Chemistry 104 Laboratory

This Week

Answer Keys

Gen Chem Lab

Periodic Table

· Expt 7- Kinetics of the Fading of Phenolphthalein (Parts A, C only)

o Preliminary Assignment due

· Lab Check Out

Notes for Today

The Phenolphthalein Fading Reaction:

P^2-	+	OH^-	->	POH^3-
red		colorless		colorless

rate = k[OH^-]^m[P^2-]ⁿ

rate = k₁[P^2-]ⁿ when k₁ = k[OH^-]^m

• Overall Goals
• Find the rate law (understand dependence of rate on concentration)
• Learn how the rxn occurs (understand the molecularity of each step)
• Find the activation energy

• Part A: Find n and k₁

Find n

• Monitor [P^2-] vs. time (at constant [OH-], temperature)
• Use line fitting techniques to identify n
• Zero order: [P^2-] vs time is linear; n = 0
• First order: ln[P^2-] vs time is linear; n = 1
• Second order: 1/[P^2-] vs time is linear; n = 2

• Do the [P^2-] vs. time data shown below obey zero order kinetics?

Find k₁

• Use linear plot to find k₁
• What information from the plot yields k₁?

• Part C: Find E_A
• Monitor k₁ vs. temperature (at constant [OH-])
• Use the Arrhenius relationship to find E_A

ln k₁ = (-E_A/R) (1/T_K) + constant

y = m x + b

• What information from the plot yields E_A?

• Safety
• Goggles required during solution preparation

More About Molecularity and Reaction Mechanism

• Many rxns occur in a multistep process
• One step will be the rate-controlling step (slow step)
• Other steps do not affect the overall rate
• Coefficients of the overall balanced equation do not yield the rxn order BUT rxn orders are (usually) the coefficients of the slow step
• Example: If experiment shows that n=0, m=1

· [P^2-] does not affect reaction rate (since[P^2-]⁰ = 1)

· no P^2- is involved in the slow step

· one OH^- is probably involved in slow step (since rate = f([OH^-]¹))

· molecularity = n + m = 1

· slow step is unimolecular

• Molecularity gives clues about the reaction mechanism
• Rxn orders are (usually) the coefficients of the slow step

• Example: If experiment shows that n=0, m=1

• Slow step might be: 1OH- -> intermediate (slow step)
• Overall reaction must involve additional steps
• Steps must sum to give overall reaction

• One possible two-step mechanism:
• (slow) 1OH^- -> intermediate
• (fast) intermediate + P^2- -> POH^3-