A study of equilibrium system

Question

Assignment:

A study of equilibrium system

Purpose - to study the effect of varying concentrations of the hydrogen ion on the equilibrium system for chromate/dichromate ions.

Intoduction - the equilibrium to be considered in this experiment is represented by the equation

H⁺(aq) + 2CrO₄^2-(aq) ←→ Cr₂O₇^2-(aq) + OH^-(aq)

In Part 1 study the changes in the concentration of H⁺(aq) and OH^-(aq) to determine the effects of the alterations on the system, and to judge the reversibility of the reaction. The colour changes are from orange to orange-yellow.

Part II deals with a further equilibrium between the ions of BaCrO₄ and the solid, as the concentration of the CrO₄^2-(aq) is changed in the system

BaCrO₄(s) ←→ Ba²⁺(aq) + CrO₄^2-(aq)

Part III is interesting in that it studies the effects on the equilibrium of other acids and bases, and of a typical organic compound, ethanol.

Procedure:

Part I: The CrO₄^2-/Cr₂O₇^2- Ion Equilibrium

1. Place ¼ test-tube of K₂CrO₇ solutions in separate test-tubes. Note and record the colour of each solution. Add, one drop at a time, alternately to each solution, the NaOH solution until a colour change is noted in one of the solutions. Note the change in colour. Retain these test tubes for step 4.

2. Repeat step 1 with fresh solutions ion clean test-tubes, but add HCl solution, drop by drop, alternately to each test-tube. Note the colour change. Retain these test-tubes for step 3.

3. Add NaOH, drop by drop, either of the test tubes in step until a change is noted.

4. Add HCl, drop by drop, either of the test-tubes in step 1 until a change is noted.

Part II: The Equilibrium of BaSO₄(s) with Ba²⁺(aq) and CrO₄^2-(aq) Ions

5. To ¼ test-tube of K₂CrO₄ solution, in a clean test-tube, add 2 drops of NaOH solution. Add the Ba(NO₃)₂. Retain this test-tube solution, drop by drop, until a change is noted. Retain this test-tube for step 7.

6. To ¼ test-tube of K₂Cr₂O₇ solution, add 2 drops of HCl, then add about 10 drops of Ba(NO₃)₂. Retain this test-tube for step 8. Record your conclusions about the relative solubilities of BaCrO₄ and BaCr₂O₇.

7. To the test-tube from step 5 add HCl, drop by drop, until a change is noted.

8. To the test-tube from step 6 add NaOH, drop by drop, until a change is noted.

9. Add a few drops of Ba(NO₃)₂ to ¼ test-tubes of K₂CrO₄ and K₂Cr₂O₇. The latter reactions may be a bit slow.

10. To ¼ test-tube of K₂CrO₄ and K₂Cr₂O₇, in separate test-tubes, add HNO₃ solution, one drop at a time, alternately to each solution, until a colour change is noted on one of the solutions. Using fresh solutions of K₂CrO₄ and K₂Cr₂O₇, repeat this procedure to determine the effect of the following substances: CH₃COOH, K₂SO₄, KOH, NH₃, Ca(OH)₂, and C₂H₅OH.

Observations:

	Solution of K2CrO4 (yellow)	Solution of K2CrO7 (orange)
1	stays yellow	becomes yellow
2	becomes orange	stays orange
3	becomes yellow	becomes yellow
4	becomes orange	becomes orange
5	forms a yellow preciptate
6		no precipitate
7	precipitate dissolves and becomes orange
8		forms a yellow precipitate
9	large amount of yellow precipitate formed	small amount of yellow precipitate formed
10	CH3COOH becomes orange	stays orange
	H2SO4 becomes orange	stays orange
	KOH stays yellow	becomes yellow
	NH3 stays yellow	becomes yellow
	Ca(OH)2 stays yellow	becomes very pale
	C2H5OH no change	no change

Questions:

Q1. How does the (H⁺) of the solution affect the CrO₄^2-/Cr₂O₇^2- eqiuilibrium?

Q2. What poof was shown that CrO₄^2- ions exist in a solution of K₂Cr₂O₇.

Q3. In what way does OH^- affect the equilibrium? Give two methods by which this ion upsets the equilibrium?

Q4. Write the equations for the equilibrium, showing

a) the reaction between H⁺ and CrO₄^2- to produce Cr₂O₇^2- and H₂O

b) the reaction between OH^- and Cr₂O₇^2- to produce CrO₄^2- and H₂O

c) a summation equation showing both H⁺ and OH ^- in the equilibrium reaction between CrO₄^2- and Cr₂O₇^2- ions.

Q5. Explain fully what happens when H⁺ is added to a mixture containing BaCrO₄(s) and CrO₄^2-(aq).

Q6. In effect what was occurring in steps 7 and 8?

Q7. Explain the general effect of the acids and bases added in part III.

Q8. If all the bases have OH^- (aq) in common, then explain why NH₃(aq) acts like a base.

Q9. Explain why Ca(OH)₂ has little effect on the equilibrium.

Is it because it is moderately soluble?

Q10. Why does C₂H₅OH not affect the equilibrium?

Q11. List the conclusions for this experiment, based on le Chatelier's principle.

Provide complete and step by step solution for the question and show calculations and use formulas.