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
NaOH(aq)
H2O
K2Cr2O7(aq
Well 1
0
5
5 drops
Well 2
1
4
5
Well 3
2
3
5
Well 4
3
2
5
Well 5
4
1
5
Well 6
5
0
5
PROCEDURE#2:
In this study you will add progressively more HCl to the system.This will add H+,
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.
which
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
HCl
H2O
K2Cr2O7(aq
Well 1
0
5
5 drops
Well 2
1
4
5
Well 3
2
3
5
Well 4
3
2
5
Well 5
4
1
5
Well 6
5
0
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
NaOH(aq)
H2O
K2CrO4(aq)
Well 1
0
5
5 drops
Well 2
1
4
5
Well 3
2
3
5
Well 4
3
2
5
Well 5
4
1
5
Well 6
5
0
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
HCl
H2O
K2CrO4(aq)
Well 1
0
5
5 drops
Well 2
1
4
5
Well 3
2
3
5
Well 4
3
2
5
Well 5
4
1
5
Well 6
5
0
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.
.
SIMMULATION
OF A K2Cr2O7
SOLUTION
SIMMULATION
OF A K2CrO4
SOLUTION