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In many syntheses stuff like calcium carbonate is used to neutralise acids, but this seems annoying, as you have to add it very slowly to avoid too much fizzing.

Could someone explain why anyone would voluntarily use carbonates over something like $\ce{NaOH}$ or $\ce{Ca(OH)2}$?

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    $\begingroup$ They buffer near neutral if you add a little bit excess. With NaOH, the pH goes up really high if you overshoot even a little bit if no other buffer is present $\endgroup$
    – Andrew
    Sep 3, 2021 at 23:22
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    $\begingroup$ Related to the previous comment: excess hydroxide is a reasonably good nucleophike and may trigger side reactions. Carbonate would be weaker. $\endgroup$ Sep 4, 2021 at 0:34
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    $\begingroup$ And there are expense and safety issues, too. $\endgroup$ Sep 5, 2021 at 2:36

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It depends, because the neutralization with $\ce{Na2CO3}$ offers the byproduct of $\ce{CO2}$, while the neutralization with lye does not.

If you perform the neutralization in an open system, you may monitor the evolution of the gas typically leaving the reaction mixture (you have to account for some $\ce{CO2}$ being soluted in water, like in a seltzer, though). Thus, provided there was enough time for the neutralization reaction, the cease of gas evolution is a cheap indicator that $\ce{HCl}$ was neutralized and any additional addition of aqueous $\ce{Na2CO3}$ only increases the pH value of the aqueous phase much above 7. Contrasting to this, if you use lye, the determination of the endpoint of titration requires either a) the addition of an indicator to an aliquot of your reaction/titration, b) adding an aqueous droplet of your reaction mixture on strip of pH paper, or c) an electrochemical determination of the pH value.

In organic chemistry, the use of $\ce{Na2CO3}$/$\ce{K2CO3}$ e.g. in a malonic ester condensation equally has the advantage that $\ce{CO2}$ escaping the open reaction mixture 1) drives the deprotonation to completion and 2) inhibits the back reaction, which otherwise would yield an equilibrium between acid (and corresponding conjugate base) and added base (and its corresponding conjugate acid).

For a neutralization in a closed system, the generation of $\ce{CO2}$ and generated pressure is a potential safety issue and may be a no-go. Balanced with other parameters (e.g., price of the reagent per equivalent to quench one acidic function, process parameters and scale of reaction, OSHA/safety for the employees), lye then may be an alternative.

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It's simple. Hydroxides are often strong bases, strong enough to react rapidly with e.g. human skin, should any be spilled on an operator's hands. In contrast, carbonates are weak bases- they don't react readily with human skin, so the operator can just wash the carbonate out with water (attempting to do that for hydroxide would result in an even faster reaction) without any noticeable harm.

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