Experiment 36 Disturbing Equilibrium Aim: To observe the effect of a change in conditions on a system at equilibrium and explain the effects observed through Le Ch^atelier's principle. Equipment: 5 medium test tubes 1 test-tube rack 1 100-mL beaker 1 250 mL beaker 1 dropper pipet 1 spatula 1 50-mL graduated cylinder 1 white card, to record Materials: Saturated potassium nitrate solution, KNO 3 0. 1 M Iron (III) chloride, FeCl 3 0. 1 M Potassium thiocyanate, KSCN Potassium chloride crystals, KCl Potassium nitrate crystals, KNO 3 Ice Distilled water Procedure: Part A. Effect of Temperature on a Physical Equilibrium 1.

Add 2- 3 mL of saturated potassium nitrate solution to a clean test tube. Using a spatula, add one crystal of potassium nitrate to the solution to act as a seed crystal. 2. Cool the test tube in a 250 mL beaker of ice for 10 minutes. Record results. 3.

Remove the test tube from the ice water and place it in the test-tube rack. Record what happens as the solution warms to room temperature. Part B. Common Ion Effect on a Chemical Equilibrium 4. Use a graduated cylinder to add 50 mL of distilled water to a 100 mL beaker. Add 1 mL of 0.

1 M iron (III) chloride and 1 mL of potassium thiocyanate to the water; stir. 5. Label four identical, dry test tubes. Pour 5 mL of the mixture from step 4 into each. Hold the tubes over a white background and look down into them and they should appear equally dark. 6.

Tube 1 is the control in this experiment. To tube 2, add 20 drops of 0. 1 M iron (III) chloride. To tube 3, add 20 drops of 0.

1 M potassium thiocyanate. Flick each tube to mix the solutions. To tube 4, add 1 g of potassium chloride crystals. Flick the tube to dissolve the crystals. Compare the colours of the solutions in tubes 2, 3 and 4 with the colour of the solution in the control tube (tube 1).

Record your observations. Results/Observations System Observation KNO 3 (sat. ) (cooled) Turned to ice crystals, forming up the tube. KNO 3 (sat. ) (warmed) Decomposed to ice flakes, a small amount of liquid. Fe 3+/SCN- reaction Red concentration, (Control test tube).

Fe 3+/SCN- Mixture + additional Fe 3+ Slightly darker from the control test tube. Fe 3+/SCN- Mixture + additional SCN- Significantly darker from the control test tube. Fe 3+/SCN- Mixture + KCl (s) Lighter from the control test tube. Almost orange Conclusion: 1. Write a balanced equation for the equilibrium that existed before the saturated potassium nitrate was cooled.

KNO 3 + KNO 3 2. Did lowering the temperature (step 2) affect the equilibrium? Explain your answer. Lowering the temperature does effect the equilibrium, because this disrupts the balance and therefore has to make adjustments needed to restore the equilibrium. This reaction is an exothermic reaction, because when the temperature is reduced it releases heat and the equilibrium shifts and favours the products. 3. Did increasing the temperature (step 3) disturb the equilibrium? What evidence do you have for your answer? Increasing the temperature does effect the equilibrium.

The equilibrium favours the side that absorbs heat, in this case the reactants. Evidence shows when increasing the temperature, the ice crystals decomposed and were less dense and liquid was visible. This exothermic reaction disrupted the balance of the equilibrium and shifted to balance the stress of the system. 4. Explain what happened in the potassium nitrate system in terms of Le Ch^atelier's principle. The Le Ch^atelier's principle states if a stress is applied to a system in a dynamic equilibrium, the system changes to relieve the stress.

In reducing and increasing the temperature in the potassium nitrate solution, this is applied stress to the equilibrium. The equilibrium relieves this stress by shifting to favour a side, to readjust and balance the reaction. 5. Write a balanced equation for the equilibrium that existed after the ferric and thiocyanate were combined in the beaker. FeCl 3 + 3 KSCN Fe SCN 3 + 3 KCl 6. What evidence was there that the equilibrium shifted when iron (III) chloride was added? In which direction did it shift? The evidence shown that the equilibrium adjusted, was the change in colour.

This concentration changed colour slightly from the controlled test tube (tube 1). This displayed, that when iron (III) chloride concentration was added the equilibrium shifted to favour the products. Iron (III) chloride is a reactant and adding more of this concentration produces more of the products. 7.

What evidence was there that the equilibrium shifted when potassium thiocyanate was added? In which direction did it shift? The evidence also shown that the equilibrium adjusted, was the change in colour. This concentration changed colour significantly from the controlled test tube (tube 1). The controlled test tube was a red colour and changed to a very deep red. This displayed, when potassium thiocyanate concentration was added the equilibrium shifted to favour the products. 8.

Explain the effect of adding potassium chloride to the system. The effect of adding potassium chloride is that the equilibrium will have to adjust to the stress of the added concentration. The equilibrium will shift to favour the reactants. Adding the potassium chloride changes the colour and this indicates the equilibrium has shifted to balance the stress of the system. 9. Explain the changes observed in the Ferrothiocyanate ion system in terms of Le Ch^atelier's principle.

The changes involved in the system was the large amount of colour change due to the added concentrations. In tube 1 was the control of the experiment, with the colour a red. In tube 2 the colour went slightly darker when the iron (III) chloride was added. In tube 3 the colour of this concentration changed significantly darker, and tube 4 changed lighter.

The Ferrothiocyanate solution has this applied stress of the added concentrations. This disrupts the balance of the equilibrium and the equilibrium relieves this stress by re-adjusting and favouring a side to balance the reaction to reach dynamic equilibrium.