Question

Re-write the chemical reaction for the equilibrium established between the chromate and dichromate ion. Label the colors expected for the chromate and dichromate ions. You may refer to the experimental procedure.

2CrO₄^2- (aq) + 2H₃O⁺ (aq) Û Cr₂O₇^2- (aq) + 3H₂O (l)

(chromate-yellow)                   (dichromate-yellow)

A completed data table #1.

Change to the Reaction	Drops Added	Visual Observations	Shift in the reaction (to products, reactants, or no change)
K2CrO4 solution only		The color after the change to the reaction is yellow. There are no solids, or bubbles that are produced. A transparent yellow color occurs.
First addition of H2SO4	5	No solids or bubbles occurred after the reaction. The color simply changed from a transparent yellow, to a transparent reddish/orange color.
Addition of NaOH	5	The color stayed almost the same, while the color was still orange I could observe that it was slightly more yellow. No precipitates were formed, the solution was still transparent.
Second addition of H2SO4	6	The solution became a deeper red-orange color. The test tube holding the solution became warmer, meaning it was an exothermic reaction.. The solution only subtly changed in color.

3. Using equation 2, interpret your observations in terms of Le Châtelier’s principle when you added sulfuric acid (the first time) to the test tube containing potassium chromate.

need the answer to the shift in reaction, and to #3.  

Answer 1

Given reaction:

2 CrO4^2- + 2H3O+ ------- 2Cr2O7^2- + 3H2O

(chromate-yellow) (dichromate-orange)

Change to the reaction	Drops added	Visual observations	Shift to the reaction
K2CrO4 only		yellow	Reactant, chromate
First addition H2SO4	5	orange	Product, dichromate
Addition of NaOH	5	yellow	Reactant, chromate
Second addition H2SO4	5	orange	Product, dichromate

3. Addition of H2SO4 for the first time creates a higher concentration of CrO42- ions. In order to re-establish the equilibrium the reaction shifts to the products thereby increasing the concentration of Cr2O7^2- ions giving an orange color