[Sheikh {/} Nykaa]

The aqa A2 text book really sucks! so I'm still unclear of how a basic buffer resists changes in ph...any help would be much appreciated

[killa78]

(Original post by Sheikh {/} Nykaa)
The aqa A2 text book really sucks! so I'm still unclear of how a basic buffer resists changes in ph...any help would be much appreciated

http://alevelchem.com/aqa_a_level_ch...4/s3403/05.htm

Buffers are confusing, but as long as you understand the reversible reactions, you should be fine.

[arvin_infinity]

(Original post by Sheikh {/} Nykaa)
The aqa A2 text book really sucks! so I'm still unclear of how a basic buffer resists changes in ph...any help would be much appreciated

not 100% sure but v v similar if not identical to acid buffers!

weak base+ its salt!

I guess you use kb for that

[clownfish]

Taking ammonia/ammonium chloride as an example:

NH₃ + H⁺ <---> NH₄⁺

and

NH₄Cl ---> NH₄⁺ + Cl-

So there is far more ammonia and ammonium ion than H+.



If acid is added it will combine with ammonia:
NH₃ + H⁺ ---> NH₄⁺

If base is added it will combine with ammonium:
NH₄⁺ + OH- --> NH₃ + H₂O


The net of both of these equations is the concentration of H+ does not change, and hence the pH is not altered.

[chemicalX]

Buffer solutions are solutions that resist change in Hydronium ion and the hydroxide ion concentration (and consequently pH) upon addition of small amounts of acid or base, or upon dilution. Buffer solutions consist of a weak acid and its conjugate base (more common) or a weak base and its conjugate acid (less common). The resistive action is the result of the equilibrium between the weak acid (HA) and its conjugate base (A−):

HA(aq) + H2O(l) → H3O+(aq) + A−(aq)

Any alkali added to the solution is consumed by the Hydronium ions. These ions are mostly regenerated as the equilibrium moves to the right and some of the acid dissociates into Hydronium ions and the conjugate base. If a strong acid is added, the conjugate base is protonated, and the pH is almost entirely restored. This is an example of Le Chatelier's principle and the common ion effect. This contrasts with solutions of strong acids or strong bases, where any additional strong acid or base can greatly change the pH. This may be easier to see by comparing two graphs when an strong acid is titrated with a strong base the curve will have a large gradient throughout showing that a small addition of base/acid will have a large effect compared to a weak acid/strong base titration curve which will have a smaller gradient near the pKa.
Titration of a weak acid with a strong base the flat region at pKa is the buffering region.
Titration of a weak acid with a strong base the flat region at pKa is the buffering region.
Titration of a strong acid with a strong base. Note the sharp rise in pH: this solution can not buffer.
Titration of a strong acid with a strong base. Note the sharp rise in pH: this solution can not buffer.

When writing about buffer systems they can be represented as salt of conjugate base/acid, or base/salt of conjugate acid. It should be noted that here buffer solutions are presented in terms of the Brønsted-Lowry notion of acids and bases, as opposed to the Lewis acid-base theory (see acid-base reaction theories). Omitted here are buffer solutions prepared with solvents other than water.


Source(s):
http://en.wikipedia.org/