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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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Friday, April 27, 2012

Hydrolysis of Big-Charge Cations

There's another reason why Al(NO3)3 has the value of pH < 7 in its solution. You will think the reason of it is Al3+ will be hydrolyzed and release H+ ions. No, that is not the real reason that makes a lower pH of Al(NO3)3 solution. Al3+ will form a complex ion with water first in its solution [Al(H2O)6]3+. These ligand that make a lower pH. See the figure as follows as the illustration:

 
(Source: http://www.wou.edu/las/physci/ch412/hydrxn2.jpg)


Especially, the presence of the hydronium ions that make a lower pH of the solution. Now can you explain which cation has the lowest pH in its solution: Co2+ or Fe3+ ?

Mostly, the coordination number of a complex ion is 2 times the oxidation number of the cation. So that we can assume that the only species that exist in Co2+ solution is [Co(H2O)4]2+ and [Fe(H2O)6]3+ for Fe3+ ion. Because the amount of H2O of Fe3+ is greater than Co2+, it makes the presence of hydronium ions of Fe3+ greater than Co2+ too so that the pH value of Fe3+ < Co2+.

Thank you.

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