Given the following information: nitrous acid = HNO2 hydrocyanic acid = HCN HNO2

Question

Given the following information:

nitrous acid = HNO2

hydrocyanic acid = HCN

HNO2 is a stronger acid than HCN

Write the net ionic equation for the reaction that occurs when equal volumes of 0.054 M aqueous nitrous acid and sodium cyanide are mixed. It is not necessary to include states such as (aq) or (s).

{color{Red}OK. I know the answer is: }

{color{Red}My question is why cant you write it in reverse (it is an equilibrium reaction afterall) }

{color{Red}If you write it in the reverse reaction the homework says its wrong, and of course this could adversely affect later calculations, such as KSP. So, my question is HOW do you know which to put as reactants and which to put as products? }

Explanation / Answer

in this solution

there two acids

HN02 and HCN

we know that

HN02 is a stronger acid

so it reacts with water

HN02 + OH- ---> H20 + N02-

now

there are two bases

N02- and CN-

as HCN is a weak acid than HN02

for their conjugates , it will reverse

so

CN- will be a strong base than N02-

so

CN- will react with H+

the reaction is

CN- + H+ ---> HCN

also

consider the dissociation of water

H20 ---> H+   + OH-

now

put all the reactions together

HN02 + OH- ---> H20 + N02-

CN- + H+ ---> HCN

H20 ---> H+ + OH-

add all the reactions

we get

HN02 + CN- ---> N02- + HCN

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