Why Does CN- Cause pH to Increase in Hydrolysis?

[april.burials]

On our study guide, our teacher gave us some problems. This one baffles me for some reason.

1. When solid NaCN is added to water, the pH ________________.

a.remains at 7
b.becomes greater than 7 because of the hydrolysis of Na+
c.becomes less than 7 because of the hydrolysis of Na+
d.becomes greater than 7 because of the hydrolysis of CN−
e.becomes less than 7 because of the hydrolysis of CN−

Answer: d. becomes greater than 7 because of the hydrolysis of CN−


He gave us the answers as well, but I'm having a hard time conceptualizing this.

Why does CN hydrolyze? I thought hydrolysis was the splitting of H2O, and obviously Na got a good chuck of that as well. Didn't Na hydrolyze too?


Obviously, the equation goes:

NaCN + H2O => NaOH + HCN


but that doesn't help me.

 

[Bohrok]

There is a more technical answer, but this should help:

CN^- hydrolyzes to form its conjugate acid, HCN, because HCN is a weak acid; weak acids want to form, unlike strong acids. Because the CN^- picks up H⁺ ions, it decreases the amount of H⁺ in solution and so increases the pH.

Na⁺ doesn't exactly pick up OH^- because NaOH is a strong base. Like strong acids, strong bases don't want to form. So the Na⁺ doesn't hydrolyze. Your equation is correct, but it doesn't really happen like that in real life.

 

[april.burials]

Thanks so much for your reply! I think I keep forgetting that this is only theoretical.

 

[Borek]

Not sure what you mean - hydrolysis is as real as anything can be.

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methods

 

[april.burials]

I didn't mean Hydrolysis wasn't real! Rather, I meant that salts and water don't really react to make acid and a base. Right?

 

[Borek]

Sure they do, in 0.001M solution of KCN over 13% of the cyanide is in the form of HCN - that is, it reacted with water producing acid and OH^- (hence solution is basic, with pH slightly over 10).

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methods

 

[april.burials]

I must have misunderstood my teacher. Is that only because HCN is a weak acid? I'm kind of weak when it comes to math.

How would you go backwards with the Ka = [H][A]/[HA] equation? Or, how do you come up with 13%?