ShmoopTube

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Here’s your Shmoop du jour, brought to you by glow sticks. [Glow stick bends]

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It’s all fun and games until somebody bites into one. [Man bites glow stick]

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Don’t do it, guys.

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They taste terrible… according to our friend, of course.

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We'd never be dumb enough to try that….ahem… [Man and friend with green faces]

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Here’s today’s question:

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Raving Rodney knows that his glow sticks glow only after he cracks them.

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What he doesn't know is that the reaction that occurs in the glow stick is called chemi-luminescence,

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and it is a first order reaction.

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What fraction of a reactant remains after four half-lives of a first order reaction?

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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]

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doesn’t mean you can’t learn from his mistakes.

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Seriously, who changes their name to “Raving”…?

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Anyway, to answer today’s question, we need to remember that the “half-life” is defined [Student highlights in a book]

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as the amount of time it takes for something to be reduced to half its initial value.

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Here, the molecules inside Rodney’s glow sticks that are emitting light are decreasing [Molecules in Rodney's glow stick]

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in concentration.

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After one half-life, the concentration of these molecules is reduced to half its initial

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value.

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After two half-lives, the concentration is decreased by half again, leaving Rodney with

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1/4 of the initial concentration.

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After three half-lives, we’re down to 1/8 the initial concentration. [Table of elapsed half lives]

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After four half-lives, the concentration is decreased by to 1/16 of its initial value.

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You guys get it, right? [Rodney holding a green glow stick with a green face]

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We know you

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You're smart.

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You would never bite into glow sticks.

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…Neither would we….aheeeem.

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Anyway, our handy dandy table shows us that the correct answer is (D): only 1/16 of the

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initial concentration of a reactant remains after four-half lives.

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So, yeah, keep that in mind next time you crack a glowstick. [Rodney cracks a glow stick]

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Which we know you’ll only be doing at wholesome, supervised parties, unlike Raving Rodney. [Rodney and friends at a party with glow sticks]

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Right?

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…Right?

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