Chemical Forums

Chemistry Forums for Students => High School Chemistry Forum => Topic started by: jsmith613 on December 17, 2009, 07:19:30 AM

Title: Conjugate base pair
Post by: jsmith613 on December 17, 2009, 07:19:30 AM

what is the conjugate base pair of Ca2+

Title: Re: Conjugate base pair
Post by: kimyacı on December 17, 2009, 03:13:35 PM

 Dear Friend, I think this uestion is  incomplate or wrong.because Ca+2 is not acid,so it has not conjugate base.

Title: Re: Conjugate base pair
Post by: jsmith613 on December 17, 2009, 03:51:04 PM

Quote from: kimyacı on December 17, 2009, 03:13:35 PM

Dear Friend, I think this uestion is  incomplate or wrong.because Ca+2 is not acid,so it has not conjugate base.

the conjujagte acid for Cl- = HCl
the conjugate base for Na+ = NaOH

therefore Ca2+ must also have a conjugate base

Title: Re: Conjugate base pair
Post by: Grundalizer on December 17, 2009, 04:08:15 PM

One usually says somthing like, Cl^- is the conjugate base of HCl, not the conjugate acid for Cl^- is HCl

So I would also say the question is worded a little bit unusual, but Ca(OH)₂ is the base which dissociates into hydroxide and Ca²⁺ in water (not as strong as NaOH or KOH)

Title: Re: Conjugate base pair
Post by: cliverlong on December 17, 2009, 04:25:54 PM

I would take this approach (similar to Grundalizer)

Ca(OH)₂ dissolves in water to provide two types of ion: Ca²⁺ and OH^-

It is the OH^- that acts as a base by taking a proton from water H₂0. Water is acting as the acid in this case by donating the proton to the OH^-.

The equilibrium equation is

OH^- + H₂0  ::equil:: H₂0 + OH^-

Cunningly, the species are the same on both sides. In the H⁺/OH^- equilibrium, OH^- dominates, so the solution is strongly alkaline. Ca²⁺ does not appear (it specatates) and is neither acid nor base.

Clive

Title: Re: Conjugate base pair
Post by: Borek on December 17, 2009, 04:57:54 PM

Quote from: cliverlong on December 17, 2009, 04:25:54 PM

Cunningly, the species are the same on both sides. In the H⁺/OH^- equilibrium, OH^- dominates, so the solution is strongly alkaline. Ca²⁺ does not appear (it specatates) and is neither acid nor base.

This is an important reaction taking place in alkalic solutions of Ca²⁺:

Ca²⁺ + OH^- -> CaOH⁺

It also takes place in neutral solutions, although the stability constant for the complex is not that high, so the effect on pH is relatively small:

Ca²⁺ + H₂O -> CaOH⁺ + H⁺

Still, it exists and can't be neglected. CaOH⁺ is a conjugate base of Ca²⁺.

Title: Re: Conjugate base pair
Post by: jsmith613 on December 17, 2009, 05:06:24 PM

so in that case, is CaCl₂ neutral becasue
both the Ca²⁺ and the 2Cl- spectate therefore are neutral

BUT
CaCO₃ is basic becasue
Ca = neutral
CO3- + H2O --> HCO3- + OH-
therefore as HCO3- is weak and OH- is strong the equillibria lies towards a higher pH

Title: Re: Conjugate base pair
Post by: cliverlong on December 17, 2009, 05:31:22 PM

Quote from: Borek on December 17, 2009, 04:57:54 PM

Quote from: cliverlong on December 17, 2009, 04:25:54 PM

Cunningly, the species are the same on both sides. In the H⁺/OH^- equilibrium, OH^- dominates, so the solution is strongly alkaline. Ca²⁺ does not appear (it specatates) and is neither acid nor base.

This is an important reaction taking place in alkalic solutions of Ca²⁺:

Ca²⁺ + OH^- -> CaOH⁺

It also takes place in neutral solutions, although the stability constant for the complex is not that high, so the effect on pH is relatively small:

Ca²⁺ + H₂O -> CaOH⁺ + H⁺

Still, it exists and can't be neglected. CaOH⁺ is a conjugate base of Ca²⁺.

Well there you go.

There is always something new to learn.

So there is a meaningful answer to the original question !

Clive

Title: Re: Conjugate base pair
Post by: kimyacı on December 18, 2009, 06:07:01 AM

 Thank you all, these are good explanations really.

Title: Re: Conjugate base pair
Post by: jsmith613 on December 19, 2009, 01:53:18 PM

Quote

Still, it exists and can't be neglected. CaOH⁺ is a conjugate base of Ca²⁺.

surely CaOH⁺ is an acid

Title: Re: Conjugate base pair
Post by: Borek on December 19, 2009, 02:00:50 PM

Quote from: jsmith613 on December 19, 2009, 01:53:18 PM

Quote

Still, it exists and can't be neglected. CaOH⁺ is a conjugate base of Ca²⁺.

surely CaOH⁺ is an acid

Actually it is both base and acid, in a way similarly to HCO₃^-. This is called amphiprotic substance.