Equilibrium Lab #1
In aqueous solution, the iron(III) ions will react with thiocyanate ions
according to the equation:
Fe3+ + SCN- <-----> [FeSCN]2+
An equilibrium is established among these species of ions.
An established equilibrium may be altered by applying a stress to the
system. One such stress that can be applied to an ionic equilibrium in
solution is a decrease or increase in the concentrations of one of the species
of ions.
The principle of Le Châtelier states: When a stress is brought to bear on a
system in equilibrium, the system tends to change so as to relieve the
stress." Thus we would then expect anything that changes the
concentration of any of the components of a system in equilibrium to alter
the equilibrium.
Since Fe3+ is yellow, SCN- is colourless, and the [FeSCN]2+ is a very deep
blood red, we would expect to notice changes in colour if changes in the
concentration of the red [FeSCN]2+ ion. IF it decreases, we would expect a
lighter red to be produced and if it increases, a darker red. You will
investigate the effects upon this equilibrium of the addition of several
compounds which will have the effect of increasing or decreasing one of the
component ions of the equilibrium mixture.
Another equilibrium system that you will investigate is one involving the
sparingly soluble calcium oxalate:
CaC2O4(s) <-------> Ca2+(aq) + C2O42-(aq)
Any substance added to this system which can tie up the C2O42-, thereby
decreasing its concentration, will cause more calcium oxalate to dissolve.
You will see the effects of addition of hydrochloric acid on this equilibrium.
Temperature changes may also affect equilibria. You study the effect of
temperature on the following equilibrium:
[Co(H2O)6]2+ + 4 Cl- <-----> [CoCl4]2- + 6 H2O
Procedure
A. Iron-thiocyanate equilibrium
1. Into a clean 250-mL beaker, pipet 0.3 mL of 1 M iron(III) nitrate,
Fe(NO3)3, and 0.3 mL of 1 M ammonium thiocyanate, NH4SCN. Add about
75 mL of distilled water. Mix well.
2. Divide the solution, just prepared, into approximately equal portions into
four 6-inch test tubes and number the test tubes, 1 through 4.
3. To tube number 1, add 5 drops of distilled water. Stopper this tube and
invert it several times to mix it. Use this tube for comparison as your
control.
4. To tube number 2, add, dropwise, 5 drops of 1 M iron(III) nitrate. Put a
cork stopper in the tube and mix it by inverting several times. Note and
record any change in colour.
5. To tube number 3, add, dropwise, 5 drops of 1 M ammonium
thiocyanate. Mix as before. Note and record any colour changes.
6. To tube number 4, add, dropwise, 5 drops of 1 M ammonia solution
(NH3). Mix as before. Note any change from the original colour.
B. Calcium oxalate equilibrium
1. In a clean 6-inch test tube, put 5 mL of 0.1 M calcium chloride, CaCl2,
and add 5 mL of 0.1 M sodium oxalate, Na2C2O4. Mix by stoppering and
inverting the tube several times. Note the formation of a white precipitate
of calcium oxalate. The following equilibrium is established.
Ca2+ + C2O42- <----> CaC2O4
2. To the above tube containing the white precipitate of calcium oxalate,
add concentrated hydrochloric acid, dropwise, shaking the tube after each
addition of acid. Note any changes.
C. Cobalt chloride equilibrium
Heat 2 mL of a saturated aqueous solution of CoCl2 to boiling in a 6-inch
test tube. Record the colour of the solution before and after heating. Now,
place the tube in ice water and record any change in colour observed.
Questions
These questions are to be answered when you have completed the Lab
report. Space is provided for the answers following the report.
1. Fe3+ in the presence of hydroxide ions forms an insoluble gelatinous
precipitate. Did this happen when you added the ammonia solution to the
equilibrium system? How do you know?
2. How did this affect the equilibrium?
3. Given the information that oxalic acid is a weak acid, can you explain
briefly why calcium oxalate dissolves when hydrochloric acid is added to
that equilibrium system?
4. If you were given the information that the [CoCl4]2- ion is blue, how is the
third equilibrium altered by heating.
Introduction to Equilibrium
Lab #1 Report
Prelab Study Quiz
Name:____________________
Date:____________________
1. If a reagent added to the first equilibrium to be studied causes the colour
to go from deep red to orange, how has the equilibrium been altered?
_____________________________________________________________
2. If calcium chloride solution were added to the second equilibrium, would
you expect an increase in the precipitate or a decrease in the amount of
precipitate?
_____________________________________________________________
3. How would addition of sodium chloride to the third equilibrium to be
studied alter the equilibrium?
_____________________________________________________________
Chemical Equilibrium Lab Data
A. Iron(III) thiocyanate equilibrium
Colour of control tube ________________________________
Colour change in tube 2 ________________________________
Colour change in tube 3 _______________________________
Colour change in tube 4 ________________________________
B. Calcium oxalate equilibrium
Change __________________________________________________
C. Cobalt(III) chloride equilibrium
Colour of the solution before heating ___________________
Colour of the solution after heating ___________________
Colour of the solution upon cooling ___________________
The purpose of this study is:
_________________________________________________________
_________________________________________________________
_________________________________________________________
Results and Conclusions
Addition of the following solutions alter the equilibrium
Fe3+ + SCN- <----> [FeSCN]2+ as given
Added Reagent
[FeSCN]2+ increases
[FeSCN]2+ decreases
Ferric nitrate
__________________
____________________
Ammonium thiocyanate __________________
____________________
Ammonium hydroxide
____________________
__________________
Addition of HCl to the equilibrium Ca2+ + C2O42- <-------------> CaC2O4
alters the equilibrium to ( increase, decrease ) to amount of solid CaC2O4.
Temperature (has, does not have) a definite effect on the
[Co(H2O)6]2+ + 4 Cl- <--------> [CoCl4]2- + 6 H2O equilibrium.
Equilibrium is affected by the following changes:
______________________________________________________________
______________________________________________________________
Answers to the Questions
1. ___________________________________________________________
___________________________________________________________
2. ___________________________________________________________
___________________________________________________________
3. ___________________________________________________________
___________________________________________________________
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