Why is the pH of something at its equivelance point not always 7

[run-bandit]

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?

[slothpomba]

I think this belongs in the chemistry section.

[stonecold]

strong acid + weak base = equivilance point < 7
weak acid + strong base = equivilance point > 7
stong acid + strong base = equivilance point @ 7

[qshyrn]

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?

[Gloamglozer]

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.

[vexx]

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?

[naved_s9994]

[H30+] = [OH-]

Then we get it.. 
In most cases 7, but there are exceptions.

[kenhung123]

Because of the different strengths some may partially ionise and some complete ionise

[Mao]

When an acid (HA) reacts with a base (B), the neutralization reaction is:



The equivalence point is defined that n(HA) = n(B). The explanation for 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.