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Factors Affecting the Rate of Catalytic Decomposition of Hydrogen Peroxide.

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Introduction

Chemistry Science 1 GCSE Coursework: Factors Affecting the Rate of Catalytic Decomposition of Hydrogen Peroxide What is a reaction ? A reaction occurs when two fast moving particles collide to produce a product. This product is only produced if the two particles have enough energy to either join or decompose, this amount of energy is called the activation energy. Possibilities when particles collide Reaction A B No reaction A B Particles bounce apart Hydrogen Peroxide decomposes in a slow, exothermic reaction to form oxygen and water. As the reaction is slow, a catalyst is used to speed it up. A suitable catalyst for this investigation is Manganese Dioxide (Mn02 - a black powder). The definition of a catalyst is : - A substance that increases the rate of a chemical reaction but undergoes no permanent chemical change itself The equation for the reaction is : Mn02 Catalyst Hydrogen Peroxide Oxygen + Water (+ Heat) Mn02 Catalyst H202 O2 + H20 (+Heat) The reaction occurs when the H202 particles collide with each other. Some rebound off eachother, whilst some have the activation energy to react. The catalyst holds the Hydrogen Peroxide molecules on its surface in such a manner that their activation energies are hence lowered, and more can react. Although there is still some uncertainty as to what exactly the Manganese Dioxide catalyst does, it is believed that believed that the hydrogen peroxide becomes adsorbed (chemically absorbed and held on) onto the surface of the manganese dioxide, and then some temporary bonds are formed. The structural formula of Hydrogen Peroxide is : H-O-O-H It is thought that the temporary bonds formed with the catalyst (which is an active catalyst- i.e. it will only work if it collides with the Hydrogen Peroxide) help to break/weaken the H-H and O-H and O-O bonds contained in the Hydrogen Peroxid - hence lowering the amount of energy needed to break the bonds and so the amount of energy needed for the Hydrogen Peroxide to decompose. ...read more.

Middle

* H202 in increments of 2 Vol. from 4 Vol. to 20 Vol. - chosen because gives a suitable range of results whilst keeping the experiment safe - not too much heat is given out * Spatula - this is used to measure out the catalyst, prevents having to use the hands (a safety issue) and also allows smaller masses to be taken out at a time * Manganese Dioxide - this was chosen as a suitable catalyst, and although the reaction can happen without this, it is too slow to be measurable * Stop clock - this measures time elapsed, it is reasonably accurate though there is the threat always of human error, but is also quick and easy to use, and economical * Paper "bucket" - this was used to contain the mass and to pour it into the hydrogen peroxide, it is easy to use and has negligible effect on the scales (though these should be tared anyway) * 10 ml measuring cylinder - used to measure volumes, chosen because it is easy to use whilst maintaining a high level of accuracy * 100 ml beaker - the reaction occurred in this beaker, it did not contain the reaction too much as to make it dangerous but was not so large that the catalyst would miss the liquid Detailed Method 1. Tare the top-pan balance with the paper "bucket" on top of it at the time 2. Weigh out the 0.25g of catalyst, and leave it on the balance 3. Measure out 25cm3 of whichever concentration is needed of hydrogen peroxide using a measuring cylinder 4. Place the beaker onto the top-pan balance next to the bucket full of catalyst 5. Tare the top-pan balance, and add the catalyst to the hydrogen peroxide 6. Start the stop-clock at the instant of contact (N.B. remember to replace the paper bucket onto the top-pan balance) ...read more.

Conclusion

If the graph is redrawn with error bars, the following is shown: As the anomalies can be accounted for, and most points lie pretty near the best fit line, and when readings were taken from the graph and tested for proportionality they confirmed the prediction, my results are sufficient to justify the prediction for this range. The results are reliable because repeats were taken to make sure they were not flukes, and because they all lie reasonably close to the best fit line - and anomalies have a clear reason for their presence. However, in order to 100% justify the prediction, the experiment would have to be repeated with certain improvements. Improvements to the experiment if it were to be redone: * Investigate a larger range of concentrations (this can be used to determine whether there really is a terminal rate of reaction/mass loss) * Use more accurate top-pan balance, and use a burette to more accurately measure volume * Conduct the experiment in a vacuum where there is no atmospheric pressure, and contain the temperature using a temperature probe, taking a reading to make sure the temperature never alters. There will also be no people moving around the top pan balances which causes fluctuation and uncertainty over reliability of readings * Further investigate rate of reaction using a different variable * Extend this experiment by using an alternate method - see below * Do the experiment was using a gas syringe to collect the oxygen gas in, so that the volume given off could be plotted against the time taken for that volume to be collected. Do the experiment in a boiling tube with a rubber bung on top, through which there can be some tubing. This led to the gas syringe, which can measure how much gas has been collected. The rate of reaction could then be calculated by drawing curves for each concentration, and then dividing volume collected by the time it took to collect. Hugo Steckelmacher 5J1 ...read more.

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