Romix
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Dissolution of NaOH in H2O
Why dissolution of NaOH in H2O is exothermic?
What happening when NaOH dissolves in H2O?
And what happens when Al reacts with this solution?
Diluting solution of sodium aluminate precipicates Aluminun hydroxide? Correct?
Once I heated AlCl3, it decomposed, I smelled HCl.
[Edited on 21-6-2015 by Romix]
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kecskesajt
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Strong base / Strong acid protonates the water and thats exothermic.I can't write down in english but the energy in the molecules are raising.If this
makes any sense.Where was you when the class studied about this.We had first chem in 7th class and we studied in 8th.
[Edited on 20-6-2015 by kecskesajt]
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papaya
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Then I want to ask why dissolution of KOH in water is not that exothermic, if it's exothermic at all (from my experience)?
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vmelkon
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The process is called solvation.
https://en.wikipedia.org/wiki/Solvation
When the solvent has a dipole (one side has a + charge and the other side has a - charge), it will dissolve ionic compounds like NaOH.
At first, energy is required to break away individual ions from the solid NaOH lattice. Individual Na+ are pulled out of the solid NaOH lattice.
Individual OH- are also pulled out of the solid NaOH lattice.
When the Na+ get hydrated, energy is released. I guess OH- is also hydrated and energy is released. The energy release feedbacks into step 1 and more
Na+ and OH- are pulled out.
As for the NaOH vs KOH energy release issue, they think that the ratio of charge to cation size has relevance. The bigger the cation is, the less
attraction results between the cation and the H2O because of the shielding effect of the many electron orbitals.
Interestingly, this is also the reason why as you go down the periodic table, there is less and less non-metals. The last orbital electrons are more
weekly attracted to the nucleus and so they are free to roam in the solid, thus giving metallic properties to the element.
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kecskesajt
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Wikipedia says that the dissolution of KOH in water is highly exothermic.
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blogfast25
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Quote: Originally posted by papaya  | | Then I want to ask why dissolution of KOH in water is not that exothermic, if it's exothermic at all (from my experience)? |
Unless you've compared e.g. NaOH and KOH back-to-back ('all other things being equal') that's a useless statement. Or at least have some data from a
controlled experiment: dissolving a very small amount of KOH in a lot of water e.g. won't have much effect.
Dissolution of KOH certainly is exothermic.
| Quote: Originally posted by vmelkon |
As for the NaOH vs KOH energy release issue, they think that the ratio of charge to cation size has relevance. The bigger the cation is, the less
attraction results between the cation and the H2O because of the shielding effect of the many electron orbitals.
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Yes, theory predicts that the lattice energy of ionic compounds where there is a strong difference in cation/anion size is smaller. There are many
examples of highly soluble compounds made up of small cations and very large anions. Large cations are of course less numerous.
When dissolving an ionic compound firstly the lattice energy needs to be overcome (energy has to be expended to dismantle the lattice, this energy in
endothermic). If solvation enthalpy (exothermic) is larger than lattice energy, then the overall process will be exothermic (see NaOH and KOH).
Sometimes (e.g. KCl) dissolution is endothermic: that's because entropy ALWAYS favours dissolution.
BTW, acc. this source:
http://sites.chem.colostate.edu/diverdi/all_courses/CRC%20re...
The dissolution of KOH is slightly more exothermic than that of NaOH.
[Edited on 20-6-2015 by blogfast25]
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blogfast25
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| Quote: Originally posted by Romix |
And what happens when Al reacts with this solution?
Diluting solution of sodium aluminate precipicates Aluminun hydroxide? Correct?
Once I heated AlCl3, it decomposed, I smelled HCl.
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Strongly alkaline solutions dissolve Al to aluminate and hydrogen gas:
Al(s) + OH<sup>-</sup>(aq) + 3 H<sub>2</sub>O(l) === > Al(OH)<sub>4</sub><sup>-</sup>(aq) + 3/2
H<sub>2</sub>(g)
On very strong dilution such aluminate solutions precipitate Al(OH)<sub>3</sub>. The Al(OH)<sub>3</sub> can be redissolved in
more alkali or dissolved in acid (Al is so-called 'amphoteric').
AlCl<sub>3</sub> only sheds HCl on heating when it's a hydrate, due to hydrolysis. Anhydrous AlCl<sub>3</sub> simply sublimes
(> 180 C).
[Edited on 20-6-2015 by blogfast25]
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Romix
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Google says that hydrogens on the water molecule are relatively positive and are atracted to the OH- ions. Oxygen end of water molecule is relatively
negative and attracted to Na+.
Why?
Hydrogens 1s1 have 1 proton with + charge, and 1 electron with - charge which it covalently shares with oxygen to form water molecule.
Oxygen have 8 protons +, 8 electrons -, 2 in first shell, 6 in the second. 1s2 2s2 2p4
[Edited on 21-6-2015 by Romix]
[Edited on 21-6-2015 by Romix]
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blogfast25
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| Quote: Originally posted by Romix | Google says that hydrogens on the water molecule are relatively positive and are atracted to the OH- ions. Oxygen end of water molecule is relatively
negative and attracted to Na+.
Why?
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Simple electrostatic attraction.
http://www.gcsescience.com/pse3-electrostatic-charge-attract...
The H-O bond in water molecules is permanently polarised because O is far more electronegative than H. H thus carries a partial positive electrical
charge, O a partial negative one.
[Edited on 21-6-2015 by blogfast25]
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