Essay Preview: Chemistry
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This investigation is about rates of reaction and what affects them. In this case I am going to look at hydrochloric acid and sodium thiosulphate which is a precipitation reaction. They react as in the equations below:
Sodium thiosulphate + hydrochloric acid -> sodium chloride + sulphur + sulphur dioxide + water
My prediction for the experiment of mixing the 2 compounds Sodium thiosulphate and hydrochloric acid together is when the higher the concentration of sodium thiosulphate the shorter the time it will take for the reaction to be completed in are case for the cross not to be visible. Where as the lower the concentration the longer it will take for the reaction. A reaction will only occur where the particles of the reactants meet and combine. This is called the collision theory. Therefore it stands to reason that to increase the rate of reaction it is necessary to cause more particles to collide harder and make it happen more often. There are several ways to do this and these make up the variables for this experiment. They are listed below along with predictions as to their affect on the reaction.
Increasing the pressure. By reducing the volume in which the same amount of particles exists the pressure is increased. Once the same number of particles is in a smaller area there is less space in which to move and so the particles are more likely to hit each other. It is therefore possible to predict that increasing the pressure will result in an increase in the rate of reaction. I will not test this variable because the school doesnt have the facilities to test it. However pressure is a continuous variable.
Using a catalyst is another method that could affect the time. A catalyst is a separate substance which speeds up a reaction. After the reaction has happened it gets left behind. This makes this variable unsuitable for the type of experiment I am going to do. A catalyst is also a discontinuous variable with only one likely useful catalyst emerging.
Energy. By giving the particles extra energy they will move faster. This means that they cover more ground and are therefore more likely to hit each other which in turn make the reaction faster. Also when the particles do collide with more energy its more likely of a reaction. The best way to give energy to a particle is as heat and so I can predict that raising temperature will increase the rate of reaction. This is a continuous, independent variable. I shall test this variable. I predict that temperature is proportional to rate of reaction. Eg the faster pupils run in a corridor the more difficult and the more space they will run making they chance of hitting some one higher. It will be harder to dodge some one so a collision is at high risk and the faster they are running the more it will hurt when a collision takes place
Concentration. Just as increasing the pressure will increase the number of particles colliding, so will the concentration. By putting more particles into the reaction, the chance of them colliding increases and so the rate increases. This variable is continuous and independent. I shall test this variable. I predict that by doubling the concentration of the acid, the rate of reaction will double. Eg the more pupils in a corridor the more likely they will collide because more space is taken up.
Surface area. Particles can only collide when the two sorts can meet. Therefore a reaction can only occur on the surface of the material. Therefore by increasing the area of the material which is available to collide the speed of the reaction will increase. I predict that doubling the surface area will double the speed of the reaction. This variable is continuous but I shall not test it because it is hard to control the exact surface area of the two reactants as they both come in an aqueous solution.
All of these processes could cause the reaction to speed up or to react slower so the time taken depends on these things. But in this investigation we are not using all of these. Although we are taking in to consideration of the temp of the room, the concentration, and the energy the particles will have all these are the main subjects because they are necessary for this investigation.
I have to make this experiment fair to this I need to use equal amounts of each sodium thiosulphate concentration added to the equal amounts of hydrochloric acid. Also the room in which I do the experiment in should stay the same temperature because it could cause inaccurate results by causing the reaction to go slower or faster. The same sized beakers should be used because the volume in side the beaker could be different making the collision of particle more or less likely. The beakers and measuring cylinders should be washed after use this is because the different concentration could get mixed, speeding or slowing the reaction, or if the sodium thiosulphate is mixed with a quantity of hydrochloric acid could cause a big difference on the time, giving the reaction a head start. I will no when to stop the watch which is when the reaction is at the right point which is when the cross under the beaker becomes obscured, this is because the mixture is to cloudy.
I am going to do this experiment 4 times giving me reasonable results, the more the better but have time on my hands plus if I do too many and cant finish all of the results that day, when I come back another day the results could be affected by the temperature.
The more times each concentration is added with hydrochloric acid it means that the average result will be more average and accurate. And it will be visible to read if there is a mistake made by you or the stopwatch by comparing them with all of the other results. If I only do this experiment once it will mean inaccurate results, if a mistake is made either when timing you wont recognise it, or if a bit of another concentration has been added. It will change the whole investigation and could end up being wrong, even a few seconds makes a big difference.
Another reason I am doing this experiment 4 times s because we cannot
depend on are eyes to tell us when the cross is totally disappeared or at least naked to the eye, although they will be around the same time so this is why we are having an average result.
I am going to test the two variables concentration and temperature. Both of these are independent, continuous variables. I think that concentration will have the biggest affect because