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Teaching Methods

Kinetics - First Order Half Life

Started 7 months ago by Kimberly Duncan.

We had a question concerning first order half life during the "Lessons Learned from the 2018 AP Chemistry Exam" webinar last week. I thought that I would post it to see if anyone out there has some words of advice. Please post a response! FRQ 7 - how do you know that radioactive decay follows the 1st order half life equation?


  • Christine Taylor

    Posted 7 months ago

    Agree with those above. Radioactive decay is always first order. I tell my students to think of it this way, there is one reactant in a nuclear decay reaction. There is no way to predict when one atom will decay so we use half-life as the measure of rate of decay. Half of a sample will decay in this time period. Students should be able to determine from graph or data table.

  • Paul Price

    Posted 7 months ago

    Radioactive decay is one of the exemplar discussions for LO 4.3  In fact it is the only time decay is discussed in the entire CED.  Students need to be exposed to decay data as part of discussions on kinetics.

  • Linda Cummings

    Posted 7 months ago

    Radioactive decay is always a first-order process. First-order processes can often be identified from data or graphs by a constant half-life. Constant half-life (not dependent on concentration) = first order.

  • Heather Weck

    Posted 7 months ago

    I think it's based on experimental evidence showing that reaction rate is proportional to reactant concentration (to a power of one). If you use the link below and scroll down to " Radioactive Decay Rates," it shows the derivation of the first-order rate equation. https://chem.libretexts.org/Textbook_Maps/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Nuclear_Chemistry/Nuclear_Kinetics/Half-Lives_and_Radioactive_Decay_Kinetics