LAB 15: THE EQUILIBRIUM OF K2Cr2O7 AND K2CrO4

2 CrO42-(aq) + 2 H+ (aq) Cr2O72-(aq) + H2O (l)

All solutions of K2Cr2O7(aq) contain small amounts of the CrO4-2 ion with which it exists in equilibrium. Likewise a solution of K2CrO4(aq) contains small quantities of ions of Cr2O7-2 in an equilibrium. The above reaction summarizes this equilibrium and some factors that can influence it. In this lab you will be given 2 solutions in which either ion predominates at the equilibrium point. You will stress the system by adding H+ ions (from HCl) or removing H+ ions ( by reacting with NaOH). You can follow the equilibrium by the color of the solution, yellow indicates the CrO4-2(aq) ion, which dissociates from K2CrO4(aq) while orange indicates the Cr2O7-2 (aq), which dissociates from the K2Cr2O7(aq) salt. Please note that the water liquid does not influence the equilibrium as it is the solvent and considered to be a constant. Remember that liquids and solids do not affect equilibrium.

MATERIALS:

reaction microplate,

5 micropipettes,

K2Cr2O7(aq)-is orange in color,

K2CrO4(aq)- is yellow in color,

NaOH(aq),

HCl(aq)

PROCEDURE#1:

In this study you will add progressively more NaOH to system, this will neutralize the H+ effectively removing it from the system. The system will reverse direction (shifting the equilibrium to the left) to replace the hydrogen ions. Predict the color change of the reaction.

1)  NaOH and K2Cr2O7(aq) Study. Using a micropipette to dispense each solution, mix the reagents as indicated in the table below in the small wells of a microplate.

Reagent / Well 1 / Well 2 / Well 3 / Well 4 / Well 5 / Well 6
NaOH(aq) / 0 / 1 / 2 / 3 / 4 / 5
H2O / 5 / 4 / 3 / 2 / 1 / 0
K2Cr2O7(aq / 5 drops / 5 / 5 / 5 / 5 / 5

PROCEDURE#2:

In this study you will add progressively more HCl to the system.This will add H+, which the system will try to consume by shifting the equilibrium to the right. As the reaction goes forward, some of the added hydrogen will be consumed.

2) HCl and K2Cr2O7(aq) Study. Using a micropipette to dispense each solution, mix the reagents as indicated in the table below in the small wells of a microplate

Reagent / Well 1 / Well 2 / Well 3 / Well 4 / Well 5 / Well 6
HCl / 0 / 1 / 2 / 3 / 4 / 5
H2O / 5 / 4 / 3 / 2 / 1 / 0
K2Cr2O7(aq / 5 drops / 5 / 5 / 5 / 5 / 5

PROCEDURE#3:

In this study you will add progressively more NaOH to the system. This will remove H+ which the system will try to consume by shifting the equilibrium to the right. As the reaction goes forward, some of the added hydrogen will be consumed.

2) NaOH and K2CrO4(aq) Study. Using a micropipette to dispense each solution, mix the reagents as indicated in the table below in the small wells of a microplate

Reagent / Well 1 / Well 2 / Well 3 / Well 4 / Well 5 / Well 6
NaOH(aq) / 0 / 1 / 2 / 3 / 4 / 5
H2O / 5 / 4 / 3 / 2 / 1 / 0
K2CrO4(aq) / 5 drops / 5 / 5 / 5 / 5 / 5

PROCEDURE#4:

In this study you will add progressively more HCl to the system. This will add H+ which the system will try to consume by shifting the equilibrium to the right. As the reaction goes forward, some of the added hydrogen will be consumed.

2)  HCl and K2CrO4(aq) Study. Using a micropipette to dispense each solution, mix the reagents as indicated in the table below in the small wells of a microplate

Reagent / Well 1 / Well 2 / Well 3 / Well 4 / Well 5 / Well 6
HCl / 0 / 1 / 2 / 3 / 4 / 5
H2O / 5 / 4 / 3 / 2 / 1 / 0
K2CrO4(aq) / 5 drops / 5 / 5 / 5 / 5 / 5

PROCEDURE#5:

To 5 ml each of BOTH solutions, slowly add small amounts of BaNO3, which will dissociate and put Ba+2(aq) into the solution. Observe what happens – HINT consult table F.

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SIMMULATION SIMMULATION

OF A K2Cr2O7 OF A K2CrO4

SOLUTION SOLUTION