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buffering effect - what is the cause of this in this solution

Submitted by Anonymous (not verified) on Sun, 03/08/2009 - 12:09

in this solution what is responsible for the buffering effect?

Al(OH3)(s) + 3HCl(aq) ? AlCl3(aq) + 3H2O(l)   

is it hydrogen and hydroxide?

Al(OH3) is a weak base and HCl is a strong acid, an acidic solution will be formed in water.  is this right??and why???
why acidic..? the ions affect the equilibrium of the water.. but why is an acidic solution formed in the water?


The aluminum ion reacts with the H+ to form a complex ion: Al(H2O)6 +3

The hexahydrate Aluminun can then react with water to form hydronium ion:
             Al(H2O)6 +3  + 2 H2O  --> Al(H2O)4(OH)-   +  2 H3O+


why does this happen??
i thought it was because the ions present in aluminum chloride affect the neutral equilibrium of water.
what do the complex ions do? and why is a hydronium ion formed and what does that do?

spock wrote:

The aluminum ion reacts with the H+ to form a complex ion: Al(H2O)6 +3

The hexahydrate Aluminun can then react with water to form hydronium ion:
             Al(H2O)6 +3  + 2 H2O  --> Al(H2O)4(OH)-   +  2 H3O+


Oops, left out a 2 on the subscript for the hydroxide ion.  Should be
   Al(H2O)6 +3  + 2 H2O  --> Al(H2O)4(OH)2-   +  2 H3O+

The Al+3 ion has a coordination number of 6, which means it usually is found with 6 ligands bonded to it.

Al(OH)3 is really Al(OH)3(H2O)3.  It is a complex ion which means the Al+3 ion is surrounded by 3 water molecules and 3 hydroxide ions.  Al(H2O)3(OH)3 is an amphoteric substance. That means it can act as either an acid or a base.

If the Al(OH)3 accepts  3 H+ ions (which it would do if mixed with HCl), it forms the Al(H2O)6+3 ion.  This would be what happens in your question where the AlCl3 solution is reacted with HCl.  A small number of the resulting Al(H2O)6+3 ions then will act as weak acids and donate one or more of their protons to water molecules making the solution acidic

If Al(OH)3 reacts with a strong base, one of the attached water molecules will give up a proton and it forms Al(H2O)2(OH)4-.  Treating Al(OH)3 with a base would result in a slightly basic solution.

Explaining why this happens would require a more detailed explanation then I am able to give.  But the bottom line is that is the way Al(OH)3 reacts.


But is this still correct about the equilibrium and pH??
large numbers of Al3+(aq)  removed OH- from the water, and the number of H+ increased significantly and the pH dropped

or is that totally off subject?

your explanation is why the acidic solution is formed in water right??

and you say that its Al(H2O)4(OH)2- and then Al(H2O)3(OH)3.... which one is it?

and also.. what is responsible for the buffering effect in the solution??

thannkss!!

spock wrote:

Oops, left out a 2 on the subscript for the hydroxide ion.  Should be
   Al(H2O)6 +3  + 2 H2O  --> Al(H2O)4(OH)2-   +  2 H3O+

The Al+3 ion has a coordination number of 6, which means it usually is found with 6 ligands bonded to it.

Al(OH)3 is really Al(OH)3(H2O)3.  It is a complex ion which means the Al+3 ion is surrounded by 3 water molecules and 3 hydroxide ions.  Al(H2O)3(OH)3 is an amphoteric substance. That means it can act as either an acid or a base.

If the Al(OH)3 accepts  3 H+ ions (which it would do if mixed with HCl), it forms the Al(H2O)6+3 ion.  This would be what happens in your question where the AlCl3 solution is reacted with HCl.  A small number of the resulting Al(H2O)6+3 ions then will act as weak acids and donate one or more of their protons to water molecules making the solution acidic

If Al(OH)3 reacts with a strong base, one of the attached water molecules will give up a proton and it forms Al(H2O)2(OH)4-.  Treating Al(OH)3 with a base would result in a slightly basic solution.

Explaining why this happens would require a more detailed explanation then I am able to give.  But the bottom line is that is the way Al(OH)3 reacts.


nevermind about this comment :P....
and you say that its Al(H2O)4(OH)2- and then Al(H2O)3(OH)3.... which one is it?


A buffer is a mixture of a weak acid and it's conjugate base.  A buffer must have BOTH an acid and a base present.

When you react the HCl with the Al(OH)3 you form Al(H2O)6+3 ions.
In the previous post I explained how this ion acts as acid.

Some of the Al(H2O)6+3 donate their protons and form Al(H20)4(OH)2- ions. This is a weak base.

A buffer works by resisting a change in pH as a result of adding an acid or a base.  The acid portion of the buffer is capable of neutralizing addition base, while the basic protion of the buffer is capable of neutralizing additional acid

So, in your solution you will have BOTH the weak acid Al(H2O)6+3 and the weak base Al(H2O)4(OH)2- ions thus creating a buffer.


ok sounds good thanks!
but still.. does this have anything to do with the reaction....
large numbers of Al3+(aq)  removed OH- from the water, and the number of H+ increased significantly and the pH dropped.

and also what is the significance of the hydronium ion.. is it that its the ion that makes the solutiong acidic or?

spock wrote:

A buffer is a mixture of a weak acid and it's conjugate base.  A buffer must have BOTH an acid and a base present.

When you react the HCl with the Al(OH)3 you form Al(H2O)6+3 ions.
In the previous post I explained how this ion acts as acid.

Some of the Al(H2O)6+3 donate their protons and form Al(H20)4(OH)2- ions. This is a weak base.

A buffer works by resisting a change in pH as a result of adding an acid or a base.  The acid portion of the buffer is capable of neutralizing addition base, while the basic protion of the buffer is capable of neutralizing additional acid

So, in your solution you will have BOTH the weak acid Al(H2O)6+3 and the weak base Al(H2O)4(OH)2- ions thus creating a buffer.


and also is the chemical state of all these reactons aq?
  Al(H2O)6 +3  + 2 H2O  --> Al(H2O)4(OH)2-  +  2 H3O+

spock wrote:

A buffer is a mixture of a weak acid and it's conjugate base.  A buffer must have BOTH an acid and a base present.

When you react the HCl with the Al(OH)3 you form Al(H2O)6+3 ions.
In the previous post I explained how this ion acts as acid.

Some of the Al(H2O)6+3 donate their protons and form Al(H20)4(OH)2- ions. This is a weak base.

A buffer works by resisting a change in pH as a result of adding an acid or a base.  The acid portion of the buffer is capable of neutralizing addition base, while the basic protion of the buffer is capable of neutralizing additional acid

So, in your solution you will have BOTH the weak acid Al(H2O)6+3 and the weak base Al(H2O)4(OH)2- ions thus creating a buffer.