1 / 11

Half lives

Half lives. The half life is the time it takes for the concentration of a substance to decline to half its initial value. The general concept is the same as in radioactivity. After two half lives ; ½ + ½ ( ½ ) = ¾ will have reacted. Leaving only ¼ of the initial concentration.

gayle
Download Presentation

Half lives

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Half lives The half life is the time it takes for the concentration of a substance to decline to half its initial value.

  2. The general concept is the same as in radioactivity

  3. After two half lives; • ½ + ½ (½ ) = ¾ will have reacted. • Leaving only ¼ of the initial concentration. • After three half lives; • ½ + ½ (½ ) + ½[½ (½ )] = 7/8 will have reacted. • Leaving only 1/8 of the initial value. • After four half lives; • ½ + ½ (½ ) + ½[½ (½ )] +½{½[½ (½ )]} =15/16 will have reacted. • Leaving only 1/16 of the initial value.

  4. Eg; The isotope Carbon-14 is radioactive. • It has a half life of 5,730 years • This means that after 5,730 years have passed only half of the original amount of C14 will remain. • After 2 x 5730 = 11,460 years there will be ½ x ½ = ¼. • After 3 x 5730 = 17190 years there will be ½ x ½ x ½ = 1/8…..

  5. C14 is made by the action of cosmic rays. • Life is based on carbon. • Whilst organisms are alive C14 will be absorbed at the same rate as C12. • As C14 decays it is continually replaced. • But after death no more C14 is absorbed. • It is as if a stop clock is started. • Archaeologists have only to measure the C14 in bones, wood, hair…. to date them.

  6. The technique can be used for objects up to 48,000 years old. • But when the ages of historic artefacts several millennia old were compared to the radiocarbon dates they were found to be too young. • It seems that C14 is not always produced at the same rate. • So the dates have been calibrated using the wood of the Bristlecone Pine, which lives for over 7,000 years!

  7. Half lives of first order reactions • For a first order reaction the half life is constant. • The time taken for the concentration to fall from the initial value to ½, from ½ to ¼, from ¼ to 1/8 …is exactly the same. • t½= 0.693 / k • Where k = rate constant. • Half lives can be determined by plotting concentration of a reactant against time then measuring the time take for the initial concentration to halve.

  8. Eg; The decarboxylation of 2,4,6 trinitrobenzoic acid.

  9. Half lives of second order reactions. • Second order half lives are not constant. • A basic plot of concentration against time starts off as a much steeper curve, then levels off. • This means that the half lives become progressively longer.

  10. Zero order • NB For a zero order reaction the rate is independent of the concentration. • Thus a plot of concentration against time is a straight line, rather than a curve.

  11. Concentration/time graph for a zero order reaction.

More Related