AP Chemistry 3.4 Chemical Reaction Rates
AP Chemistry 3.4 Chemical Reaction Rates. What fraction of a reactant remains after four half-lives of a first order reaction?
|AP Chemistry||Chemical Reaction Rates|
Chemical and Physical Properties from Structure and Arrangement
|Test Prep||AP Chemistry|
Here’s today’s question:
Raving Rodney knows that his glow sticks glow only after he cracks them.
What he doesn't know is that the reaction that occurs in the glow stick is called chemi-luminescence,
and it is a first order reaction.
What fraction of a reactant remains after four half-lives of a first order reaction?
And here are your potential answers: Raving Rodney might be a lost cause, but that [Raving Rodney appears by a woman sitting at a desk]
doesn’t mean you can’t learn from his mistakes.
Seriously, who changes their name to “Raving”…?
Anyway, to answer today’s question, we need to remember that the “half-life” is defined [Student highlights in a book]
as the amount of time it takes for something to be reduced to half its initial value.
Here, the molecules inside Rodney’s glow sticks that are emitting light are decreasing [Molecules in Rodney's glow stick]
After one half-life, the concentration of these molecules is reduced to half its initial
After two half-lives, the concentration is decreased by half again, leaving Rodney with
1/4 of the initial concentration.
After three half-lives, we’re down to 1/8 the initial concentration. [Table of elapsed half lives]
After four half-lives, the concentration is decreased by to 1/16 of its initial value.
You guys get it, right? [Rodney holding a green glow stick with a green face]
We know you
You would never bite into glow sticks.
…Neither would we….aheeeem.
Anyway, our handy dandy table shows us that the correct answer is (D): only 1/16 of the
initial concentration of a reactant remains after four-half lives.
So, yeah, keep that in mind next time you crack a glowstick. [Rodney cracks a glow stick]
Which we know you’ll only be doing at wholesome, supervised parties, unlike Raving Rodney. [Rodney and friends at a party with glow sticks]