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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Nescafe on September 20, 2012, 08:01:31 AM

Title: Potassium and Cesium carbonates - Pka
Post by: Nescafe on September 20, 2012, 08:01:31 AM

Hi,

Anyone know the Pka of these bases? I am guessing the pkaof Cs2CO3 is around 10-11? that of potassium carbonate is around 7-8?

Nescafe.

Title: Re: Potassium and Cesium carbonates - Pka
Post by: Dan on September 20, 2012, 08:55:50 AM

For a compound to have a pKa, it must have a proton:

AH  ::equil:: A^- + H⁺

I assume your question is referring to the pKa of the conjugate acids, i.e. CsHCO₃ and KHCO₃ (i.e. bicarbonate)?

Quote

I am guessing the pkaof Cs2CO3 is around 10-11? that of potassium carbonate is around 7-8?

Why?

Title: Re: Potassium and Cesium carbonates - Pka
Post by: Nescafe on September 20, 2012, 06:22:24 PM

Quote from: Dan on September 20, 2012, 08:55:50 AM

For a compound to have a pKa, it must have a proton:

AH  ::equil:: A^- + H⁺

I assume your question is referring to the pKa of the conjugate acids, i.e. CsHCO₃ and KHCO₃ (i.e. bicarbonate)?

Quote

I am guessing the pkaof Cs2CO3 is around 10-11? that of potassium carbonate is around 7-8?

Why?

Yes my assumption was that when you ask for a pKa of a compound with no protons it is asking for its conjugate acid or as Clayden likes to write, pKah. We have two negative charges, when the first proton is picked up I imagine it will become less basic than before.

My reasoning is simply comparative. acetic acid has a pKa of ~4.3. I assumed if if we have two negative charges close by it would be more unstable than just having a methyl, and therefore more basic, even after one gets protonated it should still be basic but less than it initially was. So by some imagination I assumed the pKah to be ~10-12. I also know it is not as strong as lets say potassium tert butoxide which forms a stable complex (pkah ~18) after protonation. Thus why I went for the middle. So if you know what it is I would love to know as I plan to use it as a base in my SN2.

Nescafe.

Title: Re: Potassium and Cesium carbonates - Pka
Post by: Babcock_Hall on September 20, 2012, 06:52:17 PM

Potassium carbonate is K₂CO₃, whereas cesium carbonate is Cs₂CO₃.  My impression is that potassium ions and cesium ions are not very different from each other, and I would be surprised if there was a big difference in their basicities.  Are you thinking of bicarbonates?

Title: Re: Potassium and Cesium carbonates - Pka
Post by: Nescafe on September 20, 2012, 07:50:40 PM

Yes the bicarbonates, any idea of their Pkas?

Title: Re: Potassium and Cesium carbonates - Pka
Post by: Dan on September 21, 2012, 02:59:10 AM

Quote

I am guessing the pkaof Cs2CO3 is around 10-11? that of potassium carbonate is around 7-8?

OK, my question is: why do you guess that cesium carbonate is 1000 times more basic than potassium carbonate?

Title: Re: Potassium and Cesium carbonates - Pka
Post by: AWK on September 21, 2012, 04:27:51 AM

Quote from: Nescafe on September 20, 2012, 08:01:31 AM

Hi,

Anyone know the Pka of these bases? I am guessing the pkaof Cs2CO3 is around 10-11? that of potassium carbonate is around 7-8?

Nescafe.

As a bases work CO₃^2- or HCO₃^-. Its protolysis constants can be easily calculated from K_a2 and K_a1 of carbonic acid, respectively.