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Rate of Reaction?

  1. Nov 30, 2013 #1
    1. The problem statement, all variables and given/known data
    Hello all,

    I'm currently in high school and I'm doing a biology experiment with catalase reacting with hydrogen peroxide. I have measured the total amount of gas produced in terms of mL during the reaction which took place over 30 seconds.

    Throughout the experiment I'm testing different concentrations of hydrogen peroxide - 3%, 2.4%, 1.8%, 1.2% and 0.6%.

    2. Relevant equations
    If I would like to calculate the average rate of reaction do I simply divide the total amount of gas produced with time?

    We were not allowed to use a pressure sensor by our teacher.

    3. The attempt at a solution
    For example:

    Hydrogen Peroxide Concentration: 3%
    Gas Produced: 60 mL of O2
    Time: 30 sec
    Rate of reaction: 2 mL O2/sec

    Is this correct? Is the unit correct?

    - Flomer
     
  2. jcsd
  3. Nov 30, 2013 #2

    Borek

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    Staff: Mentor

    This is a correct average rate. Doesn't mean it was constant and identical all the time.

    Besides, you can be required to give the answer in mol/sec. But what you wrote is perfectly acceptable.
     
  4. Nov 30, 2013 #3

    epenguin

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    Homework Helper
    Gold Member

    Your calculation is correct. Data is often written up preliminarily in the laboratory in the way you have.

    Then in formal presentations in e.g. published scientific papers you'd probably recalculate in terms of more standard units such as moles O2 produced per litre reaction solution per sec. (you have not told us the volume you did the reaction in). But if you don't yet know what moles are or how to calculate them I wouldn't regard this as essential.

    Reactions do not necessarily continue at the same rate so measuring amount of product after one fixed time is a somewhat unreliable measurement of rate if the rate slows a lot during the time of the measured reaction - if possible measure the same thing at say 30, 60 and 90 sec. I am curious how you started reaction, measured and timed, I mean it takes a few sec to mix - did you measure a difference in volumes over 30 sec?

    We will be interested to see your results when you are ready.

    Edit: I hadn't seen Borek's reply but I see we both say the same things!
     
  5. Nov 30, 2013 #4
    Thanks for the replies! We have not learnt about mol yet and I don't think it is included in the standard level syllabus.

    I understand that my calculation will only be the average rate and as you guys mentioned, it does not indicate how the rate of reaction changed over the 30 seconds. Should I mention this in my conclusion/evaluation?

    The experiment is for my biology class and our teacher said that time had to be one of the control variables. But I do understand your point, Epenguin!

    The volume for the reaction was 50 ml.

    The experiment was carried out the following way:

    • A water bath was filled with water.
    • A graduated cylinder, filled with water, was inverted and placed in the bath. (So the water stayed in the upside-down cylinder)
    • I then added hydrogen peroxide into an Erlenmeyer Flask.
    • I then added the catalase.
    • The flask was stirred and the reaction was allowed to start for 10 sec.
    • A gas delivery tube was then connected between the flask and cylinder.
    • The reaction was allowed to take place over 30 sec (measured with a stop watch from the point when the delivery tube was connected).
    • After 30 seconds the delivery tube was disconnected and then I measured how much water "had left" the cylinder as the oxygen entered.

    I had to go about it this way since we were not allowed to use a pressure sensor and instead we had to measure the total gas produced. Lastly, this method was the only one possible due to the materials my school offers. I know there are several improvements that can be made to this method and that there are unreliable parts of it - I'm planning to include this in my evaluation.

    I hope this make sense. English is not my mother-tongue so if I need to clarify anything please tell me!
     
  6. Nov 30, 2013 #5

    epenguin

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    Homework Helper
    Gold Member

    OK, school experiments often have limitations.
    Since, if I understand, you measured volume of oxygen by looking at the meniscus, it would have been little extra trouble to take different time measurements. But you may have some impression from the way it was bubbling if the reaction seemed to be continuing about the same at the end?

    Delay between mixing and start of measurement, which there usually is, doesn't matter if reaction rate is constant. Don't worry about the moles, it is still an experiment which can give a conclusion.

    Anyway what did the results look like, how did it depend on peroxide %?
     
  7. Nov 30, 2013 #6
    I noticed especially with the trails using some of the lower concentrations there were fewer and fewer bubbles towards the end of the 30 seconds.

    I have still to calculate most of the data - but the general trend is that the gas produced (rate of reaction) decreases as concentrations of peroxide decreases.

    For example two of my results were the following:
    3% peroxide - 60 mL gas
    0.6% peroxide - 20 mL gas
     
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