common ion effect vs small change assumption (1 Viewer)

[yodela123]

y'all im so confused... what's the difference between the two? I've been seeing people use them interchangeably (?)

 

[bruhwth]

A small change assumption is an assumption as the name suggest, wherein you neglect the changes to the concentration of a species.

The common ion effect is the lack of further dissolution of a salt for example into a solution already containing a high concentration of ions that make up the salt. I.e try to dissolve table salt (NaCl) into a beaker with a high conc. of Na+

You apply the Small change assumption in cases of common ions being present in solution because like i said above, if you try to mix more salt into already salt water it ain't happening. So you assume only a small change in the conc. of the salt.

Also the small change assumption can be applied to highly insoluble chemicals (like salts or acids with a very small Keq). So yh people use the small change assumption to solve the common ion effect q, but it is also applicable in other situations where the equilibrium of a system lies considerably to the left.

Basically, common ion is the situation. Small change is the action you take when faced with the situation.

Idk how to explain it better lmao sorry.

 

[Run hard@thehsc]

you just assume the change of the common ion is negligible (i.e. it equals zero).... using this you can multiply the known concentration of the common ion, times that of the unknown variable equating it with the solubility constant...... then you can find the molar solubility of the substance!

 

[yodela123]

A small change assumption is an assumption as the name suggest, wherein you neglect the changes to the concentration of a species.

The common ion effect is the lack of further dissolution of a salt for example into a solution already containing a high concentration of ions that make up the salt. I.e try to dissolve table salt (NaCl) into a beaker with a high conc. of Na+

You apply the Small change assumption in cases of common ions being present in solution because like i said above, if you try to mix more salt into already salt water it ain't happening. So you assume only a small change in the conc. of the salt.

Also the small change assumption can be applied to highly insoluble chemicals (like salts or acids with a very small Keq). So yh people use the small change assumption to solve the common ion effect q, but it is also applicable in other situations where the equilibrium of a system lies considerably to the left.

Basically, common ion is the situation. Small change is the action you take when faced with the situation.

Idk how to explain it better lmao sorry.

thanks! so for working out I'm meant to quote small change assumption?

 

[user1818]

thanks! so for working out I'm meant to quote small change assumption?

I mean if youre faced with an ICE table type Q where you have some quadratic and the small change assumption is viable just write "assume x is small since [reactants] initial. ≈ [reactant] eqm." : ))

 

[bruhwth]

thanks! so for working out I'm meant to quote small change assumption?

yh i usually write a sentences or two justifying my use of the assumption. I'll give an example:

Common Ion Situtation: "Since there is a preexisting high concentration of A ion in solution, further dissolution of AB (s) will drive the equilibrium to the left precipitating out ions. Therefore assume only a small change in the conc of AB. i.e. 0.054-X ~ (idk how to write the approximate sign) 0.054."

Insoluble chemical: "Since the equilibrium constant lies to the far left, assume only a minimal change in the conc. of [species]. i.e 0.033-x ~ 0.034"

Something like that just to show the marker yk what your on about