ostaur
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mass balance question
What does it mean an excess of reaction products ?
For exemple: if we have the following reaction
A + B = C + D
the final mass (g) of (C+D) is higher than the initial mass (g)(A+B). Does any exterior factor affect the final result ?
Normally, according Lavoisier, the final product mass should be egal to the initial reactants mass.
Any idea about this ?
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Engager
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All chemical reactions are stopped at equlibrium point. That means that all substances A, B, C and D are always present. The question is in that how
quantity of products compares to quantity of reagents in equlibrium. If quantity of reagents is lesser then quantity of products that means that
products are in excess. Mathematicaly that's the condition:
Kc = [C]*[D] / [A]*[B]
Kc is equilibrium constant in terms of concentration. If Kc > 1 that means the product excess, if Kc < 1 that means reagent excess, if Kc = 1
that means that quantity of reagents and products are equal.
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ostaur
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thanks Engager !
I got your answer about my first question, but about the second I'm still confused.
If we have the right reagents stoechiometric ratio why we have an excess in reaction products ?
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Engager
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| Quote: | Originally posted by ostaur
thanks Engager !
I got your answer about my first question, but about the second I'm still confused.
If we have the right reagents stoechiometric ratio why we have an excess in reaction products ? |
You can imagine product excess in such way. You have for example 1 mol/liter of A and B. Half of that concentration is 0.5 mol/l. If more then 0.5
mol/l of A and B will be converted to C and D that will mean that larger part of reagents is converted to products then remain intact. So you have the
products excess. If less then 0.5 mol/l of A and B is converted to C and D, that will mean that the part of converted reagents is smaller then part
remaining intact, so you have a reagent excess.
Still confused?
[Edited on 24-8-2006 by Engager]
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ostaur
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I have the reaction FeCl3+3NaOH=Fe(OH)3+3NaCl
I know the quantity of reactants: 60 g FeCl3 and 110 NaOH
and I obtain 172 g products of reaction under mixture form
how do you calculate the Ke ?
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jack-sparrow
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You don't calculate the Ke. You can find it in chemical thermodynamics tables.
If the reaction is complete and the reagents are in the right stoechiometry, then the mass of A+B is equal to C+D.
In your case of iron chloride reaction, it seems complete so you dont need a Ke value.
The Ke value is for incomplete reactions that are at equilibrium.
Like H2O <=> H+ + OH-
Ke = [H][OH]/[H2O]
if the reaction is complete then Ke = infinity because the divider is close to zero.
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