Some basic concept problems

[Mohit Pandey]

Please explain pointwise.

1.In the chemical changes which I have studied till now , precipitate appears and after adding more reactant, it dissolves. For example--

when ammonium hydroxide is added to copper sulphate solution, a blue precipitate appears. When more ammonium hydroxide is added, blue precipitate disappears.

I can't understand the chemical change.Can all precipitate be dissolved in such manner?

 

2.Dry ammonia has no action on litmus. But a solution of ammonia in water turns red litmus blue. Is it a base when it is dry? How can we check it without getting it wet?

 

3.Concentrated nitric acid oxidises sulphur to sulphuric acid.

Contact process, which is a longer than the above, is used for commercial production of sulphuric acid . Shouldn't it be wise if we use oxidation of sulphur using nitric acid for commercial puporses?

 

4.

In my textbook it is written that

H2SO4 is a dibasic acid. It reacts with alkalis to form acid salts called hydrogensulphates and normal salts called sulphates.

Can anyone explain this? I have already Googled it without any satisfactory results.

One more question.

NaOH + H2SO4(aq) ==NaHSO4(aq) + H2O

2NaOH + H2SO4(aq) == Na2SO4(aq) +H2O

Do these reactions occur at same time?

[eric l]

That's too many questions at once, because the answers will be quite different.

This post will be limited to question # 1 about copper sulphate and ammonium hydroxide.

First you have to understand that even if a precipitate is formed, not all of the product precipitates, even "insolluble" products are always slithghtly (or very slightly solluble.

What happens with copper sulphate and ammonium hydroxide is that Cu²⁺ forms a complex ion with NH₃ and that the resulting products is simply much more solluble.

There is a good deal more about complex ions in Wikipedia (Complex (chemistry) - Wikipedia, the free encyclopedia)

So wath happens is that the "insolluble" Cu(OH)₂ is transformed into solluble Cu(NH₃)_n(OH)₂

I would have to check what exactly the value of n would be in this case

 

A more or less similar case would be if you add NaOH to AlCl₃ : first you will have a gel like precipitation of alluminium hydroxide, but by further addition of NaOH you form solluble sodium alluminate. But this is not a case of forming complex ions, it is that Al is "amphoteric" and able to form acids as well as bases. See also Amphoterism - Wikipedia, the free encyclopedia

 

I'll leave the other questions for someone else - or for a next occasion.

[Mohit Pandey]

#1 is now clear. Move ahead.

[eric l]

delete

[eric l]

Nobody else trying to answer, while the question may be important to other students as well, I give it a try with question # 2.

The colour of lithmus (or other pH-indicator substances) is determined by the ratio between the concentration of H⁺ (or rather H₃O⁺) and OH^-. That means there must be water around.

Dry ammonia has neither of these ions, but a wetted paper has, and if this wetted paper absorbs dry ammonia the ratio between the concentrations of these ions changes.

For the same reason, you can not really determine the pH of e.g. concentrated sulphuric acid.

Be patient for the answers on questions 3 and 4.

[Mohit Pandey]

Nobody else trying to answer, while the question may be important to other students as well.

Yes, you are correct :xparty:. They are just patiently waiting and just viewing the posts. They are passive participants. Come on! guys.

Post your doubts here and answer others.

 

#2 is also clear.

[modest]

3.Concentrated nitric acid oxidises sulphur to sulphuric acid.

Contact process, which is a longer than the above, is used for commercial production of sulphuric acid . Shouldn't it be wise if we use oxidation of sulphur using nitric acid?

I think you've got this a bit backwards. Nitric acid and sulfur combined don't make sulfuric acid. You can make nitric acid with sulfuric acid and a nitrate (like calcium nitrate). If you look at the standard electrode potentials of the half-reactions involved, (particularly that of solid sulfur and hydrogen ions vs. nitrate ions and hydrogen ions) you can predict the reaction of sulfur and nitric acid.

 

You may be thinking of a different method where sulfur or SO2 can be burned and oxidized with NO2 to make SO3 which can then be hydrated. Please don't try to do this because NO2 will kill you right quick when you breath it. Also, adding SO3 to water would be a good way to explode sulfuric acid on your face.

 

Nitric acid is much easier to make for the home chemist (not that I'm advocating that either)

 

I hope this answers your question.

[Mohit Pandey]

By Modest-Nitric acid and sulfur combined don't make sulfuric acid.

S+6HNO3==H2SO4 +6NO2 +2H2O

By Modest-You may be thinking of a different method where sulfur or SO2 can be burned and oxidized with NO2 to make SO3 which can then be hydrated. Please don't try to do this because NO2 will kill you right quick when you breath it. Also, adding SO3 to water would be a good way to explode sulfuric acid on your face.

I think you are referring to CONTACT PROCESS.

My question was somewhat different. :(

Contact process is a long process in comparison to the reaction of oxidation of Sulphur using Nitric acid. Why chemists in factories use the long Contact process?

And yes, I am not going to make it at home.

[eric l]

S+6HNO3==H2SO4 +6NO2 +2H2O

 

Contact process is a long process in comparison to the reaction of oxidation of Sulphur using Nitric acid. Why chemists in factories use the long Contact process?

And yes, I am not going to make it at home.

 

I think the fact that it is a long process is one of the reasons why the contact process is prefered to the reaction you mention : at least it is controlable on industrial scale.

What happened in Bhopal - which is, if I'm not mistaken, in your state of Madhya Pradesh - in 1984 is an example of what may happen when a chemical process runs out of controll.

The reaction of concentrated nitric acid with sulphur is pretty near explosive, even if tried at a small scale. And the nitrous fumes themselves are quite dangerous and not easy to deal with.

On top of that, nitric acid is much more expensive than sulphuric acid, which is also an important consideration when it comes to industrial production.

[modest]

S+6HNO3==H2SO4 +6NO2 +2H2O

 

I think you are referring to CONTACT PROCESS.

My question was somewhat different. :thumbs_up

Contact process is a long process in comparison to the reaction of oxidation of Sulphur using Nitric acid. Why chemists in factories use the long Contact process?

And yes, I am not going to make it at home.

 

No, the method I'm referring to is called the lead chamber method. Both methods are used in industry today - contact and lead chamber.

• Lead Chamber is an older method that creates less concentrated acid

• Contact Process is the newer method that can create concentrations of up to 98%

 

Like I said before (and posted a link and method to prove this to yourself) you can't manufacture sulfuric acid by combining nitric acid and sulfur. If you believe otherwise - post a source.

 

-modest

[Mohit Pandey]

Like I said before (and posted a link and method to prove this to yourself) you can't manufacture sulfuric acid by combining nitric acid and sulfur. If you believe otherwise - post a source.

 

-modest

 

1.Tutoring, Chemistry, Chemistry Tutoring, Group 16 (VIA) Elements-Oxygen and Sulphur, Sources of Oxygen, Sodium Peroxide, Rusting of Iron, Extraction of Sulphur, Potassium Chlorate, Frasch Process, Heat on Sulphur, Oxides - Tutorvista.com

2.Nitric acid Summary

[modest]

1.Tutoring, Chemistry, Chemistry Tutoring, Group 16 (VIA) Elements-Oxygen and Sulphur, Sources of Oxygen, Sodium Peroxide, Rusting of Iron, Extraction of Sulphur, Potassium Chlorate, Frasch Process, Heat on Sulphur, Oxides - Tutorvista.com

2.Nitric acid Summary

 

It is true that concentrated nitric acid is a good oxidizer, but the result will be

sulfur dioxide, nitrogen monoxide and water vapor. I guess in a closed system the sulfur could be further oxidized - but the result would be a little bit of weak sulfuric acid in a large amount of concentrated nitric acid and nitrates. This is not an economic setup or result. All one has done here is to contaminate perfectly good concentrated nitric acid. :shrug:

[Mohit Pandey]

#3 is clear. Move ahead.

[modest]

4.

In my textbook it is written that

H2SO4 is a dibasic acid. It reacts with alkalis to form acid salts called hydrogensulphates and normal salts called sulphates.

Can anyone explain this? I have already Googled it without any satisfactory results.

Try to google bisulfate. That is the more common way of saying it. Basically (pun intended) - sulfate has a negative 2 charge and has no hydrogen. Hydrogen sulfate has a negative 1 charge and has a hydrogen.

Dibasic just means that there are 2 hydrogen ions in each molecule.

 

One more question.

NaOH + H2SO4(aq) ==NaHSO4(aq) + H2O

2NaOH + H2SO4(aq) == Na2SO4(aq) +H2O

Do these reactions occur at same time?

 

sodium sulfate and sodium bisulfate make an equilibrium in sulfuric acid.

the way to write it would be:

 

[math] Na_{2}SO_{4}_{(aq)} + H_{2}SO_{4(aq)} \rightleftharpoons 2NaHSO_{4(aq)}[/math]

 

This is an example like your textbook says "It reacts with alkalis to form acid salts called hydrogensulphates and normal salts called sulphates"

[Mohit Pandey]

OK, everything cleared. Thank you Eric! and Modest!:)

 

My friends! if you have any doubts or views, post here.