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
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
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
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
Prelab Study Quiz Name:____________________
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
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
Equilibrium is affected by the following changes:
Answers to the Questions