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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: run-bandit on March 07, 2010, 09:06:41 pm

Title: Why is the pH of something at its equivelance point not always 7
Post by: run-bandit on March 07, 2010, 09:06:41 pm: I've been reading this exact question online but don't understand. I seem to understand when people here explain it though. Apparently only a strong acid with a strong base produces such a situation. They say that, say, a weak acid and strong base, when it reaches equilibrium in say a titration, because they become ions that are conjugate acids or bases, they will cause a change in pH in the water.

However, BOTH acid AND base will do this! And, subsequently, won't they simply cancel out once again!

All I'm hearing is circular logic!

I don't understand, could you help me?
Title: Re: Why is the pH of something at its equivelance point not always 7
Post by: slothpomba on March 07, 2010, 09:09:17 pm: I think this belongs in the chemistry section.
Title: Re: Why is the pH of something at its equivelance point not always 7
Post by: stonecold on March 07, 2010, 09:18:59 pm: strong acid + weak base = equivilance point < 7
weak acid + strong base = equivilance point > 7
stong acid + strong base = equivilance point @ 7
Title: Re: Why is the pH of something at its equivelance point not always 7
Post by: qshyrn on March 08, 2010, 12:12:53 am: Quote from: stonecold on March 07, 2010, 09:18:59 pm
strong acid + weak base = equivilance point < 7
weak acid + strong base = equivilance point > 7
stong acid + strong base = equivilance point @ 7
can you explain why?
Title: Re: Why is the pH of something at its equivelance point not always 7
Post by: Gloamglozer on March 08, 2010, 12:20:48 am: Quote from: andrewloppol on March 08, 2010, 12:06:20 am
Quote from: stonecold on March 07, 2010, 09:18:59 pm
strong acid + weak base = equivilance point < 7
weak acid + strong base = equivilance point > 7
stong acid + strong base = equivilance point @ 7

This - but not in the Biology course..

In that case, I'll move it accordingly.
Title: Re: Why is the pH of something at its equivelance point not always 7
Post by: vexx on March 08, 2010, 04:20:27 am: Quote from: qshyrn on March 08, 2010, 12:12:53 am
Quote from: stonecold on March 07, 2010, 09:18:59 pm
strong acid + weak base = equivilance point < 7
weak acid + strong base = equivilance point > 7
stong acid + strong base = equivilance point @ 7
can you explain why?

there is some reason that i read about ages ago (so hope its right) about the reaction that occurs between the acid and base (where one is strong, the other is weak), and it produces a salt, this salt completely dissociates with water to produce metal ions and non metal ions, one of which reacts with the water to create either more H+ or more OH-, which results in the overall solution being either more basic (if it was a strong base + weak acid), or more acidic (if it was a weak base + strong acid). i think we learn about this more in unit 4?
Title: Re: Why is the pH of something at its equivelance point not always 7
Post by: naved_s9994 on March 08, 2010, 09:37:19 am: [H30+] = [OH-]

Then we get it..
In most cases 7, but there are exceptions.
Title: Re: Why is the pH of something at its equivelance point not always 7
Post by: kenhung123 on March 08, 2010, 10:28:12 am: Because of the different strengths some may partially ionise and some complete ionise
Title: Re: Why is the pH of something at its equivelance point not always 7
Post by: Mao on March 08, 2010, 10:50:34 am: When an acid (HA) reacts with a base (B), the neutralization reaction is:

$HA + B \leftrightharpoons A^- + HB^+$

The equivalence point is defined that n(HA) = n(B). The explanation for $pH \neq 7$ at equivalence point [without doing any mathematics]

if HA is strong, then its conjugate base A- is neutral
if HA is weak, then its conjugate base A- is weak
if B is strong, then its conjugate acid HB+ is neutral
if B is weak, then its conjugate acid HB+ is weak

Hence, whenever you have a strong vs weak titration, you will end up with a neutral product (conjugate of the strong acid or base) and a mildly acidic/basic product. The pH of the equivalence point is affected by this mildly acidic/basic product.

A mathematical explanation would be more suited to unit 4 when acid/base equilibria is studied.

inb4 anyone asks what will happen when I react weak + weak, the answer is no one does that because it's impractical, but if anyone did, it'll depend on the relative strengths of the acid and base, and it'll be best predicted by computing the equilibrium.