18.3 pH Curves

What does this mean?

1. Complete the equilibrium for Propanoic acid in water

CH₃CH₂COOH ⇌ _________ + H⁺

What is the name of the missing ion above? _________________

When a weak acid dissolves in water most of the acid is dissociated/undissociated.

If someone added a small amount of Alkali all the H⁺ ions in the water would be used up, but the [H⁺] wouldn’t change much because _____________________________________________________

_____________________________________________________

So the pH would ________________________________________

Weak Acid equilibriums are naturally buffered in the forward/backward direction.

1. Complete the equilibrium for Propanoic acid in water

CH₃CH₂COOH ⇌ CH₃CH₂COO^- + H⁺

When a weak acid, HA, dissolves in water a low/high concentration of A^- ions is produced.

If someone added a small amount of Acid all the A^- ions in the water would be used up, and the [H⁺] would change a lot because _____________________________________________________

_____________________________________________________

So the pH would ________________________________________

Weak Acid equilibriums are not naturally buffered in the forward/backward direction.

To overcome this problem we could add, in this case, a some CH₃CH₂COO^-Na⁺ or CH₃CH₂COO^-K⁺.

What would we call these two substances?

_________________________ & _________________________

Now, if someone added a small amount of Acid there is so many A^- ions in the water that the extra [H⁺] would be used up by _____________________________________________________

_____________________________________________________

So the pH would ________________________________________

And we have successfully made a buffer.

We could add, some Lithium Propanoate, or any other Propanoate salt of course.

But what if we don’t have any Propanoate salts?

We make one!

If we carefully add some Sodium Hydroxide we would create a reservoir of _____________________.

We would just have to be careful not to completely _____________ all the Propanoic Acid because we need a reservoir of un-dissociated acid in case someone adds some _________ to the buffer.

If we slowly neutralize a strong acid with a strong base, or vice versa, we get graphs that look like this.

If you’re asked to sketch a strong-acid/strong-base curve.

· You should pick a sensible starting pH – eg 1

· You can assume the pH will gradually change because _______

______________________________________________________________________________________________________

· When the H⁺ ions are becoming less concentrated (getting closer to neutralisation) the pH will change much more markedly because ____________________________________________

___________________________________________________

· Once all the H⁺ ions have been used up the graph shoots up past 10 because __________________________________________

___________________________________________________

· The Equivalence Point is the point where there are effectively no H⁺ or OH^- ions in the water (concentrations are the same)

If we slowly neutralize a weak acid with a strong base we get graphs that look like this.

If you’re asked to sketch a weak-acid/strong-base curve.

· You should pick a sensible starting pH – eg 3

· You can assume the pH will quickly change at first because

______________________________________________________________________________________________________

· After a while a __________ solution has formed from the partial neutralization of the acid. So pH changes will be ____________

When we reach the half-equivalence point the concentration of [A^-] and [HA] are equal.

But K_a = [H⁺][A^-] / [HA] èèè K_a = [H⁺][A^-] / [HA]

And, since K_a = [H⁺], pK_a = pH⁺

If we slowly neutralize a Weak Bases with a Strong Acid we get graphs that look like this.

If you’re asked to sketch a weak-base/strong-acid curve.

· You should pick a sensible starting pH – eg ____

· You can assume the pH will quickly change at first because

______________________________________________________________________________________________________

· After a while a __________ solution has formed from the partial neutralization of the base. So pH changes will be ____________

When we reach the half-equivalence point

K_b = [OH^-], pK_b = pOH^-

If we slowly neutralize a Weak Acid with a Weak Base we get graphs that look like this.

If you’re asked to sketch a weak-acid/strong-base curve.

· You should pick a sensible starting pH – eg ____

· You can assume the pH will quickly change at first because

___________________________________________________________________________________________

· It’s not really possible to find the equivalence point by titration because ____________________________________________

Indicators are weak acids that are different colours depending on whether they are protonated (have H⁺ ions) or ionized (have lost their H⁺ ion)

We sometimes represent indicators as HIn - not HI because that could be confused with Hydrogen Iodide

HIn_(aq) ⇌ H⁺_(aq) + In^-_(aq)

COLOUR 1 COLOUR 2

· So if we added HIn_(aq) to an acidic solution the equilibrium would be forced to the ____________ and we would see Colour _____

· So if we added HIn_(aq) to an alkaline solution the equilibrium would be forced to the ____________ and we would see Colour _____

WRITE THE K_a FOR THE EQUILIBRIUM ABOVE

Remember at half-equivalence pH = pK_a.

This is also true for indicators so when the pH reaches the pK_a we’ll have an equal mix of both colours.

An indicator will usually change around pK_a +/- 1

In theory, the best indicator should change colour at exactly the equivalence point.

But, in practice they change over a range. And in practice this doesn’t really matter because there is a sudden change of at least 3 or 4 pH values on the pH curve within 1 drop.

So for Strong Acid-Strong Base both these indicators are fine.

But Methyl Red wouldn’t be good for some neutralisations.

So choose indicators that fit within the vertical section of the pH curve.