Equilibrium Lab

System I – Dichromate – Chromate Chromate and dichromate are polyatomic ions found on table E. Dichromate will dissociate into two chromate ions in the presence of water, producing the hydronium ion (acid) as well.

Acids and Bases An acidic solution contains an excess of H+ ions These ions can also be written as H3O+, the hydronium ion Acids are neutralized by bases, which are substances that produce OH- ions in water H3O+ + OH-  2H2O neutralization

Add five drops of K2Cr2O7 to each of two test tubes Add five drops of K2Cr2O7 to each of two test tubes. Fill one with water. Look down into the tubes to compare the colors. Did adding H2O cause the system to shift? Explain what you see. The hydronium ion (H3O+) is the ion of acids. We can add this ion by adding acid and remove it by adding a base. Place 10 drops of K2Cr2O7 and 10 drops of K2CrO4 in each of three 4” test tubes. Add 1” of water to each. One test tube is your color standard. Adding H3O+ - add HCl dropwise until a color change is noted. Removing H3O+ - when the OH- of a base combines with a H3O+, water is produced. Add NaOH dropwise until a color change is noted.

Species Added Color Change Direction of Shift Reason for Shift H2O H3O+ OH-

System II – Cobalt Chloride System In the presence of water, cobalt chloride, which is blue when dissolved in alcohol, turns pink. The water displaces the chloride ions to make the pink cobalt complex:

Place one of the test tubes in the hot water bath for one minute Place one of the test tubes in the hot water bath for one minute. Does the color change? Should heat be considered a reactant or a product? Let the test tube cool and note whether the system shifts back to its original state. What effect should the addition of a few crystals of CaCl2 have on the system? Pour some of the solution into each of two 4” test tubes. Add a small amount (1/2 inch on a splint) of CaCl2 to one tube. Note the change. (CaCl2 provides Cl- ions).

Add a few crystals (1/2” on a splint) of cobalt chloride, CoCl2·6H2O, to each of two dry 6” test tubes. Add 1” (not more) of ethyl alcohol to each and shake to dissolve the cobalt chloride. If the solutions are not pink, add water one drop at a time, shaking, to make the solutions just barely pink. When you added the water, did the system shift appropriately?

System III: Copper – ammonium hydroxide system When copper ions are combined with ammonium hydroxide, a complex series of reactions are possible, all of which obey LeChatelier’s principle. When a small amount of NH4OH is added to CuSO4(aq), a light blue precipitate is formed: CuSO4 + NH4OH ↔ Compound X(aq) + Compound Y(s) On your lab report sheet, write the balanced chemical equation for this reaction. Using Table F, predict what the precipitate is.

CuSO4 + NH4OH ↔ X(aq) + Y(s)

More NH4OH.. When a larger amount of ammonium hydroxide is added, the precipitate disappears and a beautiful dark blue complex of copper and ammonia is formed: Cu(OH)2 + 4 NH4OH ↔ Cu(NH3)42++ 4H2O +4OH- Both of these reactions are completely reversible – all you have to do is add acid!

Procedure Place 10 drops of CuSO4 solution in a small test tube. Add drops of ammonium hydroxide until the blue-white precipitate forms. Identify the precipitate and write the balanced equation for this reaction on your report sheet. Keep adding ammonium hydroxide until the precipitate completely disappears and is replaced by the dark blue ammonia complex. Add HCl to the tube. Describe what you see as you keep adding the acid.