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Topic: Which acid is stronger?  (Read 19618 times)

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scott_wright7

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Which acid is stronger?
« on: June 04, 2005, 03:20:27 AM »
I dont know how to tell the difference between acid strengths could someone teach me?

In which one of the following pairs is the 2nd substance a stronger acid than the 1st?
A. HCL        CH3COOH
B. H2PO4-   H3PO4
C. H2SO4    H2CO3
D. H20        OH-

Could you tell me which one is is stronger than the 1st and why?
Cheers Scott

Grafter

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Re:Which acid is stronger?
« Reply #1 on: June 04, 2005, 03:47:08 AM »
The way to work it out, is to write out a dissociation equilibrium, and then examine the bronsted lowry conjugate base.

The most stable conjugate base will have the most acidic conjugate acid.

Often, the conjugate base is an anion. The way to work out if an anion is stable is to see what kinds of stabilisation it has (eg mesomeric, induction, electronegativity).

Write out each conjugate anion and the types of stabilisation and we'll take it from there.

Offline Borek

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Re:Which acid is stronger?
« Reply #2 on: June 04, 2005, 04:22:31 AM »
The way to work it out, is to write out a dissociation equilibrium, and then examine the bronsted lowry conjugate base.

This will not apply to some of the points. Comparison between hydrochloric acid and acetic acid (or sulfuric vs carbonic) is a thing you just have to remember. Whether you happen to remember Ka values, or some general rule stating HCl is a strong inorganic and carboxylic are in general weak, or something else - doesn't matter.
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Offline xiankai

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Re:Which acid is stronger?
« Reply #3 on: June 04, 2005, 07:28:58 AM »
A) HCl and acetic acid.

of course HCl is stronger. this should be common knowledge

B)H2PO4- and H3PO4.

H3PO4 has more H+ than H2PO4-, so H3PO4 is stronger

C)H2SO4 and HCO3

of course H2SO4 is stronger. carbonic acid is a weak acid that makes our soft drinks "fizzy". i hope it is common knowledge to you also.

D) H2O and OH-

H2O has more H+ than OH-, so its stronger.

to put it simply, H+ is an indicator of acidity while OH- is an indicator of alkaline
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Grafter

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Re:Which acid is stronger?
« Reply #4 on: June 04, 2005, 08:09:26 AM »
This will not apply to some of the points. Comparison between hydrochloric acid and acetic acid (or sulfuric vs carbonic) is a thing you just have to remember. Whether you happen to remember Ka values, or some general rule stating HCl is a strong inorganic and carboxylic are in general weak, or something else - doesn't matter.

Obviously, you have to know which method of stabilisation is dominant. Evidently between AcOH and HCl, Cl- is a better stabiliser of charge than the mesomeric effects in AcO-.

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