1.16: Experiment_616_Shifting Equilibrium_1_1_3

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Student Name

Laboratory Date:

Date Report Submitted:

___________________________

Student ID

Experiment Number and Title

Experiment 616: Shifting Equilibrium

Section 1: Purpose and Summary

Determine the effect of different types of stresses to a chemical system at equilibrium.

In this experiment, students will determine the effect of adding or removing a reactant or product, or heating/cooling to a chemical system at equilibrium and apply Le Chatelier’s principle to explain the shift in the position of equilibrium.

The following chemical systems will be studied:

saturated NaCl solution: NaCl(s) ↔ Na⁺(aq) + Cl^-(aq)

acidified chromate solution: 2 CrO₄²^-(aq) + 2 H⁺(aq) ↔ Cr₂O₇²^-(aq) + H₂O(l)

yellow orange

aqueous ammonia solution: NH₃(aq) + H₂O(l) ↔ NH⁴⁺(aq) + OH^-(aq)

with phenolphthalein clear pink

cobalt(II) chloride solution: [Co(H₂O)₆]²⁺(aq) + 4 Cl^-(aq) ↔ [CoCl₄]^2-(aq) + 6 H₂O(l)

pink blue

iron(III)thiocyanate solution: Fe³⁺(aq) + SCN^-(aq) ↔ [Fe(SCN)]²⁺(aq)

lt. yellow clear deep red

Section 2: Safety Precautions and Waste Disposal

Safety Precautions:

Use of eye protection is recommended for all experimental procedures.

Waste Disposal:

While you are doing the experiment, pour your liquid waste into a beaker. When you are finished with the experiment, pour the contents of the waste beaker (liquid waste only) into the inorganic waste container in the fume hood.

Section 3: Procedure

To determine whether a reaction occurs or not, observe any color changes or precipitates. Note that the volumes of the solutions indicated are approximate.

Part 1: Saturated sodium chloride solution

Transfer about 3 mL of saturated NaCl solution into a small test tube.

Using a dropper, carefully add concentrated (12 M) HCl drop by drop to the saturated NaCl solution until a distinct change is observed. Record your observations.

Equilibrium system chemical equation:

Observations after addition of HCl:

In which direction (left or right) did the addition of HCl cause the equilibrium to shift?

Which ion caused the shift? Explain.

Part 2: Acidified chromate solution

Transfer about 3 mL of aqueous 0.1 M potassium chromate (K₂CrO₄) solution into a small test tube. Note the color of the solution.

Add an equal amount (3 mL) of 6 M nitric acid (HNO₃). Record your observations.

Then, add 10% aqueous NaOH solution drop by drop until a color change is observed. Record your observations.

Equilibrium system chemical equation:

Observations after addition of HNO₃:

In which direction (left or right) did the addition of HNO₃ cause the equilibrium to shift?

Which ion caused the shift? Explain.

Observations after addition of NaOH:

In which direction (left or right) did the addition of NaOH cause the equilibrium to shift?

Which ion caused the shift? Explain.

Part 3: Aqueous ammonia solution

For Stockroom notes only: Preparation of ammonia stock solution: Transfer 4 drops of concentrated (15 M) NH₃ solution and 3 drops of phenolphthalein indicator into a 150-mL beaker. Add 100 mL laboratory water and mix thoroughly using a stirring rod.

NOTE: Aqueous ammonia solution is sometimes written as NH₃ or as NH₄OH.

Transfer about 3 mL of the prepared ammonia stock solution into a small test tube.

Add about 1 cm³ ammonium chloride (NH₄Cl) powder into the solution. Record your observations.

Equilibrium system chemical equation:

Observations after addition of NH₄Cl:

In which direction (left or right) did the addition of NH₄Cl cause the equilibrium to shift?

Which ion caused the shift? Explain.

Part 4: Aqueous cobalt(II) chloride solution

Label three small test tubes (#1, #2, and #3).

Transfer about 3 mL of aqueous cobalt(II) chloride solution into each numbered test tube.

The solution in test tube #1 is the control. All observations to be made on test tubes 2 and 3 will be compared with test tube #1.

To test tube #2, carefully add concentrated (12 M) HCl solution drop by drop until a distinct color change occurs. Record your observations.

To test tube #3, add about 1 cm³ ammonium chloride (NH₄Cl) powder. Heat the mixture directly over a Bunsen burner, using a moderate flame. In a slant position, move the test tube back and forth for about 30 seconds, or until a distinct color change occurs. Record your observations. Let the solution cool to room temperature and observe. Record any observed changes.

Equilibrium system chemical equation:

Observations after addition of HCl:

In which direction (left or right) did the addition of HCl cause the equilibrium to shift?

Which ion caused the shift? Explain.

Observations after heating and cooling:

In which direction (left or right) did heating cause the equilibrium to shift?

In which direction (left or right) did cooling cause the equilibrium to shift?

Based on these observations, is this reaction (as written) endothermic or exothermic? Explain.

Part 5: Aqueous iron(III) thiocyanate solution

For Stockroom notes only: Preparation of iron(III) thiocyanate stock solution: Combine 1 mL of 0.1 M iron(III) chloride (FeCl₃) solution and 1 mL of 0.1 M potassium thiocyanate (KSCN) solution in a 150-mL beaker. Add 100 mL of laboratory water and mix thoroughly using a stirring rod.

Label four small test tubes (#1, #2, #3, and #4).

Transfer about 3 mL of stock iron(III) thiocyanate solution into each numbered test tube.

The solution in test tube #1 is the control. All observations to be made on test tubes 2 to 4 will be compared with test tube #1.

To test tube #2, add 1 mL of 0.1 M iron(III) chloride (FeCl₃) solution. Record your observations.

To test tube #3, add 1 mL of 0.1 M potassium thiocyanate (KSCN) solution. Record your observations.

To test tube #4, add 0.1 M silver nitrate (AgNO₃) solution drop by drop until a distinct color change occurs. Record your observations. Note that thiocyanate ion reacts with silver ion to form a precipitate: SCN^-(aq) + Ag⁺(aq) → AgSCN(s)

Equilibrium system chemical equation:

Observations after addition of FeCl₃:

In which direction (left or right) did the addition of FeCl₃ cause the equilibrium to shift?

Which ion caused the shift? Explain.

Observations after addition of KSCN:

In which direction (left or right) did the addition of KSCN cause the equilibrium to shift?

Which ion caused the shift? Explain.

Observations after addition of AgNO₃:

In which direction (left or right) did the addition of AgNO₃ cause the equilibrium to shift?

Which ion caused the shift? Explain.

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