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Topic: Sodium bicarbonate  (Read 14109 times)

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libra78wolf

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Sodium bicarbonate
« on: February 22, 2005, 10:02:33 PM »
I was askeed on a quiz to write a balanced molecular equation for solid sodium bicarbonate is heated. I got the answer wrong. Could you explain to me why my answer is wrong and what the right answer is.

Na2HCO3 -> 2Na +OH+CO2





Edit:  Just clarified the coding to make it easier to read. -Jdurg
« Last Edit: February 22, 2005, 10:08:30 PM by jdurg »
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Offline jdurg

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Re:Sodium bicarbonate
« Reply #1 on: February 22, 2005, 10:08:01 PM »
I'll just give you one hint:  What's the ionic charge of the sodium ion and what's the ionic charge of the bicarbonate ion?  Do you notice something wrong with your equation now? ;)
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libra78wolf

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Re:Sodium bicarbonate
« Reply #2 on: February 22, 2005, 10:11:48 PM »
Sodium is a +1 and HCO3 is2-
No I don't see. If sodium is a +1 and bicarbonate a 2- they switch to make
 Na2HCO3
« Last Edit: February 22, 2005, 10:16:01 PM by libra78wolf »
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Offline jdurg

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Re:Sodium bicarbonate
« Reply #3 on: February 22, 2005, 10:17:13 PM »
Quote from: libra78wolf on February 22, 2005, 10:11:48 PM
Sodium is a +1 and HCO3 is2-

Okay, let's go a little further in this.   ;)  You are correct with your sodium but incorrect with the bicarbonate.  I'll tell you that the Carbonate ion (CO3) has a 2- charge.  So based on that fact, what should the Bicarbonate (HCO3) have for a charge?  So if you look up what the proper charge is on the bicarbonate ion, you'll probably be able to see where you went wrong.  

Another hint is that sodium is a VERY reactive metal.  Would you logically think that by heating up baking soda you would make sodium metal?  :)
« Last Edit: February 22, 2005, 10:20:52 PM by jdurg »
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libra78wolf

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Re:Sodium bicarbonate
« Reply #4 on: February 22, 2005, 10:22:28 PM »
So the answer is
NAHCO3arrow Na + OH +CO2
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Offline jdurg

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Re:Sodium bicarbonate
« Reply #5 on: February 22, 2005, 11:06:16 PM »
Almost.  You now have the sodium bicarbonate correct, but your products are a bit off.  Remember, sodium metal is a HIGHLY reactive substance and would have one electron in its outer shell ready to react.  OH also isn't really a compound.  It's what's called a 'free-radical' because it has one unpaired electron in its outer shell.  Remember, electrons like to be paired.  So what do you think would be more appropriate; the Na and OH species with a lone outer shell electron, or the compound NaOH which is very stable and happy to exist?
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Offline AWK

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Re:Sodium bicarbonate
« Reply #6 on: February 23, 2005, 01:14:58 AM »
Sodium bicarbonate in solid state is composed from Na+ cations and HCO3- anions.
During heating over 100 C anions decompose to produce water and carbonate dianion.
May this help
« Last Edit: February 23, 2005, 01:15:31 AM by AWK »
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Offline jdurg

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Re:Sodium bicarbonate
« Reply #7 on: February 23, 2005, 10:26:31 AM »
Quote from: AWK on February 23, 2005, 01:14:58 AM
Sodium bicarbonate in solid state is composed from Na+ cations and HCO3- anions.
During heating over 100 C anions decompose to produce water and carbonate dianion.
May this help


That all depends on the temperature.  Above 270 C, the CO2 will be driven off leaving you with NaOH.  So it all depends on how intensely the sodium bicarbonate is heated.  (And in the heat of a bunsen burner flame, 270C is pretty easily reached).
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libra78wolf

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Re:Sodium bicarbonate
« Reply #8 on: February 23, 2005, 08:41:09 PM »
so
NaHCO3-->NAOH+CO2
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Offline jdurg

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Re:Sodium bicarbonate
« Reply #9 on: February 23, 2005, 09:10:43 PM »
Quote from: libra78wolf on February 23, 2005, 08:41:09 PM
so
NaHCO3-->NAOH+CO2

At high temperatures, yes, you are correct.  At lower temperatures, like around the boiling point of water, it will decompose into water, sodium carbonate, and carbon dioxide as AWK mentioned.  But I think if you put down what you've come up with there you'd be alright.  ;D
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Offline jdurg

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Re:Sodium bicarbonate
« Reply #10 on: February 24, 2005, 08:14:10 AM »
Quote from: Mitch on February 23, 2005, 11:57:28 PM
It's water man, read the posts.

Again, it all depends on the temperature.  In our high school labs our bunsen burner flames were pretty hot, so when we decomposed the sodium bicarbonate it was hot enough to drive off the carbon dioxide and leave sodium hydroxide.  This was tested by placing the resulting powder in a phenolphaelin(sp?) solution.  (It turned pink).  So really, either of the equations will work unless the initial question specified a temperature.
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Offline AWK

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Re:Sodium bicarbonate
« Reply #11 on: February 24, 2005, 09:09:01 AM »
Na2CO3 also change color of phenolphtalein solution (pH ~ 12).
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Offline jdurg

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Re:Sodium bicarbonate
« Reply #12 on: February 24, 2005, 09:22:38 AM »
But when we then neutralized the solution, no gasses were eluted.   ;D
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Offline AWK

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Re:Sodium bicarbonate
« Reply #13 on: February 24, 2005, 09:49:19 AM »
Na2CO3 start to decompose before melting(~800 C). Decomposition undergoes according to equation:
Na2CO3 = Na2O + CO2.
Only, when CO2 is added in close system, you can observe melting. Pure Na2CO3 can only exist in CO2 atmosphere at the temperature of melting point (860 C).
And, of cource, Na2O with water forms NaOH, but no NaOH is formed during sintering.
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Offline Donaldson Tan

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Re:Sodium bicarbonate
« Reply #14 on: February 24, 2005, 10:33:24 AM »
I always had the impression bicarbonate ion can only exist in aq solution. there is no such thing as NaHCO3 (s).
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