Determining Acid Strength

[schnappy]

Hi,
As part of choosing an indicator for an acid-base titration, you need to know the characteristics of the pH curve. So that entails you know whether its a strong acid against weak base, strong v strong, weak v weak, weak v strong... but how do you know how 'strong' they are?

And just to clarify, a 'strong' acid fully ionises in water. So, H2SO4(s?) ---H2O-> 2H+(aq) + SO4 2-(aq)... all H+ ions disassociated from the sulphate ion in solution. If we pretend sulfuric acid is weak, does that mean that possible it's only monoprotic?

Thanks

[luken93]

AFAIK, the only requirements to know are that HNO3, H2SO4 and HCl are known as strong acids?

[schnappy]

Hmm okies then, ta. What about bases?

The question is:
Phenolphthalein would be the most suitable indicator for the titration of: A(Sodium carbonate with HCl) B(Potassium dichromate with ethanol) C(ammonia with sulfuric acid) D(sodium hydroxide with ethanoic acid)

[nacho]

If we pretend sulfuric acid is weak, does that mean that possible it's only monoprotic?

I'm not sure what you're saying here, but it isn't necessary for an acid to be diprotic/triprotic to be a strong acid.
The only requirement for a strong acid is that it will ionise around 90-100%, whereas weak acids tend to ionise somewhere around 10%
The acids in between are pretty rare and not studied AFAIK.

Your question in regards to indicators can be solved by looking at the range for which the indicator responds to. This is generally given in the question (if it is a SAC) and is in the databooklet.
Phenolphthalein has a pH  detection range of (8.3 - 10.0)
Therefore, we are looking at a weak acid added to a strong base titration.
opt A is weak base, strong acid - ruled out
it helps to look at pH curves.
I would say option D.
Please correct if i am wrong.

Upon googling, i found this list of strong bases:
   * LiOH - lithium hydroxide
    * NaOH - sodium hydroxide
    * KOH - potassium hydroxide
    * RbOH - rubidium hydroxide
    * CsOH - cesium hydroxide
    * *Ca(OH)2 - calcium hydroxide
    * *Sr(OH)2 - strontium hydroxide
    * *Ba(OH)2 - barium hydroxide
( i bolded the ones that i thought were relevant, although i haven't seen LiOH in a question before) also, the common ones should be somewhere in your textbook
Should also familiarise yourself with weakbases.

Here is stuff on acids and bases if you have further difficulties:

http://www.dynamicscience.com.au/tester/solutions/chemistry/acidbase/UntitledFrameset-8.html

[schnappy]

Yes it is D.

Ok if the the -protic'ness of the molecule doesn't determine it's strength, what does? What does 'ionise' physically mean in this context? 90% of what is ionising what?

I understand pH curves I just don't understand what strength/weakness are. For the Ethanoic acid, do not all the H+ ions disassociate from the CH3COO- ion? If that's the case is it written down how strongly they ionise on the databooklet?

Thanks

[nacho]

Yes it is D.

Ok if the the -protic'ness of the molecule doesn't determine it's strength, what does? What does 'ionise' physically mean in this context? 90% of what is ionising what?

I understand pH curves I just don't understand what strength/weakness are. For the Ethanoic acid, do not all the H+ ions disassociate from the CH3COO- ion?

Thanks

Well really, if you think about the Bronsted-Lowry definiton of acids and bases, your questions is answered.
An acid is any substance that donates a proton, and a base is any substance that will take/receive this proton.
For protons to be transferred an acid and a base must be present.

If you think about the H+ ion, what is it made up of? 1 proton, 1 neutron.
You should also note that, when we talk about the strengths of acids and bases, we talk about their strength in water. A weak acid will donate all of it's protons, given that there is a base present.

In the instance where there is only water present, it will only donate ~10% of it's protons, whereas a strong acid will donate 90+% of its' protons.
The degree to ionisation just means how readily an acid will donate its' protons. If it is able to donate all or most its' protons (H+), it is a strong acid. Such as HCl, or H2SO4.
Hope this made sense, i suggest looking at the website and reading textbook for more in depth explanations.