Exp 14A: Le Chatelier’s Principle http://www.youtube.com/watch?v=4-fEvpVNTlE • When a system in chemical equilibrium is disturbed by a change in temperature, pressure or concentration (“stress”), the system shifts in equilibrium composition to relieve the effect of the change • Types of stress – Change in concentration – Change in temperature – Change in pressure – Effect of a catalyst 1

1) 2) 3) 4) Exp 14A: Le Chatelier’s Principle N 2 (

g

) + 3 H 2 (

g

)  2 NH 3 (

g

) Add H 2 right  N 2 reacts with the excess H 2 until equilibrium  shift to the Remove H  2  NH 3 will decompose to make more H 2 shift to the left, relative increase in [H 2 ] and [N 2 ] until equilibrium Add NH 3  Excess NH 3 equilibrium  will be decomposed to N shift to the left 2 and H 2 until Remove NH 3 equilibrium   N 2 and H 2 shift to right will react to form more NH 3 to re-establish Add product: equilibrium shifts towards reactant (to the left) Add reactant: equilibrium shifts towards product (to the right) Remove product: equilibrium shifts towards product Remove reactant: equilibrium shifts towards reactant 2

Exp 14A: Le Chatelier’s Principle Reactions  Fe 3+ + SCN  Fe(SCN) 2+ (yellow) thiocyanate iron (III) thiocyanate (colorless) (red)  Ni 2+ + 6 NH 3  Ni(NH 3 ) 6 2+ color change  Acid-base equilibrium with indicator color change determined by acidity   Solubility of Ca(OH) 2 in H 2 O as a function of acidity Co 2+ + 4 Cl  CoCl 4 2 color change as function of temperature 3

Exp 14A: Le Chatelier’s Principle

Reaction #1

 Fe 3+ + SCN  Fe(SCN) 2+ [Fe(H 2 O) 6 ] 3+ (aq) + SCN (aq)  [Fe(H 2 O) 5 (SCN)] 2+ (aq) + H 2 O(l) (yellow) thiocyanate iron(III) thiocyanate (colorless) (red) 1) Mark 3 test tubes 2) Add 20 mL diH 2 O to a 100-mL beaker 3) Add 20 drops of 0.1 M Fe(NO 3 ) 3 + 20 drops 0.1 M KSCN.

Mix/Stir

4) Add 3 mL of the solution to each test tube 5) Add 20 drops 0.1 M Fe(NO 3 ) 3 to tube 1. Mix gently 6) Add 20 drops 0.1 M KSCN to tube 2. Mix gently 7) Add 20 drops dH 2 O to tube 3 (

reference tube

). Mix gently 8) Compare the colors in all 3 tubes. View down the tubes to a piece of white filter paper 9) Record observations on the Results Sheet (p. 269) 4

Exp 14A: Le Chatelier’s Principle

Reaction #2

 Ni 2+ + 6 NH 3  [Ni(NH 3 ) 6 ] 2+ [Ni(H 2 O) 6 ] 2+ (aq) + 6 NH 3 (aq)  [Ni(NH 3 ) 6 ] 2+ (aq) + 6 H 2 O(l) color change 1) 2) 3) 4) Add 10 drops of 0.1 M Ni(NO 3 ) 2 to a clean test tube o What is the color?

Add drops of 6 M NH 3 until the color changes and intensifies o What is the color?

Add drops of 6 M HCl until the color changes again o  What is the color?

The acid has reacted with NH 3 to form NH 4 + Record observation on p. 269 5

Exp 14A: Le Chatelier’s Principle

Reaction #3

 Acid-base equilibrium with indicator H 3 O + (

aq

) + OH (

aq

)  H 2 O(

l

) 1) 2) 3) 4) 5) 6) 7) Mark 2 small beakers with A (acid) and B (base) Add 10 mL of dH 2 O and 4 drops of 6 M HCl to beaker A. Swirl & mix Add 10 mL of dH 2 O and 4 drops of 6 M NaOH to beaker B. Swirl & mix Add 1 mL of dH 2 O to a clean test tube Add 2 drops of indicator solution and 2 drops of dilute acid from beaker A. Mix gently and record color Add drops of dilute base solution from beaker B until color changes. Shake gently and record color on page 269 Add drops of dilute acid until color changes again. Shake gently and record color on page 269 6

Exp 14A: Le Chatelier’s Principle

Reaction #3

 Acid-base equilibrium with indicator

Methyl orange

1) Methyl orange is one of the indicators commonly used in titrations. Its structure is simple enough to be able to see what is happening as it loses and gains hydrogen ions. 2) You have the same sort of equilibrium as in the litmus case but the colors are different. 3) You should be able to work out for yourself why the color changes when you add an acid or an alkali. The explanation is identical to the litmus case - all that differs are the colors.

7

Acidic Basic 8

9

Exp 14A: Le Chatelier’s Principle

Reaction #4

: Solubility of Ca(OH) 2  Ca 2+ (aq) + 2 OH (aq)  in H 2 Ca(OH) O 2 (s) Ca(OH) 2 solubility changes with acidity 1) 2) 3) 4) 5) 6) 7) Put 5 mL of 6 M NaOH in small beaker using graduated cylinder Rinse cylinder thoroughly with dH 2 O Use cylinder to add 5 mL of Ca(NO 3 ) 2 to beaker Stir thoroughly with stirring rod. White precipitate forms Add 5 mL 6 M HCl to beaker. Record the results on page 269 Add 5 mL NaOH to the beaker. Record the results on page 269 Which substance is formed in the reaction(s)?

11

Exp 14A: Le Chatelier’s Principle

Reaction #5

 Co 2+ + 4 Cl [Co(H 2 O) 6 ] 2+ pink  + 4Cl CoCl 4 2  [CoCl 4 ] 2 + 6H 2 O blue 5) 6) 7) 8) 9) 1) 2) 3) 4) Set up ring stand, ring, metal gauze, and bunsen burner Place 50 mL beaker with dH 2 O and bring to gentle boil Add

5 drops CoCl

2

to a clean test tube. Record color (p. 270) Add

5-10

drops of

concentrated HCl

(

12 M

) until the solution changes color. Shake test tube and record color on p. 270. This is the color of CoCl 4 2 Add

5

drops of dH 2 O until the solution changes color. Add 5-10 drops of water to the tube. Shake gently and record color (p. 270) Place tube in boiling water and wait until color changes. Record color Cool tube in ice water until color changes. Record color Repeat steps 6 & 7 as often as you want 12

Next Monday, Oct 22 • Postlab for Expt. 14A • Prelab assignment for Expt. 14B

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