5. A CHM 126 student examined how stressing a second equilibrium can affect equi

Question

5. A CHM 126 student examined how stressing a second equilibrium can affect equilibrium. Into a clean test tube the student placed about 2 mL of 0.5 M K CrOs solution. The student then added 5 drops of 1.0 M Ba(NO,)2 solution and noted the formation of a solid from the following equilibrium: Equilibrium eli Ba.. (aq) CrO/. (aq) Badro. (s) + - The student then added 20 drops of 6 M hydrochloric acid and observed a change in the mixture from the following equilibrium: -Crop (aq) H20 (6 + Equilibrium. 2Cro.2-(aq) + 2H. (aq) Explain what should be observed when 20 drops of hydrochloric acid are added to the test tube. Also, explain the chemical concepts behind this observation. Points will be deducted if you do not note which equilibrium and which compound you are referring to in your explanation Use the following color descriptions in your explanation and observation: K CrO4 solution: yellow solution Ba(NO solutions: colorless solution BaCrOs solid: light yellow solid K Cr O, solution: orange solution Predicted observation upon addition of 6 M hydrochloric acid to the mixture (2 points) Explanation of observation on adition of 6 M hydrochloric acid to the mixture (3 points)

Explanation / Answer

5. When Ba(NO3)2 was mixed with K2CrO4 we get solid BaCrO4 (light yellow) formation.

This is shown in equilibrium 1

When HCl is added to this test solution, we have formation of BaCr2O7 (green solution). this is because the soluble CrO4^2- in equilibrium 1 reaction is reacting with added H+ in solution to form BaCr2O7. this is according to the LeChatellier;s principle, which states that if a change occurs in a equilibrium reaction, the reaction would shift to that direction so as to minimize that change adn reestablish equilibrium condition. This is shown by equilibrium 2.

-So addition of HCl would shift direction of equilibrium reaction 2, towards product end and more of Cr2O7^2- is formed.

- This as explained above is according to the Lechatellier's principle.

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