Le Chatelier’s Principle Quiz

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5 Questions

What is the effect of adding NH4SCN to the test tube?

Formation of red color due to the reaction with Fe3+

What happens when a test tube containing the reaction mixture is placed in hot water?

Red color fades as the reaction shifts backward

What is the result of adding AgNO3 to the test tube?

Decrease in red color and formation of white precipitate

What happens when a test tube containing the reaction mixture is placed in ice?

Red color becomes more intense as the reaction shifts forward

What is the effect of adding Oxalic acid to the test tube?

Decrease in red color due to the formation of ferric oxalate

Study Notes

Le Chatelier's Principle

  • States that a system at equilibrium will undergo a change to relieve stress when placed under stress
  • Applies to reversible reactions

Application of Le Chatelier's Principle

  • Can be demonstrated using an aqueous solution of iron(III) ions and thiocyanate ions
  • Reaction: Fe3+ + 6SCN- ⇋ [Fe(SCN)6]3- (blood red color)
  • Equilibrium can be changed by decreasing or increasing the concentration of one of the reactants and products

Effect of Concentration on Equilibrium

  • Increasing the concentration of SCN- ions (e.g. adding 0.5ml of SCN- to the solution) shifts the reaction forward, resulting in a deeper red color
  • Decreasing the concentration of Fe3+ ions (e.g. adding NH4Cl to the solution) shifts the reaction backward, resulting in a decrease in the red color
  • Adding a substance that reacts with one of the reactants or products (e.g. adding AgNO3 to the solution, which reacts with SCN- to form a white precipitate of AgSCN) shifts the reaction backward

Experimental Procedure

  • Prepare 5 test tubes containing 4ml of Fe3+ and 3 drops of SCN-
  • Perform different experiments to demonstrate the effect of concentration on equilibrium:
    • Add 0.5ml of SCN- to one tube
    • Add 0.5ml of NH4Cl to another tube
    • Add 0.5ml of AgNO3 to another tube

Test your understanding of Le Chatelier’s principle by applying it to reversible reactions under different stress conditions, such as changes in concentration and temperature. Explore the effects of these stresses on a system at equilibrium.

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