Skip to main content

Using le chatelier's principle find under what situation equilibrium will shift towards reactant side

Submitted by mang05 on Sun, 07/15/2012 - 03:13

Which reagent, HCl or NaOH, should you use to shift this equilibrium towards the reactants?


Hello. 

To shift the equilbrium, le chatelier's principle says that stress must be applied, either by increasing the concentration of BaF2 or by decreasing the concerntration of the reactants. In this case, adding NaOH will decrease the concentration of Ba2+, because barium will actively replace sodium (according to the activity series), and because barium hydroxide is only slightly soluble. The decrease in moles of barium ions availible for reaction will stress the system, and to attempt to restore the previous state, the equilibrium will shift backwards—to the left—as BaF2 decomposes more into its ionic components. 

Source

http://sp.eths.k12.il.us/science/ssc/Documents/Chemistry/ActivitySeriesChart.pdf
http://www.chemguide.co.uk/physical/equilibria/lechatelier.html
http://www.csudh.edu/oliver/chemdata/solrules.htm


There is a bit of a problem in this explanation:

"In this case, adding NaOH will decrease the concentration of Ba2+, because barium will actively replace sodium (according to the activity series), and because barium hydroxide is only slightly soluble."

While it is true that Ba is more active than Na, this is not a case where Ba atoms are replacing Na+ ions (a single replacement reaction).  Therefore this part of the explanation is not correct.

However, the 2nd part of the answer is correct.  The hydroxide ions are combining with the Ba2+ ions in a double replacement reaction forming the slightly soluble Ba(OH)2.  

                    Ba2+  (ag)    +    2 OH- (aq)    -->    Ba(OH)2 (s)

This does lower the concentration of the Ba2+ ions and causes the original equilibrium reaction:

                        Ba2+ (aq)   +   2  F- (aq)      <===>    BaF2 (s)

to shift towards the reactants causing BaF2 (s) to dissolve.