Describe with equations, how a solution of NaCN with HCN functions as a buffer?

Question

Would this be correct?

H+ + HCN + NaCN --------> H2CN + NaCN

Thanks!

or would it be

H^+ + HCN + NaCN -----> H2CN^- + NaCN ?

Answer 1

A buffer is a solution which resists change in pH.
HCN dissociates to a small degree, as H+ & CN-

HCN <<<---> H+ + CN-
NaCN <---->>> Na+ + CN-
Add a little more acid and the extra H+ ions are 'mopped up' by the CN- ions because the equilibrium lies to the left.

Ka = [H+][CN-] / [HCN]
The value of Ka remains the same when NaCN is added.
On adding NaCN this dissociates fully as Na+ & CN-
These will now be a large quantity of CN- ions in the solution, but only a small amount of H+ ions.
If a small quantity of acid is added, the H+ ions will reassociate with the large excess of CN- ions, hence removing H+ ions from the solution, and some of the CN- ions to form HCN in undissociated form.
So ultimately, the CN- ions are slightly less in quantity, and HCN is slightly more in quantity.

Answer 2

NaCN is a strong electrolyte as most sodium salts are.
NaCN >> Na+ + CN-
HCN is a strong acid and CN- is its conjugate base.
If we add H+ CN- will react with H+ to give HCN
The effect of the added H+ will increase the moles of HCN and will decrease the moles of CN-
pH = pKa + log [CN-]/ [HCN]

Answer 3

Hcn Nacn