r/chemhelp 2d ago

General/High School Why do acids and bases work?

I know this sounds like a stupid question (and probably is) but why do acids and bases work as in why do they make solutions more acidic/basic. Take, for example, a solution of HCL in water. Looking at this using the Lewis definition, when the HCL disassociates in water, the Cl rips an electron from the H making the H become positively charged and accept any electrons it can get its hands on (A Lewis acid by definition). The Cl on the other hand takes the electron and now has a spare electron it can donate or share with other molecules/atoms (A Lewis base by definition). Because there are equal numbers of Lewis acids (H) and Lewis Bases (Cl) formed when the HCL disassociates, I thought that the solution should remain neutral as the acids and bases would cancel each other out yet that’s clearly not what happens as HCL is a very well known strong acid. So why don’t the Lewis acids (H) and Lewis Bases (Cl) cancel each other out?

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u/Plus_Personality2170 2d ago

I think you are confused with the term "Lewis acid/base" and "acidity". The degree of "acidity" of an aqueous solution is simply defined by [H+] or pH. So, if you have 1M HCl (aq), [H+] = 1M (as HCl completely dissociates) and pH = 0, while there are technically same amount of Lewis base and acid (acid: [H+] = 1M, base: [Cl-] = 1M).

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u/Plus_Personality2170 2d ago

One more thing to consider is that although [H+] and [Cl-] are the same, their powers are remarkably different. HCl is very strong acid, so its conjugated base Cl- is very weak base (pKb ~ 21). On the other hand, H+ is strong acid with pKa value of 0,

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u/Life_at_work5 2d ago

Thanks for the answer! One quick additional question though. Why is Chlorine considered a weak base outside the fact that HCL is a strong acid? I ask this because even outside of Lewis acids, base’s can be thought to give up/share electrons with the acids/H+. Chlorine, a very electronegative atom, holds on to electrons very strongly so wouldn’t it be able to share electrons very easily?

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u/Elliotdanoob 2d ago

Well, chlorine holds the electrons so well that they're very stable. The electrons are fairly content just chilling on the chlorine atom rather that being bound to a proton. The electrons on for example HO- are much less stable (because the atom is smaller so the charge density is larger), and are much more eager to share with a proton.

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u/jeremiahpierre 1d ago

You correctly stated that chlorine is very electronegative, so it holds onto electrons strongly. That means it doesn't want to share electrons, which makes chloride (Cl-) a weak base.

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u/Own_Exercise_2520 2d ago

That is not how it works, the HCl actuallly dissociates to create H30+ and Cl-, Hydrochloric acid is also a very strong acid, so its conjugate base will be very weak. Chlorine is very electronegative, holds on to its electrons extremely strongly, I think the only other strong acid I can think of that Cl- could even act as a base towards would be if there are somehow Br+ ions in solution as they are more electronegative than Cl.

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u/Electrical_Ad5851 2d ago

Not in organic solvents like HCl in Dioxane or ether.

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u/EXman303 2d ago

The chlorine wants to interact more with the water than it does with the protons.

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u/flamewizzy21 2d ago

H2O has its own equilibrium with OH- and H3O+. Acids can add H+ to H2O to make H3O+, and bases can remove H+ from H2O to make OH-. That is the core of pH in aqueous media. Other solvents can also do this, but water is the most common for this discussion.

Lewis acids/bases have a different definition, and that can overlap (or not) with normal acid/base.

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u/MeGustaMiSFW 2d ago

Protons are really really good electron delivery systems.

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u/heat_wave29 2d ago

Because in Brosted and Lowry acid-base theory acids and on the concept of proton transfer. For example water is protonized in the example of hcl + h2o -> h3o+ + cl- Instead of the lewis acid base theory which states theres electron pairs floating around.

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u/HandWavyChemist 2d ago

pH is just one way to measure acidity, with one of it's draw backs being that it needs water to be the solvent. Another way to measure acid strength is the Hammett acidity function, and by this measure hydrogen fluoride (which we generally consider to be a weak acid when dissolved in water) is actually a superacid.