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I'm not an expert teacher or lecturer of chemistry. I was only a student from SMA NEGERI 15 SURABAYA who had been one of the Bronze Medalist Participants of Olimpiade Sains Nasional X (2011) of Chemistry In Manado, North Sulawesi, 11 - 16 September 2011 and graduated in 2012. Now, I'm studying at Universitas Airlangga in Surabaya, Indonesia. I do love chemistry and I would like to help them who had difficulties in studying chemistry. That's why, please understand me if you found some misconcepts in my entries. Suggestions are always necessary in order to develop this blog. And I'm sorry because my English isn't so well.

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Monday, December 19, 2011

Hydrolysis of Salts

We know that not all salts are really a neutral salt. The most popular neutral salt we know is Sodium Chloride (NaCl). Why it is called as a neutral salt? And why there are many un-neutral salts? Let's remember this: Water is a beautiful and powerful solvent. Also, In this post, the activities are neglected.

Sodium Chloride when diluted, is ionized to become Sodium ions and Chloride ions. The electrostatic force that can attract them is blocked by the solvation of the ions with water molecules. See the picture below:



Source: http://guweb2.gonzaga.edu/faculty/cronk/chemistry/images/aquated_ions.jpg

Now let's take another example. When we diluted Sodium acetate in water, the acetate ions not only solvated but also hydrolyzed to form Acetic Acid and Hydroxide Ions. This is the reason why sodium acetate solution has the pH above 7.

Why Acetate ions hydrolyzed but chloride ions not?
Acetic acid is a weak acid, and acetate ions are a strong conjugate base. This ability makes those ions can attract with the hydrogen atoms of water and make a bonding with it so that the acetic acid is formed.
Meanwhile, Chloride acid is a strong acid, and chloride ions are a weak conjugate base so that these ions are tend to not attract the hydrogen atoms of water. It just solvated not hydrolyzed.

In Senior High School, we know there is a formula to calculate the pH of Hydrolyzed Salts. Take an example: Sodium Acetate.

My question is: Where does it come from?
Now let's answer my question.

See the picture below:


PS: Only in this post, means equilibrium reaction, not a resonance structure.

We found an equilibrium reaction, so that there is an equilibrium constant (that is not neglected).

Rearrange that formula and we get:

The concentration of acetic acid is the same with hydroxide ions because the acid doesn't have initial concentration so that the concentration can be concluded = .

Is it over? Not yet. See the reactions below:


Remember that acetic acid can be ionized in water with Ka about 1.8 x 10-5 at RTP and remember that water can be autoionized to become Hydronium ions and Hydroxide ions.

From the description above, we got Ka and Kw. Now see the picture below:


The relation between Ka, Kw, and Kh is:


Now we know where this formula below comes from:


pH of Base-Salts

With the same way, we found the formula to calculate the pH of Acid-Salts (Example: Amonnium Sulfate):



How to calculate the pH of Salts which are its cations and anions can be hydrolyzed by water? Take an example: Ammonium acetate.







We already discuss 3 types of Hydrolyzed Salts, and for the last what if a complex ion hydrolyzed in water?

Take an example: Aluminium ions in water will form Hexaaquo aluminium ions.



Hexaaquo Aluminium Ions
(Source: http://lcta.unige.ch/~devito/images/complexe_hexaaquo.gif)

The electron of ligans will be attracted to the cation because of the higher oxidation number of Aluminium. This phenomenon makes the O - H bonds getting weaker so that it easily to be separated and the complex ions become Pentaaquo hydroxo aluminium ions.



The hydrogen ions formed will be attracted to water molecules to form Hydronium Ions that make the pH of solution to become lower than 7. We can judge there's no different between this Hydrolysis of Complex Ion phenomenon and Hydrolysis Base Salts phenomenon. That's true but the reason is different, that's the important one.


Further Reading:
Wikipedia
Riyanto, Nurdin dkk. 2009. Super Genius Olimpiade Kimia SMA. Yogyakarta: Pustaka Widyatama