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Reesab  Pathak   Lab: Application of LeChatlier’s Principle Reesab Pathak; Period 1 Pre-Lab Questions 1. The Common Ion effect comes into play. As extra KI is added, I2 is consumed from the overall reaction. This means that in equation 3, the reaction MUST SHIFT RIGHT! Because I2 is consumed in the second reaction, there is less I2 in the reaction mixture. This stresses the right side, and decreases concentration so by LeChatlier’s Principle, the reaction shifts right to counter act the stress, meaning that more I2 is converted from solid-state to an aqueous solution. 2. a. Since heat is treated as a reactant, if there is more heat, the reaction has mroe reactant. The reaction shifts to the RIGHT to counteract the stress from the left (by LC principle), meaning that the concentration of NO increases. b. If pressure increases (high pressure), then volume decreases (low volume) by Boyle’s Law. However, since the reaction is equimolar (same number of moles on each side of the equation) there will be no increase in NO concentration since there is no side that has higher partial pressures due to this change. Introductory Activity: Complex-Ion Equilibrium between Iron(III) Nitrate and Potassium Thiocyanate

Fe3+ + SCN- + heat* ! FeSCN2+ Stress

Color

Explanation

NaH2PO4

Begins lighter, then turns colorless/clear

Fe(NO3)3

Dark Red Spot

KSCN

Area near turns orange

Adding sodium phosphate monobasic shifted the equilibrium to the left because the H2PO4- ion reacted with the Fe3+ ion. The product, FeH2PO42+ is colorless as a result. Adding the iron(III) nitrate solution shifts the reaction to the right as a result, more FeSCN2+ is made which is dark red in aqueous solution. Equilibrium reaction shifts to the right since KSCN dissociates into K+ and SCN-. This second ion adds to reactants, causing the rxn to shift right to counteract stress. Since the reaction turns red, this means that the rxn shifts right. This happens if heat is a reactant (endo) Since the reaction turns orange, means rxn shifts LEFT. This happens only if heat is a reactant (endo).

Add Heat Remove Heat

Turns red Turns orange

*Reaction is ENDOTHERMIC

Major Lower Species Concentration


Reesab  Pathak   Lab: Application of LeChatlier’s Principle

Introductory Activity Questions 3+

1. Fe

+ SCN- + heat ! FeSCN2+

(orange)

(colorless)

(reddish-orange)

2. 3.

Activity A: Acid-Base Indicator Equilibrium

Stress HCl

Color

Explanation

Yellow

Cl- spectates in this reaction. HCl is a strong acid, completely dissociates, adding H+ to the products side, shifting the rxn to left, creating more yellow HIn. Na+ spectates in this reaction. NaOH is a strong base, completely dissociates. The OH- ion reacts with H+, drawing out H+, shifting the rxn to the right, creating more blue In-. Since there is both HCl and NaOH, this makes neutral water. This has no effect on the equilibrium because even if H2O autoionizes, it makes H+ and OH- in equimolar concentrations.

NaOH

Blue

Mixture

Blue

Major Lower Species Concentration

Activity B: Formation of a Copper Complex Ion

Stress NH4OH

Color Royal bluishpurple

HCl

Light Blue

More

Darker

Cu2+ + 4 NH3 ! [Cu(NH3)4]2+ Explanation Increases NH3 concentration, shifts reaction towards copper-ammonia complex ions, which are purple. Increases concentration of Cu2+ ions to make CuCl2. This removes reactants, shifting rxn to left, creating more copper ions, which react with dissociated sulfate to make CuSO4. Increases NH3 concentration, shifts reaction towards copper-ammonia

Major Lower Species Concentration


Reesab  Pathak   Lab: Application of LeChatlier’s Principle

NH4

Purple

complex ions, which are purple.

Activity C: Formation of Cobalt Complex Ions

HCl

[Co(H2O)6]2+ + 4Cl- + heat ! [CoCl4]2- + 6 H2O Color Explanation Major Lower Species Concentration Adds more chloride ions, which adds Blue

AgNO3

Pink

dH2O

Pink

CaCl2

Blue

Ice bath

Pink

Hot water

Blue

Stress

to reactants, shifting to the right. This makes more CoCl42- which is blue. Ag+ reacts with Cl- and nitrate ions react with complex hexaaquacobalt(II) ions, taking away reactants, shifting to the left, making more complex ions, which are pink. Liquid water is NEVER involved in equilibria, as a result, the existing pink is diluted to a lighter pink. Adds more chloride ions, which adds to reactants, shifting to the right. This makes more CoCl42- which is blue. Takes away heat, which is a reactant. This shifts to the left, making more complex hexaaquacobalt(II) ions, which are pink. Adds heat, which adds to reactants, shifting to the right. This makes more CoCl42- which is blue.

Activity D: Solubility of Carbon Dioxide

Stress

Color

Explanation

Smaller Volume

Darker Blue (higher pH)

Larger Volume

Lighter Blue (lower pH)

Shifts reaction to the right, because there are more gaseous moles on the reactants. This means the products increase, making it darker blue. Shifts reaction to the left, because there are more gaseous moles on the reactants. This means the reactants increase, making it lighter blue.

Major Lower Species Concentration

Activity E: Solubility of Magnesium Hydroxide

Stress

Color

Explanation

Major

Lower


Reesab  Pathak   Lab: Application of LeChatlier’s Principle

Species Concentration HCl

Green (returns to purple)

More HCl

Orange (purple to purple)

More HCl

Red

H+ ions from HCl react with the indicator to shift to the left. However, the competing reaction is the Magnesium Hydroxide reaction. This means that the color temporarily is green, but turns back to purple. H+ ions from HCl react with the indicator to shift to the left. However, the competing reaction is the Magnesium Hydroxide reaction. This means that the color temporarily is orange, but turns back to purple. At this point, the Mg(OH)2 is limiting, meaning that only the H+ is shifting the indicator’s equilibria to the left, creating a red solution.

AP Chemistry Review Questions 1. Haber Process: N2 + 3 H2 → 2 NH3 + heat 2. At lower temperatures, the reaction shifts right. At higher temperatures the reaction shifts left. This means that the reaction produces more ammonia at lower temperatures. 3. At higher pressure, the reaction shifts to the right because there are less moles. At lower pressure (higher volume by Boyle’s Law), then the reaction shifts to the left because there are mole gaseous moles. 4. If ammonia is removed, then the reaction shifts right because the products form more reactants to make up the ammonia in the equilibrium mixture. The other alternative is to use an enzyme or catalyst to speed up the reaction at a lower temperature.


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