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To investigate by the use of an experiment to measure the effect of changing one variable on the rate of a reaction

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Introduction

Chemistry practical coursework As we all know some reactions happen very quickly (e.g. rubidium in water) and on the other hand some happen very slowly (and iron nail in water). Therefore the only way of measuring the rate of a reaction is by performing an experiment. For this experiment to work I will have to work out a way of measuring either i) the rate at which reactants are used up in a reaction (most likely to be magnesium) or ii) the rate at which products are formed in a reaction (most likely to be hydrogen). I will have to experiment on one variable which affects the rate of reaction, but on the other hand keep all other variables the same, including the chronological order in which the experiment is done. The rates of reaction are all dependant on the collision theory and this is what I will use to explain how and why rates of reaction vary. More on this further. Plan Aim: To investigate by the use of an experiment to measure the effect of changing one variable on the rate of a reaction. Variables on which we could investigate on: - Surface area - Temperature - Concentration - Catalyst - Stirring > Surface area isn't very practical, as it is hard to obtain surface area, however Mg ribbon is much easier, as compared to for example CaCO3 chips. This is due to Mg ribbon being equal in width and having a defined shape, unlike CaCO3 which are all oddly shaped. For Mg ribbon I could easily get the S.A. by multiplying the width by the length and doubling the answer, whereas for CaCO3 it's virtually impossible in a classroom environment because of the random shaped chips. However I could possibly perform an experiment to investigate the effect of surface area on the rate of reaction by looking at marble chips reacting with dilute hydrochloric acid. ...read more.

Middle

to, for example the same rate of reaction of a slightly less concentration, due to a mistake in mixing the two by a very little amount. How will the other variables be controlled if they were to be used? -Surface area: Magnesium can be used in the same length from the same roll, so the width and length are kept constant. Although we are not measuring the effect of surface area on a reaction, we will still have to keep this variable the same as with each and every experiment, because we are investigating the effects of concentration on the initial rate of a reaction. -Temperature: This can be controlled in a advanced water bath however a water bath is not really going to be considered because it takes too much time. Therefore we could just possibly do it in class laboratory in the open air, measuring the temperature at the start and end of the experiment too see whether we were able to control the temperature to an acceptable degree of accuracy. Otherwise we could put the conical flask in a water bath measuring the temperature at the beginning and end of the experiment. It is considered that this reaction will be an exothermic reaction; therefore it will be suitable to perform all experiments in a trough of water. -Concentration: This is what we are going to be varying, since it is the easiest to control but also at the same time keep consistent because it comes from the same industrially produced bottle which will have the right concentration throughout accurately. We will do this by simply using water to dilute the acid. The formula we will be using will be; Concentration = 2 x volume acid / (volume acid + volume water). This however is too time consuming figuring out the volumes of acid and water. So, by simply using 100cm3 as a total of acid an water and putting the acid and water into ratios of a 100 will be much simpler and quicker, e.g. ...read more.

Conclusion

* Another little problem that occurred was that whenever the bung was inserted into the conical flask, it would cause the volume inside the gas syringe to change slightly, so that it wasn't on 0 to start of with. Another little problem I encountered was that I forgot to sometimes put the syringe back to 0 before doing the second and third runs. This although wasn't a problem which I could sort out. > The solution to the problem of the gas syringe was that the volume shown after the bung had been inserted, I would record the value on the syringe and use that as the starting point (0) and add the same volume of gas to each result noted down on the intervals (5 seconds). The problem of forgetting to put the syringe back to zero had no solution but to remember! * Another problem experienced was that the temperature was hard to keep exactly the same, because of the classroom environment and not done under any controlled temperature environments. Although there were slight changes, the general temperature of the acid before and after was roughly 20�C. > There weren't any major things I could undergo in order to keep the temperature variable as constant as possible. Although, presuming that the temperature of the tap water was fairly constant, once it had been given time to flow, I decided to try and see whether changing the tap water in the trough of water, after each concentration, would keep the temperature more constant therefore keeping the results constant hopefully. The temperature could have acted as a catalyst. A catalyst speeds up the rate of a reaction without being used up itself in the reaction. It can be reversed chemically unchanged at the end of a reaction. It works by providing an alternative reaction pathway of lower activation energy. At a given temperature , more of the collisions will have sufficient energy (Ecat) to be successful when a catalyst is present. There are more successful collisions per second, and therefore the rate is faster. ...read more.

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