Aim:
To investigate Le Chatelier’s Principle and effect of concentration changes on the position of equilibrium
Requirements:
Potassium thiocyanate, KCNS (0.5g)
Iron (III) chloride FeCL3, (0.5g)
Ammonium chloride NH4CL, (0.2g)
Potassium iodine KI, (0.2g)
Sodium hydroxide NaOH, 2M, (1mL0
Acidified hydrogen peroxide H2O2, 20 volume (1mL)
Concentrated hydrochloric acid HCL, (0.5mL)
Ammonia NH3, 2M, (1mL)
Ammonium chloride NH4CL, (2g)
Phenolphthalein (1mL)
Test tubes and rack
White tile
Spatula
Teat pipette
Procedures:
- 3mL of water was added to 2r.g. of KCNS in the test tube and it was swirled to dissolve the KCNS. 2r.g. of iron (III) chloride was added. Observations were recorded.
- 4 drops of the solution was placed on the white tile and 1 r.g. of iron ( ) chloride was added to the first, 1r.g. of ammonium chloride was added to the second, 1 r.g. of KCNS was added to the third, 1 r.g. o KL was added to the fourth. Any colour intensity changed was recorded.
- 2 drops of NaOH was added to the first drop and HCl was followed dropwise. Observations were recorded.
- Hydrogen peroxide was added to the fourth drop dropwise. Observations were recorded.
- A few drops of phenolphthalein were added to a few drops of dilute ammonium in the test tube. 1 r.g. of ammonium chloride was added. Swirled it and observed it.
- Added a second r.g. and observations were recorded.
Observations:
Steps 1 to 4:
Discussion:
Fe3+(aq) + CNS-(aq) FeCNS2+ (aq)
Kc =
When water was added to KCNS, KCNS dissolved in water. When iron (III) chloride was added, colour change occurred. It turned dark red.
When iron (III) chloride was added to the solution above, the iron would react with it, thus shifting the equilibrium to the right forming more products. So, concentration of Fe(CNS)3 increased.
When KCNS (aq) was added, by Le Chatelier’s principle, the position of equilibrium would shift to the right so as to remove some of the extra SCN-(aq). More FeCNS2+ (aq) were produced and the solution became darker red.
2I-(aq) + 2Fe3+(aq) Fe2+(aq) + I2 (aq)
Kc =
Iodine would react with Fe3+. This would reduce the concentration of Fe3+ and increased the concentration of Fe2+. According to Le Chatelier’s Principle, when the system was offset, then it would adjust to relieve the stress. Since concentration of product had increased, equilibrium would shift to the left thus producing more Fe3+. So, concentration of Fe(CNS)3 decreased.
Fe3+(aq) + OH-(aq) Fe(OH)3 (aq)
Kc =
When sodium hydroxide was added to the first drop, the colour changed to yellow. As Fe3+ had reacted with the hydroxide, more Fe3+ had to be replaced. So, equilibrium would shift to the left forming more Fe3+. Thus, concentration of Fe(CNS)3 decreased.
When hydrochloric acid was added, equilibrium shift to the left and colour became darker red. So, concentration of Fe(CNS)3 increased. The H+ ions neutralized the OH- ions.
H2O2 (aq) + 2Fe2+(aq) 2Fe3+(aq) + 2OH-(aq)
H2O2 (aq) + 2I-(aq) I2 (aq) + 2OH-(aq)
Kc =
Kc=
When hydrogen peroxide was added to the fourth drop, the colour became lighter and lighter to orange and then to yellow. Hydrogen peroxide reacted with Fe2+ to form Fe3+. Since more Fe3+ was formed, concentration of reactants increased and so the equilibrium shifts to the right forming more products. As a result, concentration of Fe(CNS)3 increased.
H-phph(aq) H+(aq) + phph-(aq)
Phenolphthalein acted as an indicator in this reaction. It was a weak acid. The weak acid is colourless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colourless. Adding hydroxide ions removes the hydrogen ions from the equilibrium which tips to the right to replace them - turning the indicator pink.
Conclusion:
Therefore, it can be concluded that when the system is offset, it will adjust to balance back the system. By increasing the concentration of products can will shift the equilibrium to the left and vice versa.