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Investigation 12C

Investigation 12C. Le Châtelier ’ s Principle. Investigation 12C: Le Châtelier ’ s Principle. When a chemical equilibrium exists, can we predict which way a chemical reaction will proceed when there is a change in concentration of a reactant?

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Investigation 12C

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  1. Investigation 12C Le Châtelier’s Principle

  2. Investigation 12C: Le Châtelier’s Principle When a chemical equilibrium exists, can we predict which way a chemical reaction will proceed when there is a change in concentration of a reactant? Can a change in the temperature of the surroundings cause one side of a chemical reaction to be favored?

  3. Fe3+(aq) + SCN–(aq) FeSCN2+(aq) clear red/orange clear light yellow Investigation 12C: Le Châtelier’s Principle There is an equilibrium between reactants and products NO3– and K+ are spectator ions; they are not involved in the reaction and are not shown in the equation In each step you will create a change that causes the equilibrium of the complex ion (FeSCN2+) to shift to the left or to the right.

  4. Investigation 12C: Le Châtelier’s Principle Part 1: Setting up 1. The control solution • In a small test tube, add 2 mL of 0.002 M ferric nitrate Fe(NO3)3 and 2 mL of 0.002 M potassium thiocyanate KSCN. • Use an empty test tube to mix the contents back and forth. • Describe the color change and keep this test tube as your control. 2. The experimental solutions • Prepare 7 additional identical test tubes. • Mix each one and compare. They should all look similar in color. If one looks different, discard it and prepare it again.

  5. Investigation 12C: Le Châtelier’s Principle Potassium hydroxide(KOH) is a strong base. It is a caustic substance to be handled with care. Avoid contact with skin.

  6. Investigation 12C: Le Châtelier’s Principle Part 2: Experiment and observations After each addition (for steps 1–5), use a clean empty test tube to mix back and forth, and record your observations. Use the control tube for color comparisons. • Add 2–3 drops of 1.0 M KSCN, a source of K+ and SCN– ions. • Add a few drops of 1.0 M Fe(NO3)3, a source of Fe3+ and NO3– ions. • Add a few drops of 6.0 M KOH, a source of K+ and OH– ions. • Add a few tiny pieces of solid K3PO4, a source of K+ and PO43– ions. • Add 5.0 mL of distilled water.

  7. Tube 7 Investigation 12C: Le Châtelier’s Principle Part 2: Experiment and observations For steps 6 and 7, be sure to wait 2–3 min before recording your observations. Use the control tube for color comparisons. • Place the reaction on ice for 2–3 min. • Place the reaction in hot water for 2–3 min. You may use hot tap water or heat a test tube of water using the Lab-Master heater. • Wipe the outside of tubes 6 and 7, and compare the two next to each other. Record your observations.

  8. Fe3+(aq) + SCN–(aq) FeSCN2+(aq) clear red/orange clear light yellow Investigation 12C: Le Châtelier’s Principle Part 3: Thinking about it • How can you determine whether or not a change occurred in your equilibrium mixture? Explain. • For each reaction in Part 2, explain how each change can be described by Le Châtelier’s Principle. Was your color lighter/more yellow or darker?

  9. Investigation 12C: Le Châtelier’s Principle Part 3: Thinking about it • Use your observations from Steps 3 and 4 from Part 2 to explain why your system became lighter or darker. Here you will need to refer back to your solubility rules (Chapter 10) and think about how a precipitate would affect your concentration of ions.

  10. Higher temperature Lower temperature Fe3+(aq) + SCN–(aq) FeSCN2+(aq) clear red/orange clear light yellow Investigation 12C: Le Châtelier’s Principle Part 3: Thinking about it • Use your observations from Steps 6 and 7 of Part 2to explain whether this equilibrium reaction is exothermic or endothermic in the forward direction.

  11. Investigation 12C: Le Châtelier’s Principle Part 4: What did you learn? • Did this experiment help you to understand Le Châtelier’s Principle? Why or why not? • Was the control test tube helpful? • Complex ions such as ferric thiocyanate often form brilliantly colored solutions. See whether you can find another complex ion using the Internet. Write down its chemical formula and its color. The red/orange color is due to the presence of FeSCN2+ complex ions

  12. Investigation 12C: Le Châtelier’s Principle Le Châtelier’s principle:principle that states that when a “change” is made to a system at equilibrium, the system will shift in a direction that partially offsets the “change”; the change can be defined as a change in temperature, concentration, volume, or pressure.

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