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Exploring Le Chatelier’s Principle Objective In this experiment you will: explore the effect of disturbing reactions that are at equilibrium, make predictions on how the equilibrium will shift as a result of the disturbance, and look for evidence that your predictions are correct.

Introduction Le Chatelier’s principle states: When a chemical system at equilibrium is disturbed, the system shifts in a direction that minimizes the disturbance. The disturbance to the reaction at equilibrium usually involves 1) changing the concentration of one of the reactant or products, or 2) changing the temperature by heating or cooling the reaction mixture.

Change in concentration The effect of a change in concentration on the equilibrium of a reaction is best understood in terms of the reaction quotient, Q. Consider the generic reaction:

aA + bB ↔ cC + dD The equilibrium constant, K for this reaction is

K =[C]c[D]d

[A]a[B]b

Where all the concentrations in the expression for K are equilibrium concentrations.

The reaction quotient, Q, is defined the same way, but the concentrations in the expression for Q can correspond to any point in the reaction, not necessarily at equilibrium. By comparing the values of Q and K at any point in the reaction, we can tell in which direction the reaction will proceed as it approaches equilibrium.

Effect of Increasing the Concentration of Reactant or Decreasing the Concentration of Product Both an increase in concentration of a reactant or a decrease in concentration of a product will cause the value of Q to be less than K. As a result, the reaction will proceed in the direction that will increase Q. In other words, the reaction will proceed to the right (to make more product).

Effect of Decreasing Concentration of Reactant or Increasing Concentration of Product Both a decrease in concentration of a reactant or an increase in concentration of a product will cause Q to be greater than K. As a result, the reaction will proceed in the direction that will decrease Q. In other words, the reaction will proceed to the left (thus using up excess product and making more of the reactants).

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Effect of Temperature Change on Equilibrium How a reaction at equilibrium responds to a change in temperature depends on whether the reaction is endothermic or exothermic. Endothermic reactions require heat as an input in order for the reaction to proceed in the forward direction. So we can write an endothermic reaction as follows:

𝑅𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠 + ℎ𝑒𝑎𝑡 ↔ 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑠

For endothermic reactions, raising the temperature (heating the reaction mixture) will favor the forward reaction, while lowering the temperature (cooling the mixture) will favor the reverse reaction.

Similarly, since an exothermic reaction is accompanied by the release of heat, we can write an exothermic reaction as:

𝑅𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠 ↔ 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑠 + ℎ𝑒𝑎𝑡

For exothermic reactions, lowering the temperature (cooling the reaction mixture) will favor the forward reaction, while raising the temperature (heating the reaction mixture) will favor the reverse reaction.

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Materials 20 test tubes Bromthymol blue indicator solution 6 M Sulfuric acid (H2SO4)

4 100 mL beakers Phenolphthalein indicator solution 0.1 M Silver nitrate (AgNO3)

2 400 mL beakers Ammonium chloride (NH4Cl) 0.1 M Copper sulfate (CuSO4)

Bunsen burner Cobalt chloride (CoCl2·6H2O) 0.1 M Potassium chromate (K2CrO4)

Ring stand 6 M Sodium Hydroxide (NaOH) 0.1 M Barium chloride (BaCl2)

Iron ring 1 M Hydrochloric acid (HCl) 0.1 M Iron(III) chloride (FeCl3)

Wire gauze 6 M Hydrochloric acid 0.1 M Antimony trichloride (SbCl3)

Disposable pipets 12 M Hydrochloric acid 0.1 M Sodium carbonate (Na2CO3)

Test tube holder 1 M Ammonium hydroxide (NH3 or NH4OH)

0.1 M Ammonium thiocyanate (NH4SCN)

Saturated Potassium Bromide (KBr)

Procedure

Part A: Effect of Changes in Concentration on Various Equilibria 1. Dissociation of the weak acid, Bromthymol blue:

Bromothymol blue (HBB) is a weak acid, that dissociates according to the equation:

HBB(𝑎𝑞) + H2O(𝑙) ↔ H3O+(𝑎𝑞) + BB−(𝑎𝑞)

(yellow) (blue)

a) Add 3 mL of distilled water to a clean test tube. To this add 2 drops of Bromthymol blue indicator. Note down the color of this equilibrium mixture in the data section.

b) Add 6M NaOH dropwise to the mixture until you see a permanent change in color. Record your observations.

c) Now add 6M HCl dropwise to the same mixture from part b, until you see a color change. Record your observations.

2. Dissociation of the weak base, Ammonia: Ammonia (NH3) is a weak base that dissociates according to the equation:

NH3(𝑎𝑞) + H2O(𝑙) ↔ NH4+(𝑎𝑞) + OH−(𝑎𝑞)

(colorless) (pink in presence of phenolphthalein)

a) Place 2 mL of distilled water in a beaker. Then add 2 mL of 1M NH3 (NH4OH) and two drops of phenolphthalein indicator. Mix the solution, and then record the color of this equilibrium mixture in the data section.

b) Divide the solution into two separate test tubes labeled A2-1 and A2-2. Keep test tube A2-1 as a reference for comparison.

c) To test tube A2-2, add solid NH4Cl until you notice a color change and gently shake the test tube or use a glass rod to mix the contents well. Compare the color against test tube A2-1, and record your observations.

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3. Solubility equilibrium of Barium carbonate: An insoluble substance such as Barium carbonate (BaCO3) dissolves in water according to the equation:

BaCO3(𝑠) + H2O(𝑙) ↔ Ba2+(𝑎𝑞) + CO32−(𝑎𝑞)

a) To a clean test tube labels as A3-1 add 2mL of 0.1M BaCl2 solution and 2mL 0.1M Na2CO3. You have just prepared the equilibrium mixture represented by the equation above. Record your observations in the data section.

b) To test tube A3-1, carefully add 6M HCl dropwise. Record you observations. (Think about what gas is being evolved).

4. Copper (II) and Ammonia An aqueous solution of Copper sulfate (CuSO4) contains Cu(II) ions in the form of the complex, Cu(H2O)4

2+, which in the presence of aqueous ammonia (NH3) results in the following equilibrium:

Cu(H2O)42+(𝑎𝑞) + NH3(𝑎𝑞) ↔ Cu(NH3)4

2+(𝑎𝑞) + H2O(𝑙)

(light blue) (dark blue)

a) Record the color of the 0.1M CuSO4 solution b) Add 3mL of 0.1M CuSO4 solution to a clean test tube labeled A4-1. Add 1M NH4OH (NH3) dropwise until you

just notice a change. Record you observations. Now you have the equilibrium mixture represented by the equation above.

c) Transfer about 1.5mL of this equilibrium mixture to another clean test tube labeled A4-2. d) To test tube A4-2, add 1M HCl dropwise until you notice a change. Compare the color to test tube A4-1, and

record your observations. e) Now add 1M NH4OH (NH3) dropwise to test tube A4-2, until you notice a change. Record your observations.

5. Iron (III) and Thiocyanate: Iron (III) ions (Fe3+) react with thiocyanate ions (SCN-) to form the iron (III) thiocyanate ion FeSCN2+ according to the equilibrium reaction:

Fe3+(𝑎𝑞) + SCN−(𝑎𝑞) ↔ Fe(SCN)2+(𝑎𝑞)

(pale yellow) (deep red)

You will set up the equilibrium and then examine the effects of adding different reagents (FeCl3, NH4SCN, and AgNO3) on this equilibrium.

a) Place 50mL of distilled water into a beaker. Then add 20 drops of 0.1M FeCl3 and 20 drops of 0.1M NH4SCN. Stir the solution until it is thoroughly mixed. Record the color of this equilibrium mixture.

b) Add 10mL of equilibrium mixture to 4 test tubes labeled A5-1, A5-2, A5-3, and A5-4. Keep test tube A5-1 as a reference for comparison in steps c, d, and e.

c) To test tube A5-2, add 20 drops of the 0.1M FeCl3 solution. Compare the color against test tube A5-1 and record your observations.

d) To test tube A5-3 add 20 drops of the 0.1M NH4SCN solution. Compare the color against test tube A5-1 and record your observations.

e) To test tube A5-4 add 20 drops 0.1M AgNO3 solution. Compare the color against test tube A5-1 and record your observations. (Think about what the precipitate might be).

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6. Chromate and Sulfuric acid: In acidic solution, the chromate ion exists in equilibrium with the dichromate ion.

2K2CrO4(𝑎𝑞) + H2SO4(𝑎𝑞) ↔ K2Cr2O7(𝑎𝑞) + K2SO4(𝑎𝑞) + H2O(𝑙)

(yellow) (orange)

a) Record the color of the 0.1M K2CrO4 solution. b) Add 2 mL of 0.1M K2CrO4 to a clean test tube. Label as test tube A6-1. Add 6M H2SO4dropwise until you

just notice a change. Record your observations. Now you have the equilibrium mixture represented by the equation above.

c) Transfer 1 mL of this equilibrium mixture to a test tube labeled A6-2. d) To test tube A6-2, add 6M NaOH solution dropwise until you notice a change. Compare to test tube A6-1,

and record your observations.

7. Antimony trichloride and Water: Antimony trichloride (SbCl3) when combined with water, results in the following equilibrium:

SbCl3(𝑎𝑞) + H2O(𝑙) ↔ SbOCl(𝑠) + 2HCl(𝑎𝑞)

a) Add 10mL distilled water to a beaker and add 2mL 0.1M SbCl3. Record your observations in the data section. Now you have the equilibrium mixture represented by the equation above.

b) To the equilibrium solution add 6M HCl dropwise until you notice a change. Record your observations. c) To the mixture in step b add distilled water dropwise until you notice a change. Record your

observations. d) Now again add 6M HCl dropwise to the mixture from the previous step until you notice a change. Record

your observations.

Part B: Effect of changes in Temperature on various equilibria In this part of the experiment, you will explore the effect of temperature on two equilibrium reactions:

1. Co(H2O)62+(𝑎𝑞) + 4Cl−(𝑎𝑞) ↔ CoCl4

2−(𝑎𝑞) + H2O(𝑙)

(pink) (blue)

2. Cu(H2O)42+(𝑎𝑞) + 4Br−(𝑎𝑞) ↔ CuBr4

2−(𝑎𝑞) + H2O(𝑙)

(blue) (yellow)

For this you will need to set up a hot water bath and an ice water bath. In the hood, set up a Bunsen burner, ring stand, iron ring, a wire gauze. Add about 250-300mL water to a 400mL beaker and bring to a boil using the Bunsen burner set-up. For the ice water bath obtain a 400mL beaker and add 3-4 ice cubes. Fill the beaker about half way with water.

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1. Cobalt and Chloride

Co(H2O)62+(𝑎𝑞) + 4Cl−(𝑎𝑞) ↔ CoCl4

2−(𝑎𝑞) + H2O(𝑙)

(pink) (blue)

a) Dissolve 1.5 g of CoCl2·6H2O in 6 mL of distilled water, in a test tube. This solution contains the species Co(H2O)6

2+. Observe the color of this solution and record it in the data section. b) Take your test tube to the fume hood, and add 12 M HCl dropwise until you just notice a change

(Caution: concentrated HCl is highly corrosive). Do not add excess HCl. Record your observations in the data section. This is the equilibrium mixture represented by the equation above.

c) Using a dropper carefully divide the equilibrium mixture between 3 test tubes labeled B1-1, B1-2, and B1-3. Use test tube B1-1 as a reference for comparison.

d) Using a test tube holder, place test tube B1-2 in the beaker with the boiling water for a few minutes and record any color change (relative to test tube B1-1) that you observe.

e) Place test tube B1-3 in the ice cold water bath for a few minutes and record any color change (relative to test tube B1-1) that you observe.

2. Copper and Bromide

Cu(H2O)42+(𝑎𝑞) + 4Br−(𝑎𝑞) ↔ CuBr4

2−(𝑎𝑞) + H2O(𝑙)

(blue) (yellow)

a) Place about 15 mL of 0.1 CuSO4 solution in a small beaker. Record its color in the data section. b) To this add 2.5 mL of saturated KBr solution. Observe the color of the solution and record it. This is the

equilibrium mixture represented by the equation above. c) Now divide the solution between 3 test tubes labeled B2-1, B2-2, and B2-3. Use test tube B2-1 as a

reference for comparison. d) Using a test tube holder, place test tube B2-2 in the hot water bath. Record any change in color relative

to test tube B2-1 that you observe. e) Place test tube B2-3 in the ice cold water bath, and again record any change in color relative to test tube

B2-1 that you observe.

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Pre-Lab Questions

1. Phenolphthalein (Hln), which is commonly used as an indicator in acid-base titrations, is a weak acid. In an aqueous solution, Hln exists in equilibrium with its conjugate base, ln- according to the equation:

Hln(𝑎𝑞) + H2O(𝑙) ↔ ln− + H3O+(𝑎𝑞)

(colorless) (deep pink)

a) Describe how the equilibrium will shift when you add concentrated HCl to the above equilibrium mixture.

b) If you carried out this experiment in the lab, what change would you look for to confirm this shift?

c) Describe how the equilibrium will shift when you add concentrated NaOH to the above equilibrium mixture.

d) If you carried out this experiment in the lab, what change would you look for to confirm this shift?

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2. Consider the following endothermic reaction:

N2O4(𝑔) + Heat ↔ 2NO2(𝑠)

(colorless) (brown)

a) In what direction would you expect the equilibrium to shift when the reaction mixture is placed in a low temperature bath?

b) If you carried out this experiment in the lab, what change would you look for to confirm this shift?

c) In what direction would you expect the equilibrium to shift when the reaction mixture is placed in a high temperature bath?

d) If you carried out this experiment in the lab, what change would you look for to confirm this shift?