Ignoring x in Chemical Equilibrium

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Hayley Hammons 1L
Posts: 26
Joined: Fri Sep 25, 2015 3:00 am

Ignoring x in Chemical Equilibrium

Postby Hayley Hammons 1L » Thu Nov 19, 2015 10:18 am

In what cases can we assume that x makes a negligible difference when calculating change in concentration for a molecule in an equilibrium expression? When can we ignore it so that it makes the math of calculating change for other molecules in the reaction easier?

Kelly Sun 3E
Posts: 28
Joined: Fri Sep 25, 2015 3:00 am

Re: Ignoring x in Chemical Equilibrium

Postby Kelly Sun 3E » Thu Nov 19, 2015 10:31 am

You can assume x makes a negligible difference when k<10^-3. Anything less than this indicates that the reaction strongly favors the reactants, so you can assume the change in concentration for the reactants is minuscule.

SubparChemist
Posts: 27
Joined: Fri Sep 25, 2015 3:00 am

Re: Ignoring x in Chemical Equilibrium

Postby SubparChemist » Thu Nov 19, 2015 3:46 pm

Going off that, is it safe to just never make that approximation? I understand there are problems where it'd be very difficult to solve without making the approximation to zero or using a graphing calculator but would we be deducted points for not utilizing this concept/trick?

Kelly Huang 3L
Posts: 18
Joined: Fri Sep 25, 2015 3:00 am

Re: Ignoring x in Chemical Equilibrium

Postby Kelly Huang 3L » Fri Nov 20, 2015 5:44 am

Yes, but it would be a waste of time and effort. The answer will be the same.

Zeynep Celikkol 2A
Posts: 9
Joined: Fri Sep 25, 2015 3:00 am

Re: Ignoring x in Chemical Equilibrium

Postby Zeynep Celikkol 2A » Fri Nov 20, 2015 3:17 pm

Also, just as a way to check if it is reasonable that you ignored x: after you find the equilibrium concentrations of the reactants, find the percentage of ionization/protonization, which is the hydrogen ion concentration at equilibrium divided by the initial concentration of acid for finding the percentage of ionization. If this percentage is less than 5% then it is reasonable that you ignored x because this means that only a small amount of the acid gives of a proton and Kc is really small.


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