# AP Chemistry 2.4 Chemical Reaction Rates

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Time to calculate some half-life. And no, you can't only half-pay attention.

AP | AP Chemistry |

AP Chemistry | Chemical Reaction Rates Chemical and Physical Properties from Structure and Arrangement |

Language | English Language |

Test Prep | AP Chemistry |

### Transcript

What is the rate constant for a first-order reaction that has a half-life of 56.0 seconds?

And here are your potential answers:

So what is “half-life,” aside from the portion of your time you spend on Netflix? [Woman with feet up watching TV]

For the sake of this problem, “half-life” is the amount of time it takes for something

to decrease to half its initial value. As the problem tells us, in chemistry, “half-life”

often means how long it takes for half of a radioactive sample to decay, but it can [Sample decays and clock ticks]

also refer to how long it takes for half of some species to be consumed in a regular chemical

reaction. In this problem, we know that some species [Woman zaps magic stick to a man on stage and he disappears]

is disappearing following a first-order rate law, and half of it is gone after 56.0 seconds.

To find the rate constant, we have to remember the equation for half-life: [Girl using calculator]

t½ = 0.693/k, where k is the first-order rate constant.

If you don’t remember this equation, you can figure it out by starting with the rate

law and solving a first order ordinary differential equation.

That is, if you want to make life way harder for yourself. [Boy studying at his desk]

Might just want to take the plunge and memorize. So, we need to take this equation and plug

in the value of the half-life, t½ or half-life= 56.0 seconds. Solving

for k, we find that the rate constant equals 0.0124 per second.

So that means that A is the correct answer. Now quick, head over to CarbonDating.com before [Scientist using tablet]

you have your half-life – we mean mid-life – crisis.