[jennielynn_1980]

Use LeChatelier's Principle to explain why the pH of a hydrogen phosphate buffer system remains constant when:
a) small amounts of acid are added
b) small amounts of base are added

The equation is:

H₂PO₄^-(aq) + H₂O(l) <--> H₃O⁺(aq) + HPO₄^2-

I know that Le Chatelier says that the equilibrium system will respond to stress by restoring equilibrium.  So if an acid is added the equilibrium will shift left to accomodate the additional H+ and if a base is added the equilibrium will shift right to accomodate the OH- ions.  The part I don't get is how the pH will stay the same. 

Thanks

[Yggdrasil]

If you add acid and the equilibrium shifts left, what happens to the acid?

If you add base and the equilibrium shifts right, what happens to the base?

The answer is simpler than you probably think.

[jennielynn_1980]

The concentrations increase?

[Yggdrasil]

If you add acid, the reaction shifts left as you mentioned.  So that means that it will favor the removal of products and production of reactants.  Since H₃O⁺ is a product, it will mostly be removed when the reaction proceeds to the left in order to reach equilibrium.  Therefore, with the addition of acid, [H₃O⁺] raises only slightly.

[jennielynn_1980]

So how does the pH manage to stay the same?  Is it because the equilibrium doesn't shift THAT much when an acid or base is added or is it because the question says that only small amounts of acid and base are added?  I guess I really don't understand how a buffer system works. 

[Yggdrasil]

Remember that pH depends on [H₃O⁺].

Now assume you add acid (HA) to pure water.  All that acid will react with water to form H₃O⁺ and lowerthe pH:

HA + H₂O --> H₃O⁺ + A^-

However, lets say you have a buffer present.  The acid will still react with the water, but most of the H₃O⁺ which is generated will be neutralized by the buffer:

H₃O⁺ + HPO₄^2- --> H₂O + H₂PO₄^-

(remember that when acid is added the backward reaction will be the predominant reaction)
So, [H₃O⁺] does not fall as much when small amounts of acid are added to a buffered system.  The pH will still fall, but not by as much as in an unbuffered system.

[enahs]

I did a quick search, and by a quick inspection this site looks like it explains it quite well:
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Buffer/Buffer.html
It talks about not only buffer systems (a little down the page), but how they work in the body (blood)!

Hope it helps, after you read that and are still unsure ask more please!

[Donaldson Tan]

So how does the pH manage to stay the same?  

Negligible change in pH

[jennielynn_1980]

A great big thanks   I finally get it.

[jennielynn_1980]

Here is my answer to the question.  Just wondering if I get the base part as well.  Thanks

H₂PO₄^-(aq) + H₂O(l) <--> H₃O⁺(aq) + HPO₄^2-
a) If a small amount of acid is added, the reaction will shift to the left meaning the formation of reactants will be favoured.  When the acid dissociates, most of the H₃O+ generated will be neutralized by the buffer.  In this case, the hydrogen ion will be used to create more H₂PO₄-.  The will cause the pH to say relatively the same because the concentration of H+ will not increase drastically.

H₃O+ + HPO₄^2- --> H₂O + H₂PO₄-

b) If a small amount of base is added, the reaction will shift right, meaning the formation of products will be favoured.  When the base becomes dissociated, the hydroxide ions (OH-) will be used up in the reaction to create more HPO₄^2- and thus the blood is neutralized and the pH stays the same.

OH-(aq) + H₂PO₄-  --> H₃O+ + HPO₄^2-

[Yggdrasil]

OH-(aq) + H₂PO₄-  --> H₃O+ + HPO₄^2-

Are you sure this is the right equation?

[jennielynn_1980]

Here is my answer to the question.  Just wondering if I get the base part as well.  Thanks

H₂PO₄^-(aq) + H₂O(l) <--> H₃O⁺(aq) + HPO₄^2-
a) If a small amount of acid is added, the reaction will shift to the left meaning the formation of reactants will be favoured.  When the acid dissociates, most of the H₃O+ generated will be neutralized by the buffer.  In this case, the hydrogen ion will be used to create more H₂PO₄-.  The will cause the pH to say relatively the same because the concentration of H+ will not increase drastically.

H₃O+ + HPO₄^2- --> H₂O + H₂PO₄-

b) If a small amount of base is added, the reaction will shift right, meaning the formation of products will be favoured.  When the base becomes dissociated, the hydroxide ions (OH-) will be used up in the reaction to create more HPO₄^2- and thus the blood is neutralized and the pH stays the same.

OH-(aq) + H₂PO₄-  --> H₂O + HPO₄^2-

Oops, I didn't mean to do that.  Is this correct now?

[Yggdrasil]

Looks good now.

[english]

Also, what I found that usually helped me was to think of what "buffer" means.  Apply this terminology to what an acid/base buffer does, and you've got your answer.

A buffer essentially resists changes, in any context.