Le Chatelier's Principle

Le Chatelier's Principle

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Le Chatelier's Principle


Le Chatelier's Principle states that if a change is made to a system
in equilibrium, the system reacts in such a way as to tend to oppose
the change, and a new equilibrium is formed. For example, whatever is
done to the equilibrium, the system does the opposite. If something is
added to a system at equilibrium, the system will behave as to remove
it and vice versa. By increasing the concentration of a reaction, it
will result in an increase of the rate of reaction. The new
equilibrium will have a higher concentration of products too. The
system has shifted to relieve the stress. Therefore, the forward and
backward reactions have a balanced equilibrium system. For example: A
+ B [IMAGE] C + D

If the concentration of the reactants B is increased, the equilibrium
will shift to the right, therefore forming more products. If
concentration of C increases, equilibrium will shift to the left
forming more A and B.


To investigate Le Chatelier's Principle and effect of concentration
changes on the position of equilibrium


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


Teat pipette


1. 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.

2. 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.

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g. of KCNS was added to the
third, 1 r.g. o KL was added to the fourth. Any colour intensity
changed was recorded.

3. 2 drops of NaOH was added to the first drop and HCl was followed
dropwise. Observations were recorded.

4. Hydrogen peroxide was added to the fourth drop dropwise.
Observations were recorded.

5. 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.

6. Added a second r.g. and observations were recorded.


Steps 1 to 4:

Agent added

Colour change

Shift in Fe(CNS)3 conc.

Iron (III) chloride

Dark red


Ammonium chloride

Light red



Darker red



Lighter red





Hydrochloric acid

Dark Red


Hydrogen peroxide





Ammonium chloride

Green then Colourless


Fe3+(aq) + CNS-(aq) [IMAGE]FeCNS2+ (aq)

Kc = [IMAGE]

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) [IMAGE] Fe2+(aq) + I2 (aq)

Kc = [IMAGE]

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) [IMAGE] Fe(OH)3 (aq)

Kc = [IMAGE]

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) [IMAGE] 2Fe3+(aq) + 2OH-(aq)

H2O2 (aq) + 2I-(aq) [IMAGE] I2 (aq) + 2OH-(aq)

Kc = [IMAGE]


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

H-phph(aq) [IMAGE]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.


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.
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