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Topic: Hess's Law with net ionic equations  (Read 81162 times)

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Offline Lt.Pliskin

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Hess's Law with net ionic equations
« on: October 07, 2005, 12:14:56 AM »
Alright, I have three equations I carried out during a lab I did.

Combining HCl and NaOH

HCl + NaOH --> NaCl + H2O
H+(aq) + Cl -(aq) + Na+(aq) + OH-(aq) -->  Na+(aq) + Cl-(aq) + H2O(liq)
H+(aq) + OH-(aq) --> H2O(liq)


Combining Ammonium Chloride and Sodium Hydroxide

NH4Cl + NaOH --> NH3 + NaCl + H2O
NH3(aq) + H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) --> NH3(aq) + Na+(aq) + Cl-(aq) + H20 (liq)
H+(aq) + OH-(aq) --> H2O(liq)



Combining a solution of ammonia (NH3OH) and HCl

NH4OH +HCl --> NH4Cl + H20
NH3(aq) + OH-(aq) + 2H+(aq) + Cl-(aq) --> NH3(aq) + H+(aq) + Cl-(aq) + H2O(liq)
H+(aq) + OH-(aq) --> H20(liq)


Alright, using the heats of reactions obtained from the lab, I came up with:

H+(aq) + OH-(aq) --> H20(liq)            -158.23 kJ/mol

H+(aq) + OH-(aq) --> H20(liq)            -80.65 kJ/mol      
------------------------------------
H+(aq) + OH-(aq) --> H20(liq)            -267.54 kJ/mol


My question is, how would I use hess's law to verify that the third one can be found using the heats of reactions of the first two. Usually, a Hess's law problem involves rearranging the reactants and products to get the final quation. How would that work with these?

Any help is greatly appreciated!  
« Last Edit: October 07, 2005, 12:15:55 AM by Lt.Pliskin »

Offline mike

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Re:Hess's Law with net ionic equations
« Reply #1 on: October 07, 2005, 12:29:21 AM »
I think maybe all you should have to do is reverse the second equation and add it to the first equation.
There is no science without fancy, and no art without facts.

Offline mike

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Re:Hess's Law with net ionic equations
« Reply #2 on: October 07, 2005, 12:30:11 AM »
NH4OH is just NH3 and H2O
There is no science without fancy, and no art without facts.

Offline Lt.Pliskin

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Re:Hess's Law with net ionic equations
« Reply #3 on: October 07, 2005, 12:35:00 AM »
So, it would be

Combining a solution of ammonia (NH3OH) and HCl

NH4OH +HCl --> NH4Cl + H20
NH3(aq) + H2O(liq) + H+(aq) + Cl-(aq) --> NH3(aq) + H+(aq) + Cl-(aq) + H2O(liq)
Nothing ---> Nothing?


H+(aq) + OH-(aq) --> H20(liq)            -158.23 kJ/mol

H20(liq) -->   H+(aq) + OH-(aq)           80.65 kJ/mol      
------------------------------------
Nothing --> nothing                           -267.54 kJ/mol

The two equations cancel each other out, so Im assuming that when NH4OH is added to HCL, no reaction occurs?
« Last Edit: October 07, 2005, 12:37:55 AM by Lt.Pliskin »

Offline mike

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Re:Hess's Law with net ionic equations
« Reply #4 on: October 07, 2005, 12:36:59 AM »
HCl + NaOH ---> NaCl + H2O

NH4OH + NaCl ---> NH4Cl + NaOH

----------------------------------------

NH4OH + HCl ---> NH4Cl + H2O
There is no science without fancy, and no art without facts.

Offline mike

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Re:Hess's Law with net ionic equations
« Reply #5 on: October 07, 2005, 12:38:46 AM »
This is like:

NH3 + HCl ---> NH4+ + Cl-
There is no science without fancy, and no art without facts.

Offline Lt.Pliskin

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Re:Hess's Law with net ionic equations
« Reply #6 on: October 07, 2005, 12:49:37 AM »
OH, ok, I see, thanks!

Now, I have to use net ionic equations witht he calculations, how would those look?

I got:

 H+ + Cl- + Na+ + OH- -->  H20 + Na+ + Cl-

 NH3 + H20 + Na+ + Cl- --> NH3+ H20 + Na+ + Cl-
-----------------------------------------------------------
NH3 + H20 + H+ + Cl- --> NH3 + H+ + Cl-  + H2O      which basically means nothing happens :-\


Does the NH4OH dissasociate into those ions? How about the NH4Cl?

Thankyou for your help thus far, I think Ive almost got it! :)

Offline Lt.Pliskin

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Re:Hess's Law with net ionic equations
« Reply #7 on: October 07, 2005, 12:52:47 AM »
sorry, we seem to keep missing each other

H+ OH- -->  H20

H20 --> H20
-----------------------------------------------------------
H+ OH- --> H2O  

Is this right

**modified to remove spectator ions**
« Last Edit: October 07, 2005, 12:57:17 AM by Lt.Pliskin »

Offline mike

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Re:Hess's Law with net ionic equations
« Reply #8 on: October 07, 2005, 01:09:04 AM »
H+ + NH3 ---> NH4+

You are reacting H+ with NH3 not OH-
There is no science without fancy, and no art without facts.

Offline Lt.Pliskin

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Re:Hess's Law with net ionic equations
« Reply #9 on: October 07, 2005, 01:20:53 AM »
I am totally, lost...

Thanks for your patience with me thus far.

What is the net ionic equation for the NH4CL + NaOH -->  NH4OH  + NaCl?

Offline mike

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Re:Hess's Law with net ionic equations
« Reply #10 on: October 07, 2005, 01:29:52 AM »
NH4+ + Cl- + Na+ + OH- ----> NH3 + Na+ + Cl- + H2O
There is no science without fancy, and no art without facts.

Offline Lt.Pliskin

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Re:Hess's Law with net ionic equations
« Reply #11 on: October 07, 2005, 01:39:12 AM »
:D Thank you so much! I hope I havent been too much of a bother. I can finally get some sleep now!

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