Diagram
Variables
Other variables which may affect the outcome of this experiment, if not kept constant, are as follows:
Temperature Of Sodium Thiosulphate - I believe that the hotter the temperature, the faster the rate of reaction. This is because when a substance is heated up, each particle has more energy, resulting in more collisions between the free moving particles in a liquid, causing more particles to react, therefore increasing the overall rate of reaction. The second effect this has is that the particles collide with more force, increasing the chance of the particles reacting
Concentration Of Hydrochloric Acid - The more concentrated the hydrochloric acid, the faster the reaction will be, because there will be more particles in the same volume. The particles will be closer together, colliding more often, therefore increasing the speed of the overall reaction. I will keep the concentration of hydrochloric acid constant, by using it from the same bottle each time, which has a fixed concentration.
Size Of Conical Flask - Although this does not affect the rate of reaction, in that it doesn't affect the chemical reaction itself, but what this variable does affect is our measurement of how long the reaction has taken to complete. I will be judging the time taken for the reaction to finish as the time it takes for the reacted mixture of sodium thiosulphate to go completely cloudy so that I cannot see a cross drawn underneath the flask. If the conical flask is wider then the cloudiness will appear less concentrated, because it will be spread across the larger area of the bottom of the conical flask, and so it will take longer for the cross to disappear, and the results will increase. I will keep this variable constant by using the same conical flask for each experiment. Previous experiments have shown that the size of conical flask I will be using is adequate when gauging turbidity with this volume of chemicals (60cm³).
Cross - The material I use draw the cross underneath the conical flask, which I use in determining when the reaction has finished, could affect how easily the cross is seen. This in turn could affect the results because if the cross is seen more easily, if it was drawn with a thick marker, our measurement of how long the reaction took to complete would be longer and so the rate of reaction would seem slower. Equally, if the cross was hard to see, then it would disappear more quickly and the rate of reaction would seem quicker. I will keep this constant by using the same black pen to draw the cross each time, if I are unable to use the same cross each time, which is preferable.
Volume Of Sodium Thiosulphate And Hydrochloric Acid - The larger the volume of sodium thiosulphate and/or hydrochloric acid, the greater total number of particles in the solution. This means that to completely react, and go cloudy, it will take longer. I will keep this variable constant by using 50cm³ of sodium thiosulphate solution with 10cm³ of hydrochloric acid each time. I have found from some preliminary tests that this volume (total of 60cm³) is most suitable when gauging the turbidity of the mixture, because if too little is used then it is hard to see changes in it.
Movement Of Reacting Mixture - If I shake the conical flask while it is reacting, then the reaction may speed up and therefore the rate of reaction would be quicker. Likewise, if I stir it then it will increase the rate of reaction, because there will be a greater chance of the particles colliding with other particles which they can react with because the two types of particles will be more mixed together and also, will have more kinetic energy because they will be moving while being stirred. I will prevent this from affecting the results by not stirring or shaking the mixture in any way.
Prediction
I think that as I increase the concentration of sodium thiosulphate, provided that no other variables affect the results, the rate of reaction will also increase. I believe that this will happen because of the way in which the two chemicals react. When the sodium thiosulphate and hydrochloric acid are placed together in the conical flask, their respective molecules get mixed together, moving freely. When molecules from each chemical collide, they react, forming new compounds. The concentration will increase the rate of reaction because if the concentration is higher, then there will be more molecules of sodium thiosulphate in a given volume, and so it is reasonable to say that it greatly increases the chances of one of those molecules colliding with the hydrochloric acid molecules. This means that more of the molecules will be colliding and reacting within a given time, and so the rate of the reaction has speeded up.
Results
The results which I have collected, without any changes to the planned method, are shown below. I have also plotted them onto a graph (see separate sheet) so that the trend can be seen more easily.
Conclusion
The results, and the graph appear to show roughly what I predicted. As the concentration of sodium thiosulphate is increased, then the rate of reaction increases also. What I believe to be happening is that as the concentration decreases, there are less sodium thiosulphate molecules around in the reacting mixture, meaning that it will take longer for each hydrochloric acid molecule to ‘find’ a sodium thiosulphate for it to react with. Overall, this decreases the rate of reaction because it takes longer for the mixture as a whole to react fully, because of the extra time it takes for the molecules to collide with each other. The results support this theory in that the relationship. This also supports the idea that it takes time for the molecules to ‘find’ each other in order to react, because if the concentration is less, then the molecules of sodium thiosulphate will be more spaced out and there will be more space between themselves and the hydrochloric acid molecules. If the molecules move around in the liquids at the same speed each time (because they have a constant temperature), then the further they have to travel to reach another molecule, the longer it will take for them to reach that molecule and then react.
Evaluation
The results of this experiment appear to be quite accurate and so I can make a definite conclusion. From this, I can speculate that the experiment was conducted well, and that the results have only been affected by other variables by a little. However, looking at the graph, I can see that there are some results, two specifically, that do not follow the general trend, and show a higher or lower rate of reaction than would be expected at that point. I believe that perhaps this was caused by a change in another variable, or an inaccuracy in the measuring of the input variable. Either the reacting mixture was shaken accidentally, moving the molecules around more, increasing their chances of reacting, or more likely, the concentrations of sodium thiosulphate were incorrectly measured, and too much sodium thiosulphate was used. This would reduce the distances between the molecules, and increase the rate of reaction. I think that the only flaws in the experiment were in the measuring of the concentrations, and in the judging of when the reaction had completed. It was very hard to get an exact volume of water/sodium thiosulphate to get the right concentration, and so this may have thrown those three results off the trend. Also, there was not really any sure way of knowing when the reaction had finished. My rudimentary method of looking at the turbidity of the mixture is very inaccurate, and leads to confusion, especially, for example, if you look away for a moment after thinking the reaction is over, and then look again and realise it hasn’t. If I was to repeat the experiment, I would use a much more accurate way of measuring the volumes of the reactants, and I would find a completely different way of measuring when the reaction has completed. Perhaps a Light Dependant Resistor could have been connected to a stop clock which would stop once the LDR stopped receiving direct light.
