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Ionic Equilibria should be clear

Ionic Equilibria should be clear

by Shashank Todwal -
Number of replies: 5

In all the books it is written that a buffer solution can be made with weak acid(e.g.CH3COOH) and strong base (e.g.NaOH).Suppose i have some CH3COOH solution.Then I have foll. equilibriums established:

CH3COOH----><----CH3COO- + H+      -----(1) ---Ka(small)                      AND

CH3COO-+H20----><----CH3COOH+OH-  ----------(2)----Kh(small)

if i add some NaOH then reaction (2) is favoured backwards and almost all of my added NaOH gets converted to acetate ions as 1/Kh is very very large.But then b'coz of increase of acetate ions reaction (1) is favoured and almost all extra ions get converted to acid as 1/Ka is also very very large.

Then increase in acid again causes backward reaction of (2) and ions formed again and the whole process is repeated again and again.So at the end poor OH- is almost negligible.

This is just an example but if we speak in general then i can't maintain a pH of 10,11,etc as then in that case we are assuming [OH]=104etc. which i have proved above is impossible.

Plz give your suggestions on this!!!

In reply to Shashank Todwal

Re: Ionic Equilibria should be clear

by ishaan biswas -
well a ph of 10,or 11 means pOH of 3or4 =>[OH-] is of the order of 10-4 which is qi\uite possible
In reply to Shashank Todwal

Re: Ionic Equilibria should be clear

by Ankul Garg -
Well, you are partially right. But you made one mistake. You didn't see that the Kw of water has to remain constant and so, water molecules will dissociate into H+ and OH- ions making OH- in an excess. Thus, the pH of the soln. is maintained above 10 or 11 making the resulting soln. alkaline.
In reply to Shashank Todwal

Re: Ionic Equilibria should be clear

by Chandni Bhatia -

NaOH is a strong base and CH3COOH is a weak acid ,therefore their salt on reaction with water will give Na+ and CH3COO- .This anion will be reactive in nature and therefore will react with water giving acid and OH-. Also water will also dissociate into H+ and OH- ions and I think this will make the solution alkaline.

In reply to Shashank Todwal

Re: Ionic Equilibria should be clear

by Asif Zafar -

Firsty, U get a buffer solution using a weak acid and its salt of a strong base( not the base itself). Lets suppose u use

CH3COOH (weak acid) & CH3COONa (its salt of a strong base).

Ethanoic acid is a weak acid, and the position of this equilibrium will be well to the left:

Adding sodium ethanoate to this adds lots of extra ethanoate ions. According to Le Chatelier's Principle, that will tip the position of the equilibrium even further to the left.

The solution will therefore contain these important things:

  • lots of un-ionised ethanoic acid;

  • lots of ethanoate ions from the sodium ethanoate;

  • enough hydrogen ions to make the solution acidic.

Other things (like water and sodium ions) which are present aren't important to the argument.

Adding an acid to this buffer solution

The buffer solution must remove most of the new hydrogen ions otherwise the pH would drop markedly.

Hydrogen ions combine with the ethanoate ions to make ethanoic acid. Although the reaction is reversible, since the ethanoic acid is a weak acid, most of the new hydrogen ions are removed in this way.

Since most of the new hydrogen ions are removed, the pH won't change very much - but because of the equilibria involved, it will fall a little bit.

Adding an alkali to this buffer solution

Alkaline solutions contain hydroxide ions and the buffer solution removes most of these.

This time the situation is a bit more complicated because there are two processes which can remove hydroxide ions.

Removal by reacting with ethanoic acid

The most likely acidic substance which a hydroxide ion is going to collide with is an ethanoic acid molecule. They will react to form ethanoate ions and water.

Because most of the new hydroxide ions are removed, the pH doesn't increase very much.

Removal of the hydroxide ions by reacting with hydrogen ions

Remember that there are some hydrogen ions present from the ionisation of the ethanoic acid.

Hydroxide ions can combine with these to make water. As soon as this happens, the equilibrium tips to replace them. This keeps on happening until most of the hydroxide ions are removed.

Again, because you have equilibria involved, not all of the hydroxide ions are removed - just most of them. The water formed re-ionises to a very small extent to give a few hydrogen ions and hydroxide ions.


                                    

                
                
                
                                        
                
                
                                            

                                                    
                                            

                
                                        
                                

                            

                        

                    

                
        

                

                    

                        
                
                        

                            

                                        

                                                
                                        

                                

                                        In reply to Asif Zafar
                                        
Re: Ionic Equilibria should be clear

                                        

                                            by Shashank Todwal - 
                        

                                

                            

                
                            

                                        

                                        

                
                                

                                    

                                        
Dear Asif 


I have searched in all the books and even read on www.chemguide.co.uk (from where you have tried to explain me) but my question is unanswered in any of these books .Please give me some explanation which I would not read anywhere.


Bye! Best Luck