Question

Re-write the chemical reaction for the equilibrium established between the chromate and dichromate ion. Label the...

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.

2CrO42- (aq) + 2H3O+ (aq) Û Cr2O72- (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.  

Homework Answers

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

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