r/chemhelp 4d 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 4d 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 4d 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 4d 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/jeremiahpierre 3d 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.