Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: [1]   Go Down

Author Topic: Predicting Equivalence Point pH?  (Read 8161 times)

0 Members and 1 Guest are viewing this topic.

untererde

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Gender: Female
  • Posts: 5
Predicting Equivalence Point pH?
« on: July 12, 2007, 09:12:31 AM »

I'm teaching myself Gen. Chem II, and am doing alright, but when it comes to questions like the following, I'm in a bit of a bind as to anticipating how the reactions/ molecules will function:

Predict whether the equivalence point is at, above, or below pH 7.0 for
a. formic acid and NaOH
b. calcium hydroxide and perchloric acid
c. pyridine and nitric acid

I think(?) that a. is a weak acid with a strong base, and so would be >pH7.0,
that b. is a strong base with a weak acid and so would =pH 7.0, and have no real idea what to do with c. Am I thinking about this correctly, or do I need to balance the equations and (somehow?!) evaluate the products? Thanks!
Logged

enahs

  • 16-92-15-68 32-7-53-92-16
  • Retired Staff
  • Sr. Member
  • *
  • Mole Snacks: +205/-44
  • Offline Offline
  • Gender: Male
  • Posts: 2180
Re: Predicting Equivalence Point pH?
« Reply #1 on: July 12, 2007, 09:45:07 AM »

Strong Base + Strong Acid = pH 7
Weak acid + strong base = pH > 7
Weak base + strong acid = pH < 7


To predict the actual end point requires knowing the pKa values. Which are gotten from experiments and theoretical calculations based on already obtained data.

A quick search turned up this site. It only covers strong-strong, and weak acid strong base, but using the outlined procedures you can figure out weak base strong acid.
http://bouman.chem.georgetown.edu/S02/lect19/lect19.htm

And a site with more general info.
http://www.chemtutor.com/acid.htm


You can find many more by searching for "predicting ph of acid base titration". Those just happen to be the first two results and a quick glance shows to cover some good information.

Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1370/-369
  • Offline Offline
  • Gender: Male
  • Posts: 20990
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Predicting Equivalence Point pH?
« Reply #2 on: July 12, 2007, 11:40:20 AM »

See also http://www.chembuddy.com/?left=pH-calculation&right=titration-curves-calculation

b. is a strong base with a weak acid and so would =pH 7.0

Very strong acid & strong base. Ca(OH)2 has low solubility, but it is almost completely dissociated.

Quote
and have no real idea what to do with c

Acid and base - weak or strong?
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

untererde

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Gender: Female
  • Posts: 5
Re: Predicting Equivalence Point pH?
« Reply #3 on: July 12, 2007, 01:37:18 PM »

I think the major part of my difficulty here lies in the fact that I don't know how to determine whether an acid is weak or strong. I'm doing this by correspondence, so, at this point, the best thing I have to go by are the (tiny) lists of common strong/ weak acids and bases. I can't find pyridine on either. I see that HNO3 is a strong acid, but don't know where to begin reasoning about pyridine.

A second (related) question I'm working on has to do with NH3 and HCl. Unless I'm mistaken this is a case of a weak base and a strong acid?? Are there specific rules for how to deal with a titration of that configuration? (Everything seems to deal with strong a- strong b, weak a- strong b, or polyprotic acids.) Thanks!
Logged

enahs

  • 16-92-15-68 32-7-53-92-16
  • Retired Staff
  • Sr. Member
  • *
  • Mole Snacks: +205/-44
  • Offline Offline
  • Gender: Male
  • Posts: 2180
Re: Predicting Equivalence Point pH?
« Reply #4 on: July 12, 2007, 01:55:29 PM »

Basically for general chemistry, you just need to learn the major strong acids and assume everything else is a weak acid.
http://en.wikipedia.org/wiki/Strong_acid
I would not worry yet at the level you are it to be concerned with the "extremely strong acids" category.

This might seem very incomplete and counter-intuitive to what is science. But once you learn the basics you can then study more advanced topics more scientifically. And then we change the definition of an acid and base on you!  :D


To skip ahead, to verify if it is a weak/strong acid, look up on the internet (or the back of your book(s)!) the pKa value. Generaly an acid is considered strong if it's pKa is = or less then -2 (in other words, a weak acid has a pKa greater then -2).


Quote
A second (related) question I'm working on has to do with NH3 and HCl. Unless I'm mistaken this is a case of a weak base and a strong acid?? Are there specific rules for how to deal with a titration of that configuration? (Everything seems to deal with strong a- strong b, weak a- strong b, or polyprotic acids.) Thanks!

Correct. A strong base - weak acid relation, is the same as a strong acid - weak base, only in the opposite direction.

http://www.cartage.org.lb/en/themes/Sciences/Chemistry/Inorganicchemistry/AcidsBases/Acidsbasesindex/weakbasetitration.htm


You might be having trouble with titration's of weak to strong and vice versa because you are trying to learn and understand more at a faster pace your course is being taught. To truly understand it you need to have also covered the concept of buffers and buffering. Have you?
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Buffer/Buffer.html
http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch



Logged

Mitch

  • In Vivo Chemist
  • Administrator
  • Sr. Member
  • *
  • Mole Snacks: +354/-81
  • Offline Offline
  • Gender: Male
  • Posts: 5028
  • "I bring you peace." -Mr. Burns
    • Chemistry Blog
Re: Predicting Equivalence Point pH?
« Reply #5 on: July 12, 2007, 02:18:25 PM »

enahs: Great answer +2 scooby snacks. :)
Logged
Most Common Suggestions I Make on the Forums.
1. Always start by writing a balanced chemical equation.
2. Don't confuse thermodynamic stability with chemical reactivity.
3. Forum Supports LaTex

untererde

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Gender: Female
  • Posts: 5
Re: Predicting Equivalence Point pH?
« Reply #6 on: July 12, 2007, 02:22:22 PM »

Yeah. Great answer! Thank you so much. I'll check out the links, and keep working along.

Last question on this theme: am I right that if I'm given only Ka in the back of the book, I would determine pKa by taking -log Ka and then compare that number to the (extremely helpful) pKa figure you gave me?
Logged

Mitch

  • In Vivo Chemist
  • Administrator
  • Sr. Member
  • *
  • Mole Snacks: +354/-81
  • Offline Offline
  • Gender: Male
  • Posts: 5028
  • "I bring you peace." -Mr. Burns
    • Chemistry Blog
Re: Predicting Equivalence Point pH?
« Reply #7 on: July 12, 2007, 02:35:41 PM »

yes
Logged
Most Common Suggestions I Make on the Forums.
1. Always start by writing a balanced chemical equation.
2. Don't confuse thermodynamic stability with chemical reactivity.
3. Forum Supports LaTex

enahs

  • 16-92-15-68 32-7-53-92-16
  • Retired Staff
  • Sr. Member
  • *
  • Mole Snacks: +205/-44
  • Offline Offline
  • Gender: Male
  • Posts: 2180
Re: Predicting Equivalence Point pH?
« Reply #8 on: July 12, 2007, 02:39:31 PM »

Last question on this theme: am I right that if I'm given only Ka in the back of the book, I would determine pKa by taking -log Ka and then compare that number to the (extremely helpful) pKa figure you gave me?


Correct. Or you could just take that number -2 and using the rules of logs and get that number in just Ka's (and not have to convert every one in the back of your book).
If you do that, here is a math question for you. Is a strong acid then larger or smaller then that number you just calculated?
http://mathworld.wolfram.com/Logarithm.html



Borek: You just like to give people scooby snacks that link to your site!  ;)
Logged

untererde

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Gender: Female
  • Posts: 5
Re: Predicting Equivalence Point pH?
« Reply #9 on: July 12, 2007, 02:47:58 PM »

So, using logs, the magic number is .01 (or, 1x10-2), and, if my reasoning serves, a strong acid will have a Ka LARGER than 1x10-2. (?!)

I'm sure I should know why dissociation constants aren't listed for strong acids on the table I have, but I don't. Is this simply because they so rapidly dissociate fully to ions??
Logged

enahs

  • 16-92-15-68 32-7-53-92-16
  • Retired Staff
  • Sr. Member
  • *
  • Mole Snacks: +205/-44
  • Offline Offline
  • Gender: Male
  • Posts: 2180
Re: Predicting Equivalence Point pH?
« Reply #10 on: July 12, 2007, 02:55:22 PM »

So, using logs, the magic number is .01 (or, 1x10-2), and, if my reasoning serves, a strong acid will have a Ka LARGER than 1x10-2. (?!)



No, not quite.

If the pKa is the - log (kA), then the anti-log you take the anti-log of the negative of the pKa, or 10-(-2).


And yes, strong acids are said to completely dissociate. Once you cover what the actual kA and pKa's are (acid dissociation constant) it will be come quite obvious why they are generally not listed for strong acids.
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1370/-369
  • Offline Offline
  • Gender: Male
  • Posts: 20990
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Predicting Equivalence Point pH?
« Reply #11 on: July 12, 2007, 09:09:36 PM »

I'm sure I should know why dissociation constants aren't listed for strong acids on the table I have, but I don't. Is this simply because they so rapidly dissociate fully to ions??

Determination of dissociation constants for strong acids is tricky. For weak acids you may just use pKa=pH at half titration, for strong acids it doesn't work. In most cases their Ka is calculated from thermodynamic data, or from spectroscopy results. However, practical value of this data is low - you may safely assume they are always 100% dissociated.

See http://www.chembuddy.com/?left=BATE&right=dissociation_constants for some Ka values for strong acids/bases

Generaly an acid is considered strong if it's pKa is = or less then -2 (in other words, a weak acid has a pKa greater then -2).

HNO3 is condsidered to be strong acid, yet its pKa = -1. IMHO the border should be put around pKa=0 - 0.1M solution of such acid is dissociated in over 90%. Also - you may play with BATE to see how titration curves behave. Even for pKa=1 acids are usually too strong to keep up with pH=pKa for 50% titration. See http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch and discussion of dichloroacetic acid pH.
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info
Pages: [1]   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.077 seconds with 23 queries.