Question

Draw out all ions in solution for each step:

Im not really sure what they want me to do for this question, below I will list the steps for the experiment we did.

Part I. Chromate/ Dichromate Ion Equilibrium

In the presence of the hydrogen ion (H+), chromate ions (CrO42-) react to produce dichromate ions (Cr2O72-) according to the following reaction.

CrO42- (aq) + H+ (aq) Cr2O72- (aq) + H2O (l)

1. In a clean small test tube, place 1 to 2 mL of 1 M K2CrO4. Add several drops of 3 M H2SO4. Shake the test tube to mix the solution. Record your observations.

2. Add several drops of 6 M NaOH, until you observe a change (mixing after each drop).

3. Add several drops of 3 M H2SO4 until you observe a change.

4. Add several drops of 6 M NaOH until you observe a change.

Part 2:

Step 1: Place 4 mL of 0.15 M CoCl2∙(H2O)6 (dissolved in methanol) into a small test tube. Split this sample into two test tubes so one can be a control. Add 6 M HCl dropwise to one of the test tubes and record any observations.

2. Place 8 mL of 0.15 M CoCl2∙(H2O)6 (dissolved in methanol) into a small test tube. Using a dropper, add just enough water

to turn the solution pink. If you do not observe a color change in later steps, you may have added too much water and you will need to start here again.

3. Split this sample into 4 test tubes. One sample from this step, and one from step 1 will be controls for visual comparison to the other samples to monitor changes.

4. To one of the test tubes, add 6 M HCl, dropwise, until you observe a color change. Record your observations and explain your results.

5. Add water to the same sample from step 4 and record any observations. Then add HCl again dropwise and record any observations.

6. With a different sample from step 3, add H2SO4 dropwise and record any observations.

7. With the same sample from step 6, add water and record any observations.

8. Heat a different test tube from step 3 in a beaker of hot water. You can heat the beaker of water using a hot plate. Observe and record the color change.

9. If heating the solution caused the equilibrium to shift in one direction, cooling the solution is expected to shift the equilibrium in the reverse reaction. Cool the solution in an ice water bath. Observe and record the color change. Explain your results; include equations showing reactions.

Answer 1

LeChatellier's Principle : A ny change in the reaction at equilibrium causes the equilibrium to shift to that direction so as to minimize the change occured and bring back the equilibrium.

The above experiment is done to prove LeChatellier's principle

a) 2CrO4^2- (yellow) + 2H+ ----> Cr2O7^2- (orange) + H2O

So as can be seen,

we form Cr2O7^2- starting with CrO4- by the addition of acid. As acid is added, the H+ present reacts with yellow CrO4^2- to form orange colored Cr2O7^2-. This continues untill all the added H+ has reacted and equilibrium is established.

the reverse reaction is the addition of base,

Cr2O7^2- + 2OH- ---> 2CrO4^2- + H2O

So when base is added, it reacts with Cr2O7^2- (orange) present in solution and forms CrO4^2- until all the OH- has been used up and equilibrium is restablised

b) Similarly,

[Co(H2O)6]2+ (pink) + 4Cl- <==> [CoCl4]2- (blue) + 6H2O

when a solution of [Co(H2O)6]2+ is heated it turns blue due to the formation of [CoCl4]2-. This is thus an endothermic (heat absorbing) reaction. when the system is cooled, reformation of [Co(H2)6]2+ pink occurs. This is an exothermic reaction, heat is given off. When more of HCl is added, the excess Cl- reacts with [Co(H2O)6]2+ until all of acid is consumed and equilibrium is established. The reaction moves in forward direction. When water is added, the system becomes dilute and it goes in backward direction from blue to pink and forms [Co(H2O)6]2+.