Acidic or Basic: Hydrolysis of Salts

[ghostanime2001]

Homework Statement

State whether the following salts are acidic or basic in solution. Assume all salts given are soluble and 100% ionized in solution

a) KF

The Attempt at a Solution

I have one major question. How can you tell which ion is a spectator ion? because without knowing this both answers are true. The answer in the worksheet is basic. So what am I not understanding?

For example in a):

KF \rightarrow K^{+} + F^{-}

K^{+} + H_{2}O \rightleftharpoons KOH + H^{+}(The solutions is acidic - Dissociation)

F^{-} + H_{2}O \rightleftharpoons HF + OH^{-}(The solution is basic - Hydrolysis)


If the answer is basic that would mean K⁺ is a spectator ion ... WHY? and HOW do you know that? How do you figure it out?

 

[symbolipoint]

The answer in your worksheet said, "basic"; so does this tell you something about the hydrolysis of KF ? Fluoride compounds are a bit different from the other halides. "HF" is not a strong acid, or at least not as strong as the other hydrogen halides.

 

[ghostanime2001]

Do I have to look at Ka and Kb to determine if KF would make an acidic or basic solution?

 

[symbolipoint]

Do I have to look at Ka and Kb to determine if KF would make an acidic or basic solution?

That is a good idea. Look for this information about hydrofluoric acid, probably also shown as, H₂F₂.

 

[ghostanime2001]

If I wasn't given a table of Ka values let's say on an exam what else can I use to determine if KF acts as an acid or a base ?

KOH also works. What makes KF be basic and not acidic. I know only a table of Ka or Kb values would tell me that but what makes KF be basic rather than to produce a familiar compound like KOH and be acidic.

If a reaction such as this goes against my daily recognition of familiar compounds then there's something else i don't know.

&& I still do not understand why one ion would become a spectator ion. What makes anyone sure of that ? *Ill upload the worksheet here on this forum so you can see the method they used at a later time*

 

[ghostanime2001]

I think the Net ionic equation ? Maybe the ppl that wrote my worksheet assumed i already knew that and hence assumed i understood why some ion was a spectator ion perhaps ?

or

does this work most of the time and explain why KF was acidic or basic?
http://www.coolschool.ca/lor/CH12/unit4/U04L17.htm

 

[Bohrok]

If I wasn't given a table of Ka values let's say on an exam what else can I use to determine if KF acts as an acid or a base ?

KOH also works. What makes KF be basic and not acidic. I know only a table of Ka or Kb values would tell me that but what makes KF be basic rather than to produce a familiar compound like KOH and be acidic.

I would stick to using solubility rules and, especially for this problem, knowing which acids are strong or weak more than charts with Ka and Kb values.

Number 7 from the link you gave is important with KF: If the acid is strong , no hydrolysis occurs, but if the acid is weak hydrolysis does occur.
Combining some of your chemical equations from your first post:
KF + H_{2}O \rightarrow KOH + HF

HF is a weak acid, so hydrolysis occurs.

I still do not understand why one ion would become a spectator ion. What makes anyone sure of that ?

Because some ions hardly, if ever, do anything in a reaction. Another important thing to know is that the elements in Groups 1 and 2 in the periodic table generally don't react and are usually spectator ions in reactions. For the reactions where they do form something, that's where you use solubility rules. In the equation above, K⁺ and OH^- together don't form anything, and KF on the left side will ionize in water into K⁺ and F^-, so K⁺ will be a spectator ion and you can leave it out in the net ionic equation.
K^+ + F^- + H_{2}O \rightarrow K^+ + OH^- + HF
F^- + H_{2}O \rightarrow OH^- + HF (net ionic equation)

 

[ghostanime2001]

I still do not understand.

 

[symbolipoint]

THIS is what happens:

KF -----------------> K⁺ + F^- (as a simplification).

What happens to each of those in water?

K⁺ + H₂O -------------> K⁺ + H₂O (no changes here).

F^- + H₂O -------------> HF (as a simplification) + OH^-

Reminder - the compound, "HF", as written here, is a WEAK acid. It's formation from the KF dissolved in water produces hydroxide by hydrolysis, therefore making the solution alkaline, or basic.

 

[symbolipoint]

NOTE: In posts #2, 4, 9, I misunderstood the components of the fluorine-containing parts. According to a wikipedia article about hydrofluoric acid, if the aqueous solution is not close to 100%, then hydrofluoric acid is reasonably represented as HF, one formula unit of hydrogen and one formula unit of fluorine. HF it is!

 

[ghostanime2001]

THIS is what happens:

KF -----------------> K+ + F- (as a simplification).

What happens to each of those in water?

K+ + H2O -------------> K+ + H2O (no changes here).

F- + H2O -------------> HF (as a simplification) + OH-

HOW do you know "THIS is what happens"

 

[Bohrok]

The second one happens because K⁺ never reacts with H₂O or its components H⁺ or OH^-.
The third one happens because F^- will always combine with H⁺ to form HF, a weak acid. Weak acids always want to form as opposed to strong acids, like HCl.

If you want to know why a weak acid like HF will form, then it might be better to say that if we see that a particular acid always wants to form (we may not know why any acid does it exactly) we can just say that it will always do that and call it a weak acid.