Investigate Le Chatelier's Principle using three named experiments

Question 1

CoCl4^2- + 6H20 Co(H20)6 ^2 + +4Cl-
Blue Red
What colour change is observed when concentrated HCL is added to the solution and explain?

Answer 1

Causes the red colour to change to blue. This is because the HCL increases the concentration of Cl- ions and to relieve the stress, the equilibrium is shifted from right to left in accordance with Le Chateliers principle. Thus, the concentration of CoCl42- is increased and the solution turns blue

Question 2

CoCl4^2- + 6H20 Co(H20)6 ^2 + +4Cl-
Blue Red
What colour change is observed when water is added to the solution and explain?

Answer 2

Causes colour to change from blue to red due to the fact that the equilibrium is shifted from left to right in order to absorb the stress of the increased concentration of water

Question 3

CoCl4^2- + 6H20 Co(H20)6 ^2 + +4Cl-
Blue Red
What colour change is observed when the solution is placed in a beaker of hot water

Answer 3

The colour changes from red to blue, this is due to the fact the forward reaction is exothermic and in order to absorb the added head, the equilibrium is shifted in the direction of the endothermic reaction. Hence, the colour of CoCl4 ^2- (blue) predominates

Question 4

CrO7^2- + H20 2CrO4^2- + 2H+
Orange Yellow
What colour change is observed when sodium dichromate is dissolved in water?

Answer 4

Orange

Question 5

CrO7^2- + H20 2CrO4^2- + 2H+
Orange Yellow
What colour change is observed when sodium hydroxide is added to the sodium dichromate solution and explain

Answer 5

Orange colour changes to yellow
Sodium hydroxide reacts with H+ ions on the right hand side and removes them. This gives rise to a stress and the forward reaction predominates. This forward reaction produces more H+ ions and the colour changes from orange to yellow since more CrO4^2- ions are produced

Question 6

CrO7^2- + H20 2CrO4^2- + 2H+
Orange Yellow
How would you reverse the colour change from the added sodium hydroxide?

Answer 6

Add dilute hydrochloric acid until the yellow colour changes to orange, i.e the Cr2O72- predominates since equilibrium is shifted to left to absorb the excess of H+ ions

Question 7

Fe3+ + CNS- Fe(CNS)2+
Yellow Red
Describe the colour observed when a solution of potassium thiocyanate is mixed with a solution of Iron (III) chloride

Answer 7

Red colour is observed due to presence of Fe(CNS)2+

Question 8

Fe3+ + CNS- Fe(CNS)2+
Yellow Red
Describe what is observed if some dilute HCl is added to the mixture when potassium thiocyanate is mixed with iron (III) chloride

Answer 8

Red colour becomes colourless or pale yellow

Equilibrium is shifted to the left hand side in order to absorb the Cl- ions added as the Cl- ions give rise to a stress

Question 9

K2Cr2O7.2H2O, HCl, NaOH, CoCl2.6H2O and H20
Describe clearly how a student could use a selection of the chemicals listed above to establish a chemical equilibrium and write a balanced equation
Describe how to demonstrate the effect of concentration on that chemical equilibrium

Answer 9

Dissolve solution of potassium dichromate in water
CrO7^2- + H20 -> 2CrO4^2- + 2H+
Add sodium hydroxide, orange -> yellow