# Why is are weak acid-strong base and strong acid-weak base reactions irreversible?

In the case of both are strong, I can see why it's irreversible, because the acid is very willing to give a proton and the base is very willing to take one. Now here are my two main questions:

1. In the case of both are weak, we say the reaction is reversible. Does this mean the reaction happens then the products react again to give reactants, or that the reactants don't run out and the reaction doesn't happen at "full potential"?

2. In the case of a reaction between a strong base and a weak acid (or the other way around), why is it always irreversible? I mean since one of them is weak, shouldn't it give out an unstable product which is willing to react again, like

$$\ce{CH3COOH + OH- → CH3COO- + H2O}$$

The $$\ce{CH3COOH}$$ is a weak acid. The $$\ce{CH3COO-}$$ isn't stable so shouldn't it react again?

• When is a reaction considered 'irreversible'? What's the equilibrium constant of the reaction between acetic acid and sodium hydroxide? – user6376297 Jan 20 at 17:02

Yes the following reaction is reversible.

$$\ce{CH3COOH + OH- <=> CH3COO- + H2O}$$

There is an equilibrium between the species $$\ce{CH3COOH, OH-}$$ and $$\ce{CH3COO-}$$ in aqueous solution.

Thus the gist is that particular $$\ce{CH3COOH}$$ and $$\ce{CH3COO^-}$$ molecules aren't stable, but rather continuously react.

I would like to clarify your doubt...notice what happens in a reaction between a strong acid and a weak base or a strong base and a weak acid when you take the weak electrolyte in lesser amount... Suppose we cause a reaction between acetic acid and sodium hydroxide...It does feel as though it would be reversible as most ch3cooh would be undissociated... But remember that OH- is continuously coming from NaOH... And the result is that H+ are being consumed very quickly (due to them reacting with OH- and forming water which causes the equilibrium of dissociation of acetic acid to shift towards right (Le Charteliers principle) and so the weak acid starts behaving as a strong acid and does not stop the supply of H+ unlike in a normal solution of acetic acid where the concentration of H+ coming from it is fixed (Ka)...the supply of H+ continues till all acetic acid is exhausted as it is the limiting reagent and not NaOH... So this reaction basically becomes a strong acid and strong base reaction...which is well known to be irreversible... Same is the case with strong acid and weak base where in turn H+ pull the OH- out of the weak base again making it a strong acid strong base reaction... But if you take the weak electrolyte in excess the reaction stays irreversible till the OH- or the H+ coming from the strong electrolyte get exhausted... Afterward the solution becomes a buffer.

But remember this can never be the case with weak acid and weak base as both H+ and OH- are limited... According to me their reactions are always reversible.

Now addressing your doubt... Although CH3COO- is unstable... But it exists isolated in a solution due to NaCH3COO being a strong electrolyte... Remember one thing... Most salts of weak acids are good electrolytes... And it doesn't react with H+ as the CH3COO- so formed against dissociates due to the action of NaOH.

• >... So this reaction basically becomes a strong acid and strong base reaction...which is well known to be irreversible... But the reaction produces basic salt which can hydrolyze as the following: $$\ce{ CH3COO- + H2O <=> CH3COOH + OH- }$$ So this reaction becomes reversible – Adnan AL-Amleh Jan 21 at 16:00
• But see this... In this reaction a weak acid and weak base are combining to give a comparatively stronger acid and strong base... Is it a spontaneous reaction...NO, NOT AT ALL... so it does not occur unless carried in extreme conditions...and I hope you know this if you have studied thermodynamics – sauhaard batra Jan 24 at 15:29
• And I forgot to add... This reaction is not even reversible as the reaction does NOT EVEN OCCUR – sauhaard batra Jan 24 at 15:39
• >the reaction does NOT EVEN OCCUR.** Which reaction?** – Adnan AL-Amleh Jan 24 at 16:53