Why Does CN- Cause pH to Increase in Hydrolysis?

  • Thread starter Thread starter april.burials
  • Start date Start date
  • Tags Tags
    Concept
Click For Summary

Discussion Overview

The discussion revolves around the hydrolysis of sodium cyanide (NaCN) in water and its effect on pH. Participants explore the underlying chemistry of hydrolysis, the behavior of the cyanide ion (CN−), and the implications for pH changes in solution.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant questions why CN− hydrolyzes and expresses confusion about the hydrolysis process, suggesting that Na+ might also hydrolyze.
  • Another participant explains that CN− hydrolyzes to form HCN, a weak acid, which decreases H+ concentration in solution and increases pH.
  • It is noted that Na+ does not hydrolyze because it is part of a strong base (NaOH), which does not favor hydrolysis.
  • A participant mentions that in dilute solutions, a significant portion of cyanide can exist as HCN due to hydrolysis, contributing to a basic solution.
  • There is a request for clarification on the relationship between the weak acid HCN and the hydrolysis process, as well as how to calculate the percentage of HCN in solution.

Areas of Agreement / Disagreement

Participants express differing views on the nature of hydrolysis and its implications. While some agree on the role of CN− and HCN, there is uncertainty regarding the extent of hydrolysis and the behavior of Na+ in solution.

Contextual Notes

Participants discuss theoretical aspects of hydrolysis and its practical implications, highlighting limitations in their understanding of the calculations involved, particularly regarding the dissociation constant (Ka) and the percentage of HCN formed.

Who May Find This Useful

This discussion may be useful for students studying chemistry, particularly those interested in acid-base reactions, hydrolysis, and the behavior of weak acids and bases in solution.

april.burials
Messages
4
Reaction score
0
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.
 
Physics news on Phys.org
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.
 
Thanks so much for your reply! I think I keep forgetting that this is only theoretical.
 
Not sure what you mean - hydrolysis is as real as anything can be.

--
methods
 
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?
 
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).

--
methods
 
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%?
 

Similar threads

The role of diet in human evolution
  • · Replies 24 ·
Replies
24
Views
9K
Minimizing the Energy of Two Conductors Very Far Apart
  • · Replies 23 ·
Replies
23
Views
2K
Graduate Does kinetic energy increase the inertial mass?
  • · Replies 11 ·
Replies
11
Views
2K
High School Why doesn't cosmic inflation violate conservation of energy?
  • · Replies 3 ·
Replies
3
Views
4K
Undergrad Distinctions among the concepts: mass, matter, energy
  • · Replies 1 ·
Replies
1
Views
2K
High School The Drunkard's Walk: A Discussion of the Text
  • · Replies 17 ·
Replies
17
Views
4K
Why is the sun's gravitational pull on the moon greater than the Earth's?
  • · Replies 3 ·
Replies
3
Views
3K
Undergrad When should we take numerology seriously?
  • · Replies 19 ·
Replies
19
Views
7K
Does having children make you happy?
  • · Replies 93 ·
4
Replies
93
Views
16K
Graduate Why Does Gravity Around a Black Hole Remain Normal Until Extremely Close?
  • · Replies 2 ·
Replies
2
Views
3K