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Practical Eighteen-Le Chatelier's Principle

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Practical Eighteen-Le Chatelier's Principle Date: 25.10.04 Introduction: 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 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. ...read more.


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. Observations: Steps 1 to 4: Agent added Colour change Shift in Fe(CNS)3 conc. Iron (III) chloride Dark red Right Ammonium chloride Light red Left KCNS Darker red Right KI Lighter red Left NaOH Yellow Left Hydrochloric acid Dark Red Right Hydrogen peroxide Orange-yellow Left Phenolphthalein Pink Ammonium chloride Green then Colourless 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. ...read more.


+ 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. ?? ?? ?? ?? Kelly, Gavin & Siew Peng 1 ...read more.

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