Prediction
I predict that when I add more water to the sodium thiosulphate solution, the time it takes for the cross to disappear will lengthen. I predict this because, for particles to react they must collide with each other. If there is a higher concentration of sodium thiosulphate particles in a given volume there, is a higher chance of the particles colliding together and reacting. This is because they are so close together. If you weaken the solution, (so there is a much lower concentration of sodium thiosulphate particles to react with the hydrochloric acid,) then there is less chance of the particles colliding and reacting as there are less particles in the given volume, so the reaction time will slowly lengthen as the concentration goes down.
Plan
I shall have to control as many aspects of the experiment as possible, to make it a fair test. I shall make sure the total volume of the solution stays the same. I shall do this by reducing the volume of sodium thiosulphate as I add water, the total volume of the sodium thiosulphate solution and water will be 60cm3. An example would be having 40cm3 of sodium thiosulphate and having 20cm3 of water, added together their volume is 60cm3. I shall do this because if I did not then the total volume would go up and I would not get reliable results for the investigation I am doing as I am trying to find out how concentration affects the rate of reaction. I shall try to make sure that the temperature stays the same throughout the experiment as, if the temperature of the solution increases, the particles in the solution move around faster and the chances of a collision become higher, so it would not be fair. The cross that goes beneath the beaker shall stay the same throughout the experiment, because if the cross changed, it might change in size or shape and may be harder or easier to see through the cloudy solution. In addition, the same person will have to look at the cross each time as each person can see a little differently, and someone might be able to see the cross at one point, while another cannot. Therefore, the person looking must be the same for consistency and fairness. A simple precaution to make sure not to mix up would be, careful not to mix up the measuring beakers for each of the liquids, therefore I will have three different beakers and each beaker will be labelled clearly.
Apparatus Used
- 3 different beakers
- Conical flask
- A4 paper with cross in the middle
- Stopwatch
Method
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Get 60cm³ of sodium thiosulfate solution and 5cm³ of hydrochloric acid (Vary the amounts of each for the different concentration tests).
- Put each solution into a different beaker and label them.
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Put the 60cm³ sodium thiosulfate solution and 5cm³hydrochloric acid into a conical flask and place over the cross on the paper.
- Start the stopwatch and time the reaction.
- Record the time it took to make the cross disappear.
- Make sure you wear goggles, as this is a safety issue.
The Things That Must Be Kept Constant
- The temperature of the thiosulfate solution because the more molecules there are, the more collisions there will be (according to the collision theory).
- The temperature of the HCL solution, because again, the more molecules there are, the more collisions there will be. This will mean a greater chance of a successful reaction.
- I must use the same beaker all the way through, because different beakers have different surface areas and this will result in an unfair experiment.
- I must use the same cross, as one cross could have a darker shade that another. This means that the darker shaded cross would be visible for longer.
- The same person must watch the cross disappear in the experiment, as different people have different eyesight.
I shall collect the required equipment and measure the acids out using different measuring cylinders. I shall place 60cm3 of sodium thiosulphate solution into a flask with 5cm3 of hydrochloric acid. When both are placed into the conical flask, the stopwatch shall be started. I shall then watch the cross to see when it will disappear. The moment I cannot see the cross then I shall stop the stopwatch. The time taken for the cross to disappear shall be noted down in the table of results. The experiment then shall be repeated with different concentrations of sodium thiosulphate, which will be achieved by diluting the Sodium thiosulfate solution with water each time. I made a table to show my results.
My table of results:
From my results, I can see that my predictions were correct and that as the concentration of Sodium thiosulphate solution decreased, the time taken for the reaction to take place increased. In my prediction, I expected that as I added more water to the Sodium Thiosulphate solution the time it would take for the cross to disappear would lengthen. In my prediction, I expected that as I added more water to the Sodium Thiosulphate solution the time it would take for the cross to disappear would lengthen.
The collision theory suggests that if the solutions of reacting particles are made more concentrated, and there are more particles in a given volume, collisions between the reacting particles are therefore more likely to occur, and rate of reaction would then go up. This statement does appear to be true as the graphs show this.
Overall, the prediction I made is supported by the results of my experiment because they show that as concentration goes down, the rate of reaction time also goes down and the time for the cross to disappear lengthened. This would also support the collision theory as it states that if the solutions of reacting particles are made more concentrated, collisions between the reacting particles are therefore more likely to occur.