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Author Topic: Why is sodium carbonate alkaline?  (Read 26722 times)

Offline george

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Why is sodium carbonate alkaline?
« on: 10/12/2003 21:00:33 »
Hi, can one of the chemists please explain the following (in simple terms !)

- I saw the other day that the alkali added to swimming pools to keep the water in balance is "soda ash" or sodium carbonate - Na2CO3. Why is this alkaline ?

I thought you needed hydroxide to produce an alkali, like lime (CaOH) or sodium hydroxide (NaOH)



Offline Ylide

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Re: Why is sodium carbonate alkaline?
« Reply #1 on: 11/12/2003 02:49:51 »
conjugate bases (i.e. the anion) of weak acids are mildly basic.  Here's why...the conjugate base of any weak acid tends to be a somewhat strong base not in terms of causticity but in terms of affinity for a proton.  When a bicarbonate ion is placed in water, it absconds with a proton from a water molecule, leaving behind a hydroxide ion for every molecule of bicarbonate.  The reaction is as follows:

HCO3- + H2O -> H2CO3 + OH-

This is normally a reversible reaction, but H2CO3 (carbonic acid) has the tendency to lose a CO2 molecule, leaving behind some mildly basic water.  

The above principle holds true for any salt of a weak acid dissolved in water.  (acetate, carbonate, etc)  You can even predict the pH of the salt solution based on the pKa of the acid the salt is derived from (and consequently the pKb of the conjugate base)

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The Naked Scientists Forum

Re: Why is sodium carbonate alkaline?
« Reply #1 on: 11/12/2003 02:49:51 »


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