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A-level Chemistry Help

Discussion in 'Science' started by KingKong, Feb 6, 2011.

  1. KingKong

    KingKong New commenter

    Hi there i'm hoping someone out there can help.
    My son is doing a-level chemistry (OCR A). He was given a past paper question which stated 'how does increasing the concentration afftect the half life of a first order reaction?' The mark scheme it doesnt have any affect.
    Having searched the internet, all sources seem to say that increasing the concentration increases the rate of reaction. If this is the case why doesnt it affect the half life of a first order reaction?
    Hope someone out there can explain it to me in basic terms as Im a non chemist. My son has spoken to his chemistry teacher but he is still a little confused.
    Many thanks
     
  2. KingKong

    KingKong New commenter

    Hi there i'm hoping someone out there can help.
    My son is doing a-level chemistry (OCR A). He was given a past paper question which stated 'how does increasing the concentration afftect the half life of a first order reaction?' The mark scheme it doesnt have any affect.
    Having searched the internet, all sources seem to say that increasing the concentration increases the rate of reaction. If this is the case why doesnt it affect the half life of a first order reaction?
    Hope someone out there can explain it to me in basic terms as Im a non chemist. My son has spoken to his chemistry teacher but he is still a little confused.
    Many thanks
     
  3. I have a BSc degree in Chemistry with Mathematics, however I still stuggle to see why it would not affect the reaction?
    What paper was he given?
     
  4. There are two things to consider here;
    Firstly - for a reaction that is first order with respect to a particular reaction, the rate is propotional to the concentration of that reactant - if the conc of the reactant doubles, the rate does too.
    Secondly - the half life is the time taken for the concentration of a reactant to half. This is the same no matter what the concentration is .....because the time taken for the concentration to go from 2 to 1, is the same as for it to go from 1 to 0.5; and so on.
    Thus changing the starting concentration, doesn't affect the half life, although it will affect the rate, because you are measuring different things.
    Are you getting confused because you are trying to compare two different things.....?


     
  5. phlogiston

    phlogiston Star commenter

    s20blu is correct
    nasty question - the half life business distracts weaker students away from rate =k[reactant].
    P
     
  6. chemroger

    chemroger New commenter

    Not really a nasty question. I would have thought this is a standard 'A' level question. Constant half life for decrease in reactant conc for a first order reaction (or any exponential process).




     
  7. Nasty or not depends on the stage of the teaching. It would be a bit of a silly question to set early on in the topic but a few awkward things like this can make the student think and reassess their understanding.
     
  8. S20blu is correct.
    Half-life and rate are two different things. The half-life is the decrease in concentration of a reactant and will be constant for a 1st order reaction. Meaning that over a certain period of time (the half-life) the concentration will drop to 1/2 the original value, in another half-life the concentration will drop by 1/2 again so it will be 1/4 of the original value. So regardless of the original concentration it will always drop by 1/2 in the same amount of time.He will still see a graph as a curve though.
    Concentration will affect the rate. For a 1st order reaction as concentration doubles, so does the rate. A concentration rate graph would be a straight line in this case.
    For a 2nd order reaction:
    Rate is proprotianal to concentration squared. So if you double concentration, rate will increase by 4x.
    Half life increases with decreasing concentration so there will be an effect in this case.
    Just remeber that these are two different things. I have OCR exam questions and markschemes on this topic. If you PM me I will be happy to send them.
     
  9. Hi,
    The half life is the time during which the concentration of a reactant is reduced to half its initial concentration .For example if you have a 5 mol/dm3 of a gas, the time required for it to decompose to 2.5 mol/dm3 is called half life. It is denoted by t 1/2
    It is given by the expression t1/2=0.693/k for a first order reaction. Since in this equation the concentration term is not involed we can say that the half life period of a first order reaction does not depend on the initial concentration.
     
  10. Sorry, I did mean to say that, have just realised that there is no mention of time in my post [​IMG]
     

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