Why did we not use HCl instead of H2SO4?

As already written by Andrew Wolff, the strength of an acid depends on how easily a proton can break away from it. The easier it is , the stronger is the acid. HCl is a stronger acid than H2SO4 because a proton more easily separates from chloride ion than from hydrogen sulphate ion and this is reflected in their dissociation constants.

The basic question is,however, why it is easier for a proton to separate from chloride ion than from hydrogensulphate ion. Let us try to answer this basic question.

Let us consider an acid HX, where X may be an atom or a group of atoms. In HCl , X is Cl and in H2SO4 X is SO2(OH)O. As an acid HX dissociates as HX=> H+ +X-. We can assume that the more exothermic or the less endothermic is the dissociation, the stronger is the acid. To get an idea of the enthalpy change in dissociation we break the dissociation into steps for which either enthalpy change is known or can be easily estimated.

This dissociation may be considered to occur in three steps

1. HX => H + X, that is homolytic breakage of the covalent bond
2. H => H+ +e- , that is ionisation of hydrogen
3. X + e- => X-
4. Step 2 is independent of the nature of X , so, while comparing the strengths of different acids , we need not consider the enthalpy change of this step because it is the same for all acids. The difference in the strengths of acids therefore depends on the difference in enthalpies of steps (1) and (3) for the different acids. Since step (1) involves homolytic fission of H - X bond, the stronger the bond the weaker the acid. In HCl the bond dissociation enthalpy is 427 kJ/mol whereas in H2SO4 ,where O - H bond breaks , it is 467 kJ/mol. So the lower bond dissociation enthalpy of the H - Cl bond is in favour of HCl being a stronger acid than H2SO4.
5. In step 3 the electron gain enthalpy of Cl is more negative than the electron gain enthalpy of the O atom which is part of HSO4. ( I couldn't find data on a rapid search, but it is well known that O has less negative electron gain enthalpy than S which in turn has lower electron affinity than Cl) So this factor is also in favour of HCl being a stronger acid than H2SO4. Hydration energy data might favour H2SO4 but they are not available ( to me) so I have not brought them into discussion. For elementary discussion like the above enthalpy data are sufficient, but for thorough discussion free energy data are required.

Not everywhere. Certainly not pure.

Sulphuric acid is not always equivalent to hydrochloric acid. Some reasons why:

1. sulphuric acid is an oxidizer too. This may not be desirable.
2. Sulphates are not always equally soluble as chlorides, changing the dynamics of a solution
3. Hydrochloric acid is a gas dissolved in water. Sometimes the gas is used. Sulphuric acid is a liquid.

Because it certainly is!(out of no doubt)☺It is used in various industrial process and even in laboratory preparations of various acids(which are used even in your chemistry lab).for ex:- for preparation of hydrochloric acid :-NaCl+H2SO4→NaHSO4+HCl.

Thanks for reading!

You provide no context to this question. I can tell you 2 safety reasons immediately and maybe one of those is correct and true within the context that you left out.

1. HCl is a gas, H2SO4 isn’t. Is volatility an issue? Are you heating something to a high temperature? Are you trying to concentrate the chemical by removing water? getting a faceful or nose-full of HCl is bad news.
2. H2SO4 doesn’t dissolve or embrittle metals like HCl does. Are you using high temperature, high pressure reactions? H2SO4 is much safer.
3. Not safety related but H2SO4 does a better job at dehydrating organic copounds to get to the olefin/alkene.

Maybe one of these generic answers fits your question.

Permanganate will oxidize HCl to chlorine gas.

Titrations involving potassium permanganate are usually carried out in acidic medium. This is due to higher oxidizing power of permanganate ion in acidic medium than in neutral or basic medium.

Secondly, the formation of brown coloured MnO2 in basic medium interferes with the detection of the end point. For acidification of KMnO4 solution, only H2SO4 is suitable whereas the other mineral acids like HCl and HNO3 are not. HCl is not used because some of the KMnO4 will oxidize chloride ions to chlorine gas.

Nitric acid cannot be used because it is itself a strong oxidizing agent and may oxidize the reducing agent, thereby introducing error in the analysis.

First of all, as Matjaz Mraz has already pointed out, sulfuric acid withdraws water from the reaction and thus helps drive it to completion. And since HCl is normally supplied in aqueous form (31% or so), simply using it as an off-the-shelf product would also mean *starting* with a lot more water, further damaging the equilibrium.

However, just in case you were thinking that you could get around this by using molecular sieves, or some other hygroscopic compound that was otherwise less chemically aggressive than sulfuric acid, along with dry gaseous HCl, there is also a further problem. The nitronium ion that actually reacts to carry out nitrations will also tend to react with the chloride ions that HCl creates:

2NO[math]_2^+[/math] + 2Cl[math]^-[/math] → 2NO[math]_2[/math] + Cl[math]_2[/math]

…thus damaging your yields.

It should be noted, however, that many nitrations can be carried out using pure nitric acid of a very high concentration. This is somewhat more practical in cases where the nitration actually involves the formation of a nitrate ester (as for example with glycol dinitrate or nitroglycerin), since the nitronium ion is no longer a necessary intermediate. For aromatic ring nitrations, where the nitronium ion reacts electrophilically, it’s generally too slow to use nitric acid alone. Furthermore, commercial operations will still use mixed acid even for nitrate esters, for practical reasons of cost and ease of product separation.

No . From a chemistry standpoint I assume that the cement is treated with HCl .This will react with the calcium salts in the cement and produce soluble calcium chloride. These soluble salt are removed by filtration leaving any insoluble residue behind - which appears to be the purpose of the test

But H2SO4 does not produce soluble compounds when reacted with calcium salts - calcium sulphate is insoluble in water, The test will not give the correct results.

Usually because it is for a redox reaction or titration in which HCl would itself react to give chlorine. Sulfuric or phosphoric acids may be used.

That depends. What is the solvent?

Lots of answers here say “nothing happens” because they’re both strong acids. Well, it’s not that cut and dried even in water. If the two acids are dilute, less than .01 molar concentration then both acids are strong, meaning they react with water (not each other) 100%. They don’t react with each other because they react with the much more abundant water solvent. The solution will have [math]Cl^-[/math], [math]SO_4^{2-}[/math] and lots of [math]H_3O^+[/math]

If the concentration of the acids is about greater than that then only one proton will come off the sulfuric acid. The second one will only partially react with water and the [math]H_3O^+[/math] ions from the HCl will make that second proton even less likely to come off.

Interestingly, if you dissolve the HCl in pure sulfuric acid, a liquid when pure, the HCl is only a weak acid in that solvent. It only partially reacts with the sulfuric acid to produce

[math]H_3SO_4^+[/math]

This latter species is so acidic that it simply cannot exist in water at all but it can exist in liquid sulfuric acid.