3   +   H2O

The remaining gas contains a mixture of noble gases, with small traces of oxygen. When this is passed through a solution of alkaline pyrogallol, oxygen is absorbed leaving behind a mixture of noble gases .

Prior to their isolation, what evidence was there for the presence of unreactive gases in air?

Henry Cavendish discovered that air contains a small proportion of a substance that was less reactive than nitrogen

Lord Rayleigh discovered that discovered that samples of nitrogen from the air were of a different density compared to nitrogen resulting from chemical reactions

Give an account of how oxygen and nitrogen are prepared on an industrial scale. [8]

Liquefying the air

Air is filtered to remove dust, and then cooled in stages until it reaches –200°C. At this temperature it is a liquid. We say

that the air has been liquefied.

As the air liquefies:

water vapour condenses, and is removed using absorbent filters

carbon dioxide freezes at –79ºC, and is removed

oxygen liquefies at –183ºC

nitrogen liquefies at –196ºC

The liquid nitrogen and oxygen are then separated by fractional distillation.

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Account for the lack of helium in the Earth’s atmosphere. What are the main sources of helium on Earth? [4]

The reason is that there is no primordial helium in the atmosphere; due to the small mass of the atom, helium cannot be retained by the Earth's gravitational field.

Helium on Earth comes from the alpha decay of heavy elements such as uranium and thorium found in the Earth's crust, and tends to accumulate in natural gas deposits.

Notes

Rayleigh was put on the trail of argon because he used more than one method to measure the density of nitrogen.

Purifying nitrogen was mainly a matter of removing oxygen from atmospheric air. One way of doing so was to pass the air over hot copper, thus removing the oxygen as copper oxide:O2 + 2 Cu 2 CuOAnother was to bubble air through liquid ammonia and then through a hot tube:3 O2 + 4 NH3  6 H2O + 2 N2 The water produced in this reaction could then be removed by drying agents, and the nitrogen product joined nitrogen from the atmospheric sample. Because the atmosphere also contains argon (unbeknownst to anyone at that time), the proportions of nitrogen and argon were different in samples in which additional nitrogen was produced. At any rate, the moral of the story is that scientists often use more than one method to make measurements of the same quantity in order to be more confident that they really are measuring what they think they are measuring.

Rayleigh noticed small differences between methods only because of the high precision of his measurements.

When is a difference between measurements big enough to bother about? When the difference is greater than the experimental error of the measurement. A look at the results of Rayleigh's measurements from a variety of methods reveals a clear difference between two sets of data. This observation can be a springboard to a treatment of statistical significance.

Rayleigh turned to experts in disciplines outside his own to attempt to explain his anomalous results.

Rayleigh's 1892 note in Nature[5] was an admission that he was stumped by the anomalies he encountered in measuring the density of nitrogen: "I am much puzzled by some recent results as to the density of nitrogen, and shall be obliged if any of your chemical readers can offer suggestions as to the cause." Trained as a physicist, Rayleigh addressed his appeal for suggestions to chemists, that is to scientists whose expertise was different from his and who might have ideas which did not occur to him. Today's scientific journals don't have room for such communications, but cross-disciplinary consultations and collaborations are widespread in modern science.

Henry Cavendish had probably encountered argon a century earlier,[7] but he could not follow through the way Rayleigh could.

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Fractional distillation

The liquefied air is passed into the bottom of a fractionating column.

Just as in the columns used to separate oil fractions, the column is

warmer at the bottom than it is at the top.

The liquid nitrogen boils at the bottom of the column. Gaseous nitrogen

rises to the top, where it is piped off and stored. Liquid oxygen collects

at the bottom of the column. The boiling point of argon - the noble gas

that forms 0.9% of the air - is close to the boiling point of oxygen, so a

second fractionating column is often used to separate the argon from

the oxygen.

Rayleigh didn't just consult current opinion; he went back to the literature. Cavendish had passed electricity though air, absorbing the reaction products (nitrogen oxides) with a piece of potash. He was left with a residue of just under 1% of his original sample. But Cavendish was in no position to follow through on characterizing this residue for a number of reasons, both theoretical and technological. Cavendish was still operating under the phlogiston theory and was trying to characterize the principal components of the atmosphere. Furthermore, isolation of enough of the residue to study would have faced enormous technological obstacles, given that his source of electricity was a friction machine and his gas-handling apparatus was a mercury "pneumatic trough". A century later, Rayleigh used Cavendish's method "with the advantage of modern appliances", noting that, "In this Institution we have the advantage of a public supply" of electricity.[6] This episode offers an excellent example of how scientific discoveries depend at least in part on the current state of science and technology.

Rayleigh and Ramsay conducted a battery of tests to characterize the new gas physically and chemically.

Once the investigators isolated their inert residue in sufficient quantities to study it, how did they characterize it? Comparison of argon's spectrum to known spectra helped establish that the gas was previously unknown. (This mention of spectra provides an opportunity to discuss the plethora of spectroscopic characterization techniques currently in widespread use.) Measurements of constant-pressure and constant-volume heat capacities established the monatomic nature of the new substance. Finally, some tests were natural outgrowths of the investigation up to that point. For example, Rayleigh had tried to measure the density of nitrogen in the first place, so of course he measured the density of argon. Both researchers isolated argon as an unreactive residue of air, so naturally Ramsay tried to get it to react with a laundry list of reactive substances (elements, acids, bases, oxidants, and reducing agents including hydrogen, chlorine, phosphorous vapor, sulfur vapor, tellurium vapor, sodium vapor, molten sodium hydroxide, molten potassium nitrate, potassium permanganate in hydrochloric acid, sodium peroxide, bromine water, and a cocktail of nitric and hydrochloric acids).

Rayleigh recognized that the claim of elemental status for the newly discovered gas was controversial.Rayleigh told his audience that, "the subject [the assertion that argon is an element] is difficult, and one that has given rise to some difference of opinion among physicists."[6] In light of the evidence that Ramsay and Rayleigh marshaled for the elemental status of argon, and given that more than 40 elements had already been discovered in the 19 th century, why was there controversy? One reason was surely the periodic table, which had become established over the preceding quarter of a century. There was no place for argon in that table. If the periodic law and the discovery of a new inert elemental gas were both correct, then there must be a family of such elements. Ramsay arrived at that conclusion, and set about looking for other members of the family. The lesson here is that new findings must be evaluated in the context of existing knowledge. Apparent contradictions may cause a new conclusion to be greeted with skepticism (often warranted). Sometimes, however, attempts to resolve the contradictions prove scientifically fruitful.

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