Buffers? NaHCO3 and H2CO3..

Question

i'm supposed to use NaHCO3 and H2CO3 (a buffer sometimes added to swimming pools) to explain how buffers work, i've read the book and i know that it keeps the pH pretty much the same but how would i explain it though? i'm not sure how it works...

Answer 1

For our buffer, we have

HCO3- + H3O+ <---> H2CO3 + H2O

Think about the equilibrium.  Then think about the shift if small amounts of either strong acid or strong base were added, using Le Chatelier's principle to help you out.

We know that

[H3O+] = Ka(acid) * [acid/base]

Say we added some strong acid (e.g.: HCl).  This would cause the equilibrium to shift right, thus increasing the [ ] H2CO3.  However, because only a small amount of strong acid was added, according to the equation above, the ratio between acid and base will still be roughly the same as before and thus, [ H3O+] and consequently, the pH of solution will remain fairly constant.  Same thing if a strong base (e.g.: NaOH) was added.

Try to figure it out from there.

[Answer: see above]

Answer 2

1. an ideal buffer mix of those two would be a 50:50 mix

the important part to consider is that acid used in a buffer is a "weak acid", one which has released very few of its H+ ion. In this way, most of the H2CO3 molecules are still intact when dissolved, for example a 1 molar solution:

H2CO3 (99.9992%)<--> H+ & HCO3-

as a buffer, if any base is added, the added OH- ions remove the H+ ion, but the tremendous reserve of H+ ions within the H2CO3 are released to restore the H+ ions as fast as the base destroys them, until the base is consun=med & the approximate H= ion level is restored. you would not have had any H+ ions in reserve if you had used a strong acid to make the buffer

to improve the HCO3- level, a buffer has lots of the salt made from the acid, added. NaHCO3 provides tremendous amounts of HCO3- ions for the buffer's equilibrium, remember 50:50 is ideal. so if H2CO3 is 1 molar , we would like it if NaHCO3 is 1Molar:

H2CO3 (1Mlar)<--> H+ & HCO3- (1 Molar)

now, if anyone adds an acid, the added HCO3- will remove it & convert it into H2CO3, not allowing the H+ level to increase much at all

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so a buffer has two componets , one to destroy added bases ,& one to destroy added acids. a 50:50 is said to be ideal, because it has as much capacity to destroy any added bases as it has capacity to destroy any added acids

in real life, though the salt adding the negative ion in common with the acid, is added until you get the buffer pH that you want even if it takes 60/40 or 70/30, whatever