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Topic: Determine concentration of individual reactant from the conductivity  (Read 18178 times)

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Offline kurashi

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Hiz all ,

Currently, I am doing a assignment to determine the rate law of this eqn through a experiment using a batch reactor at (15s intervals) and conductivity meter.

0.01M     0.05M
NaOH + CH3COOC2H5---------->CH3COONa + C2H5OH

As from the equation, only NaOH and CHCOONa contribute to the conductivity of the solution. However, I can only obtain the total conductivity of the solution ( conductivity of NaOH + conductivity of CH3COONa ) May I know is it possible to obtain the conductivity of each individual's reactant?

Pls help

Thank you

Beat regards
Desmond

Offline Borek

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May I know is it possible to obtain the conductivity of each individual's reactant?

No, but if you assume that conductivities are additive you can find out how the reaction goes measuring total.

Hint: Na+ concentration doesn't change, [OH-] + [CH3COO-] = const.
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Offline kurashi

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But sir,
what do you mean by addictive?
My hypothesis is  the conductivity total will decrease with respect to time . Am I right?

Regds
Desmond

Offline wereworm73

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Yes, you would be right about the conductivity dropping because the reaction is replacing the highly mobile hydroxide anions with the less mobile acetate. 

Didn't you measure the conductivity of 0.01 M NaOH before adding the ethyl acetate?  That should be the solution's conductivity at t=0 sec., right?  And when the conductivity finally bottoms out, wouldn't that suggest that the reaction is complete, with the solution containing 0.01 M sodium acetate instead of 0.01M NaOH?

Anyway, since [NaOH] + [CH3COONa] = 0.01 throughout the reaction, you can say the overall conductivity of the solution is equal to ax + b(0.01-x), with a=conductivity of sodium hydroxide, b=cond. of sodium acetate, x=[NaOH] & (0.01-x) being [sodium acetate].   If you measure the conductivity of the NaOH before adding any ethyl acetate, you can easily solve for a.  Then using the final data point (assuming you allowed the reaction to completion), you can calculate b.  Once you've done that, you can find out the concentrations at any data point.

Offline kurashi

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Thanks for your reply...it has been very helpful. However, i found out that the experiment conducted for one of the reactants have to be in excess. Hence. the experiment have to be done twice for each reactant(in excess).

What i'm afraid is that the conductivity measured, includes the excess e.g(NaOH) that was not reacted. Therefore, I'm not sure whether the method/equation given by you is applicable. ???

For the experiment, I have to set the concentration of the reactants [NaOH] :[CH3COOC2H5] to the ratio of 1:5 respectively and vice versa. Hence, I have to use the method of excess to calculate the rate law.

If thats the case, how to find the conductivity of each reactant from the conductivity total?

I am supposed to calculate the conductivity of NaOH and conductivity of NaAc using the eqn below,

Conductivity total=conductivity of NaOH + conductivity of NaAc

conductivity of NaOH = 0.195[1+0.0184(T-194)][NaOH]

conductivity of NaAc  = 0.07[1+0.0284(T-194)][NaAC]


And since one of the reactants ( NaOH ) is in excess, which reactant (NaOH or CH3COOC2H5) should I put in first so as not to spoil the conductivity meter?

Thank you

Regds
Desmond
« Last Edit: July 04, 2006, 12:30:47 AM by kurashi »

Offline wereworm73

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OK, for the reaction involving 0.05M NaOH & 0.01M ethyl acetate, you can do the same thing.  Measure the conductivity of the hydroxide like before, only it will be 5x more concentrated.  Like you said in the first post, ethyl acetate doesn't contribute to the conductivity so there's really no point measuring it.  After a while, the conductivity will bottom out (though at a higher level this time since there's still 0.04M NaOH in solution).

As for calculating the rate law, just realize [NaOH]+[NaAc]= initial [NaOH]  and [EtAc]+[NaAc]= initial [EtAc].  It'll apply for both experiments so you don't need to worry about reagents being in excess.

Offline kurashi

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Sir,
If that was the case,can I say that only 0.01 M if NaOH has been reacted for the 2 experiments , the equation that you have given overall conductivity= ax+b(0.01-x) is vaild?
However, is the a and b constant? for my experiment, a and b is a variable hence is it still vaild?

Regds
Desmond
« Last Edit: July 04, 2006, 04:53:35 AM by kurashi »

Offline Borek

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overall conductivity= ax+b(0.01-x) is vaild?

I would rather go from the start for

overall conductivity= c + ax + b(0.01-x)

as you may expect some conductivity even before NaOH addition. When there is no NaOH excess, c will be small (perhaps even neglectable). In the case of NaOH excess c will be much larger, but calculations made will be in both cases identical.
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Offline kurashi

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However for my experiment, I will have a list of conductivity total at 15 s intervals, hence, from the eqn, the conductivity of NaOH and conductivity of NaAc will be a variable. However, as far as i known, the a and b is contant throughout?

Please correct me if Im wrong.

Thank you

Best regards

Desmond

Offline Borek

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a, b and c are constants.

Note that overall conductivity is linearly dependent on the x:

overall conductivity= c + ax + b(0.01-x) = (c + 0.01b) + (a-b)x = a' + b'x
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Offline kurashi

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Ok. I understood. However, currently, I have to determine the Rate law by using a batch reactor for where at each time intervals, I can only obtain the data for overall conductivity. And at each time intervals, the conductivity will change, hence, I have to calculate the concentration of NaOH from the time intervals and conductivity respectivity.

Hence, does the method still works?

Thank you

Regds
Desmond

Offline Borek

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I have to calculate the concentration of NaOH from the time intervals and conductivity respectively.

You may easily calculate x from the conductivity data.

Quote
Hence, does the method still works?

Yes. But I am not sure finding NaOH concentration is the simplest approach. I think you should concentrate on x (defined as reaction fraction).
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Offline kurashi

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Ok.. If that is the case, Lets assume X as the reducing factor or so called the conversion factor,

then, for eg,
 
at t=0,  Cond = 1       X0=0       [NaOH] not reacted = y(1-X0)
at t=1,  Cond = 0.5    X1=0.5    NaOH] not reacted =  y(1-X1)
at t=2,  Cond = 0.3    X2=0.7     [NaOH] not reacted = y(1-X2) 

is my steps vaild as i assume the conductivity data u have mentioned refers to my data from the experiment.

Thank you

Regds
Desmond

Offline Borek

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Looks OK.
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Offline kurashi

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However, from the steps i introduced, i realise that the
overall conductivity = drop in conductivity of NaOH + increase in Conductivity of CH3C00Na,

Hence, the difference of overall conductivity consist part of " some" conductivity of CH3COONa, hence, is it still possible to use it?

Best regds
Desmond

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