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Teaching Methods

Kinetics - First Order Half Life

Started almost 2 years 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?


  • Jesse Bernstein

    Posted about 1 year ago

    Kinetics is an experimental branch of chemistry. All rates of reactions and rate equations are determined from experimentation. Even if one did not know that Radioactive elements decay via first order kinetics one could determine this by observing the data and/or a graph of the decay of the element. If the 'concentration' (amount in the case of a solid) decreases by 50% in a specific time interval for each time interval then the reaction is following first order kinetics. Of course, statistics breaks down after a certain number of half-lives due to the sample size becoming rather small. So, a study of kinetics usually stops the data collection after fewer than 10 half-lives

  • Christine Taylor

    Posted almost 2 years 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 almost 2 years 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 almost 2 years 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 almost 2 years 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