Question #eb54b

1 Answer
Dec 7, 2015

The answer is (b) decreased


Hydrocyanic acid, #"HCN"#, is a weak acid, which means that it does not dissociate completely in aqueous solution to form hydronium cations and cyanide anions.

More precisely, an equilibrium will be established between the unionized molecule and the two resulting ions.

#"HCN"_text((aq]) + "H"_2"O"_text((aq]) rightleftharpoons "H"_3"O"_text((aq])^(+) + "CN"_text((aq])^(-)#

On the other hand, nitric acid, #"HNO"_3#, is a strong acid, so it will ionize completely in aqueous solution to form hydronium cations and nitrate anions

#"HNO"_text(3(aq]) + "H"_2"O"_text((aq]) -> "H"_3"O"_text((aq])^(+) + "NO"_text(3(aq])^(-)#

Now, notice that adding the nitric acid to the hydrocyano acid solution will increase the concentration of hydronium ions significantly.

As you know, chemical equilibria are governed by Le Chatelier's Principle, which states that a chemical equilibrium will react to a disturbance by shifting in the direction that will counteract that disturbance.

In this case, the disturbance in the increase in the concentration of hydronium ions. As a result, the equilibrium reaction will shift to the left, favoring the formation of hydrocyanic acid and water.

This shift will allow for the excess hydronium concentration to be consumed.

#"HCN"_text((aq]) + "H"_2"O"_text((aq]) rightleftharpoons "H"_3"O"_text((aq])^(+) + "CN"_text((aq])^(-)#

#color(white)(xxxxxxxxxx)color(red)("shift to the left")#

Since the reverse reaction is now favored, the concentration of cyanide anions will decrease and the concentration of unionized hydrocyanic acid will increase.