Concentration Changes Over Time - AP Chem Unit 5, Topic 3

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In this video, Mr. Krug shows how to determine the order of a reaction with respect to a reactant graphically. He also introduces integrated rate laws and works several examples with these important equations. He introduces and derives the first-order half-life equation and shows how this equation can be used.

[@FrenchFries879]

The fact i can understand this guy’s vids better than the AP Videos says a LOT.

[@JeremyKrug]

I try to make my videos easy to understand for everybody. Thanks for watching!

[@angd8405]

AP classroom went down the day before my unit 5 test. This is a lifesaver!

[@JeremyKrug]

I'm glad you were able to use my videos even though AP classroom went down. I hope your Unit 5 test went well!

[@ben-yn1io]

jeremy krug, the newlyfound goat of ahs

[@JeremyKrug]

A big shoutout to everybody in AP Chem at AHS. Thanks for watching!

[@Rock_Strongo]

These are way better than the videos from College Board.

[@JeremyKrug]

Thanks for the kind words; I try my best to make videos that are engaging and get you lots of examples

[@dark-death6982]

Perfect videos for studying/refreshers. Thank you for these

[@JeremyKrug]

You're very welcome. Thanks for watching!

[@harperpeel970]

Mr.Krug you are truly a lifesaver these videos always help me so much. Thank you!!!

[@JeremyKrug]

I'm so glad you find my videos helpful. Thanks so much for watching!

My son is currently going through AP Chemistry and these videos are super helpful for him! Thank you for all of the help!😊

[@JeremyKrug]

You're very welcome. I'm glad to hear your son is able to use my videos to help him in class. Best wishes as he gets ready for the AP Exam!

[@christianmartinez1913]

Thank you so much. Your explanation really helped clear the confusion.

[@JeremyKrug]

I'm very glad to hear that. Kinetics can be tough, but you can do this!

[@finleymorrison2129]

Hello 😄! I am wondering if you use the overall reaction order or the reaction order for the specific reactant you are measuring concentration change over time when determining which integrated rate law to use. Thanks for your videos.

[@JeremyKrug]

We use the reaction order for the specific reactant we're looking at. This method really works best for reactions where there is only one reactant in the reaction.

[@finleymorrison2129]

@@JeremyKrug thank u :)

[@FrenchFries879]

Btw guys for the formula: ln[A]t - ln[A]0 = -kt , you can replace the square brackets with normal brackets “()” so that your calculator doesn’t give you a syntax error.

[@JeremyKrug]

Good idea, thanks for sharing!

[@ok.1791]

at 17:55 wouldn't the half life be 160 seconds because 4.4 x 10^-3 has two sig figs?

[@JeremyKrug]

Yes, 160 s would be a better answer. Thanks!

[@thewhat2]

Does the brackets just mean concentration of a reactant for something? [A] Thank you so much!

[@JeremyKrug]

Yes, a value in brackets represents concentration in moles per liter.

[@monlonkin1157]

My ap chem teacher made this unit the final, taking it tomorrow soni gotta lock in

[@JeremyKrug]

I hope that final went well for you

[@georgiakennedy3763]

In your video, at time 14:00, how did you get the units to be s^-1? Since the question asks for seconds, would you just completely ignore the concentration unit?

[@JeremyKrug]

Good question. When you take the natural log on the left side of the equation, you end up with a 'unitless' value. Therefore, we need the same on the right side. The time unit (seconds) needs to cancel out with the reciprocal of time (s^-1). Thanks for watching!

[@chefethanguo9701]

I have a question: Where did the graphs of 0th, 1st, and 2nd order come from? How were they derived? What are the reasons why the graphs are assigned to their orders? Also, what does an order for a reaction mean? What is its definition? Thanks!

[@Josama0214]

It's because that how the graphs of their respective functions look like

[@zielonythewolf5069]

I am also asking the same question. But what I know from calculus is that the derivative of the 0th order is the ln of the 0th order. The derivative of that is the reciprocal of the 0th order because (d/dx (ln(x)) = 1/x)

[@chefethanguo9701]

@@zielonythewolf5069 There's actually an easier explanation for this. From AP Statistics, I know that you can manipulate the data on the x and y axis so that the data is now a straight line. That's what happened. Are you deriving the integrated rate laws here?

[@zielonythewolf5069]

@chefethanguo9701 I never took AP Stats, so that could explain things. I'm not totally sure what the integrated rate laws are doing, I just noticed the connection between orders and the way the y value on the graph is manipulated to get a straight line. Some questions ask to calculate the slope of the line to get the rate. Of course, I understand that the line must be linear to be able to use algebra 1 slope formula. However, calculus teaches you how you can do it for any graph. For AP Chemistry, they clearly made it easier for those who don't know by giving these linear graphs. I assume that these graphs were originally much different.

[@chefethanguo9701]

@@zielonythewolf5069 Thanks for your reply! I'm currently in AP Stats, and that was covered in our curriculum this past semester. I'm not in calculus yet (I'm a junior taking AP Precalculus), so I guess can call myself one of those who "don't know"! Haha!

[@nikitanair1107]

Thank you so much for these videos. I have 2 quick questions. We derived the half-life equation from the integrated rate law for 1st order reactions. Is this half-life equation universal, even for 0th and 2nd order reactions? I've missed quite a few points on my tests for not having the right sig figs, so I want to make sure I'm doing this right. In the last problem (plugging in values for the 2nd order integrated rate law), wouldn't the final answer have only 2 sig figs? I rounded up to 340 seconds, but you left the answer as 337 seconds. How strict is the AP exam on sig figs?

[@JeremyKrug]

Excellent question! The half-life equation derived in this video is only good for 1st order processes. There are half-life equations we can derive specifically for 0 and 2nd order processes (and any order actually), but 1st order is the only type of half-life question AP Chemistry includes in its curriculum. With regard to sig figs, on general problems, you have a leeway of + or - one sig fig. So if you should have 4 sig figs in an answer, you'll be ok with 3, 4, or 5. I tell students that when you're in doubt, always go with 3 sig figs; that will always put you within CB's tolerance level. Thanks for watching!

[@emmatarter8001]

You are an absolute lifesaver, thank you so much!

[@JeremyKrug]

So glad you’re finding my videos helpful. Thanks for watching!

[@chakjikassam5684]

Good job sir,mor grace

[@JeremyKrug]

Thanks for watching!

[@FrenchFries879]

Wait hang on how did you get 0.051 from -2.973? 😅

[@JeremyKrug]

Take the negative anti-natural log of -2.973. On your calculator, evaluate e^-(-2.973).