Half-life of a first-order reaction | Kinetics | AP Chemistry | Khan Academy

The half-life of a reaction is the time required for a reactant to reach one-half its initial concentration or pressure. For a first-order reaction, the half-life is independent of concentration and constant over time. View more lessons or practice this subject at www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:kinetics/x2eef969c74e0d802:concentration-changes-over-time/v/half-life-of-a-first-order-reaction
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[@brianm.4368]

Thank you so much. This video fills a BIG gap in youtube chemistry.

[@be4654]

This is the only teacher on Khan that I don't really like and he is in all the videos I need :/

[@austinham7108]

He’s the AP chem teacher at my high school

[@NanaNana-sz7xg]

You should put number to the concentration.

[@aloeverawang85]

Thank you! My textbook was so confusing and the derivation of the equation was not clearly explained anywhere! This made everything click

[@SillyCiliate]

had to find the rate constant for an equation using that half life formula but I got a resonance cascade instead. any idea on what I did wrong?

[@Caneladorada]

beautifully explained. thank you so much

[@jeffmartineau8479]

Great information! Really helpful! But please slow down a little!

[@sararodrigues1098]

Thank you for these amazing lessons!!

[@shodan2002]

I was looking for half life videos i have no clue wtf any of this means

[@sciencenerd7639]

thank you

[@maquensowino9588]

i loved the teaching though the video is of low quality. Peace ,from Kenya.

[@WanSyazlina]

There is this one question I don't quite understand : "Calculate half-life for first-order reaction if 68% of a substance is reacted within 66 s." So, how do i calculate this. And I wonder why must we use what is left of substance in the equation and not what is used up? ln [A] = -kt + ln [A]o ln 0.32 = - k (66 s) + ln 1 k = 0.0172642 s-1 -Why must i use 32% in the equation and not 68%? -And Why use ln (1) for ln [A]o in the equation?

[@alaen2]

1) You're calculating the current concentration percentage of [A], 68% has reacted, so you're left with 32% 2) 1 = 100/100 (100%), your initial concetration

[@naweedbabakarkhail5210]

Omg I have this exact same question on one of my tests so weird.

[@فاطمة-خ4ع8غ]

I have exactly the same question in one of my chemistry exercises !!!

[@فاطمة-خ4ع8غ]

Naweed Babakarkhail could you contact me please! As soon as possible

[@rainboi7894]

Am I the only dumb one here

[@stunningcrafts3924]

It is good but make your speed moderate

[@L_ofer12345]

bro i did type in the search ''how to use the jetpack in half life'' and this what I got.

[@finlaymartins272]

also -ln 0.5 = ln 2

[@Kerwyn204]

I pressed [log] then .5 and i got this answer -0.301029995 and he got -.693147... what am i doing wrong? do I need to press other buttons to get that answer?

[@sidewaysfcs0718]

+Kerwyn204 it's a natural logarithm, log is usually interpretend as decimal logarithm (or lg) by default, find a button that says ln instead of log

[@Kerwyn204]

perfect thanks!

[@pranjalijain3629]

K3K it is natural log of 2(and not the half) upon the rate constant (as written in all the text books) which gives the half life ..n u r right idk how he went wrong bout that concept

[@DoveStrider]

i was looking for the video game half life not this

[@yashovardhanjha9274]

very good videos

[@almiramuslih]

I. Love. This.

[@rithumanoharan7098]

Why were the initial concentrations cancelled out ?

[@edsknife]

Are you really living a life if you're not free? You're alive, but you're only living a Half-Life.

[@josh4737]

he sounds like remy from ratatouille

[@slime1167]

I genuinely hate school