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Why some acid/alkali can react with some salt, forming another acid/alkali? For example:

$\ce{2NaCl +H2SO4->Na2SO4 + 2HCl}$

$\ce{Ca(OH)2 +Na2CO3->CaCO3 +2NaOH}$

but not the reverse. What determines this? I heard someone said the weaker acid/alkali produced is favourable, but HCl is stronger than $\ce{H2SO4}$. I also heard that only if the products are insoluble or lost from the solution, the reaction happens. Someone suggested that the problem is concentration, as sulfuric acid is 98% while HCl is 38% Is that true?

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    $\begingroup$ Yes, that's the point. $\ce{CaCO3}$ is insoluble, and $\ce{HCl}$ is a gas. $\endgroup$
    – Ivan Neretin
    Aug 23, 2016 at 13:25
  • $\begingroup$ This question is difficult to understand. Also is this question similar? chemistry.stackexchange.com/questions/57814/… $\endgroup$
    – Agriculturist
    Aug 23, 2016 at 14:09
  • $\begingroup$ @ Agriculturist, I don't understand why the sulphuric acid 'displaces' hydrochloric acid from chloride salt, but hydrochloric acid can't 'displace' sulphuric acid from sulphate salt. $\endgroup$
    – chemlearner
    Aug 25, 2016 at 13:19
  • $\begingroup$ Concentrated HCl solutions can only reach about 40%. So say you add NaCl to 98% sulfuric acid. You quickly end up supersaturated with respect to dissolved HCl and it goes into the gas phase which thus very much limits the ability for that reaction to go in the reverse direction. $\endgroup$
    – airhuff
    Jan 27, 2017 at 23:55
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    $\begingroup$ The sulfuric acid reaction gives sodium bi-sulfate, maybe? $\endgroup$
    – Oscar Lanzi
    Jun 17, 2017 at 9:41

3 Answers 3

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Hydrochloric acid solutions can only readily reach about $\pu{40\%}$ concentration. So say you add $\ce{NaCl}$ to $\pu{98\%}$ sulfuric acid. You quickly end up supersaturated with respect to dissolved $\ce{HCl}$ and it goes into the gas phase which thus limits the ability for that reaction to go in the reverse direction.

The situation is similar for the second reaction, except in this case rather than removing carbonate from the system via volatilization, you are forming the insoluble $\ce{CaCO3}$, preventing the reaction from happening in the reverse direction.

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It has been observed that: A STRONGER ACID or BASE can produce a WEAKER (usually more volatile) from a reaction with given salt.

While H2SO4 is stronger (though less strong than HCl) and less volatile it can give HCl from NaCl and HNO3 from NaNO3 etc. [kind of we have seen in nitrating mixture NaNO2 + HCl to generate HNO2]

Ca(OH)2 though weaker than NaOH and being less volatile can give the latter from NaCl. [Think of NaOH giving off NH3 from all ammonium salts!]

The cases governing may be: 1. STRONG acids as well as bases give way to weaker ones (when reacted with suitable salts 2. More volatile may be given off by less volatile ones 3. Solubility

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    $\begingroup$ I disagree with your explanation. $\ce{HCl}$ is actually a stronger acid than $\ce{H2SO4}$. $\ce{HCl}$ has a $pK_a$ of -6.3 while $\ce{H2SO4}$ has a $pK_a$ of around -3, indicating that $\ce{HCl}$ is a stronger acid than $\ce{H2SO4}$. The key point is that hydrogen chloride formed is insoluble in conc. $\ce{H2SO4}$ and escapes as a vapor from the mixture, driving the reaction forward as the reverse reaction cannot occur. In the second reaction, the $\ce{CaCO3}$ is insoluble and does not dissolve into ions again, so the reverse reaction cannot occur and the reaction proceeds forward. $\endgroup$
    – rcAlex36
    Jun 17, 2017 at 2:07
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I don't think the strength of the acid is a factor. A weak acid like acetic / ethanoic acid (vinegar) can displace a strong acid (eg HCl) from its salt - if you mix salt and vinegar you end up with sodium acetate and hydrochloric acid. It has to do with the volatility. A less volatile acid displaces a more volatile one from its salt.

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