“A simplified Chem situation,
Analysis by isolation:
One species in excess
So rate law is expressed
In pseudo-nth-order notation.”
The 22 October 2021 Twitter limerick was the last in the poetic series for National Chemistry Week 2021. It summarized a technique called the isolation method, which (as with other approaches highlighted in the last few poems) is a technique used by chemists to simplify a complicated rate law.
“A simplified Chem situation, /
Analysis by isolation…”
The first two lines note that this is another simplifying scenario in the discipline of chemistry, pertaining to kinetics.
“Isolation method” is a phrase that came to mind often in 2020’s early days of the pandemic, as the terms “social distancing” and “isolation guidelines” suddenly were added to everyone’s lexicon. In the chemistry setting, though, the approach allows an investigator to examine the kinetic role of one reactant at a time as it affects a rate law.
“One species in excess /
So rate law is expressed /
In pseudo-nth-order notation.”
The last three lines summarize a typical example. One experiment I cite often in class involves the fading of a pink-colored solution over time, where the solution takes on a vibrant color because excess base is present (a case of “one species in excess”) with a chemical indicator (phenolphthalein). By monitoring the fading of the pink color, students determine information about how the reaction depends specifically on the presence of the phenolphthalein.
Rate laws are typically classified as first-order, second-order, etc. with respect to a given reactant. When the isolation method is used, the phrasing changes to pseudo-first-order, etc., acknowledging that this is a finding that has yet to be fully clarified to explore the role of the excess reactant. “Pseudo-nth-order” means the value of n is under investigation (and is a phrase that fits neatly into a metric rhythm!).
In the case of the experiment described above, the experimental finding is that the rate law is pseudo-first-order with respect to phenolphthalein. When the entire rate law is determined, it is first-order with respect to both phenolphthalein and the base, so second-order overall.
Chemphasis 