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Topic: Enthalpy change for decomposition of NaHCO3  (Read 19699 times)

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Offline Stickslikeglue

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Enthalpy change for decomposition of NaHCO3
« on: April 19, 2008, 07:51:48 AM »
Right ive looked around a bit but i havent found anything that really answers my question yet :(
 
i have the equation:

2NaHCO3 ==> Na2CO3 + H20 + CO2

i reacted NaHCO3 with HCL and did the same with Na2CO3

getting the equations:

NaHCO3 + HCl ==> NaCl + CO2 + H20 with delta H of approx.-34kj/mol

and

Na2CO3 + 2HCL ==> 2NaCl + CO2 + H2O with delta H of approx. 25kJ/mol

can i draw an enegry cycle fomt his data alone to calculate the enthalpy for the decomposition of NaHCO3 and if so how can i do it?

thanks in advance! Bill

Offline Astrokel

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Re: Enthalpy change for decomposition of NaHCO3
« Reply #1 on: April 19, 2008, 08:12:27 AM »
Yes, it's actually a simple energy cycle. Can you try it out yourself first? ;)

kelvin
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Offline Stickslikeglue

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Re: Enthalpy change for decomposition of NaHCO3
« Reply #2 on: April 19, 2008, 08:28:00 AM »
i have done! but not here ;)

erm best way to show it...

2NaHCO3 ==>Na2CO3 + H20 +CO2
     |                                               |
     |   -34kJ                                     |  +25kJ
     v                                               V

                        NaCl + CO2 +H2O   (#)


i then get delta H over all to equal 2x(-34) - (+25) to get -95kJ/mol

The bit that confuses me is the  equation labeled with the hash. the reaction with NaHCO3 and HCl or with Na2CO3 does not give this equation. 2NaHCO3 has two lots of everything and Na2CO3 has two lots of NaCl! Or is that ok because you already have one lots of water and carbon dioxide on the right hand sde of the equation already?

Hmm think ive made with as clear as mud :S hope you understand where im confused.
Thanks Again! Bill

Offline Astrokel

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Re: Enthalpy change for decomposition of NaHCO3
« Reply #3 on: April 19, 2008, 09:18:35 AM »
      2NaHCO3 ---> Na2CO3 + H20 +CO2
         |                  |
         | +2HCl         | +2HCl
         |                 |
         V                 V
      2NaCl + 2CO2 + 2H20
       

Yes, the equation with hash is wrong! You multiply by 2 with the (-34), then you should have also multiply
(NaCl + CO2 + H20) by 2 to 2NaCl + 2CO2 + 2H2O, otherwise it is not balanced.

When u do an energy cycle,

1) Ensure that the reagents you 'add' to the cycle such as in this case, HCl, is balanced. What is mean by balance is the amount of HCl 'added' in the clockwise direction is the same as the anticlockwise.

2) Ensure the final products of the clockwise and anticlockwise direction are the same, in this case, its the one highlighted in blue.


I think you are asking about this?

Kelvin

                               
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

Offline Stickslikeglue

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Re: Enthalpy change for decomposition of NaHCO3
« Reply #4 on: April 19, 2008, 09:26:26 AM »
Ahhh yes thanks alot just about makle sense now!

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