I think that the concentration of a solution effects the rate of reaction because 'the rate of reaction depends on how frequently the molecules of the reacting substances collide. A more concentrated substance has more molecules for a given volume than a more dilute substance. Because there are more molecules about, the frequency of successful collisions is greater, and the reactions happen faster.'
Safety To conduct a safe experiment I will follow some rules which include:
- Wear safety goggles to protect my eyes from chemical splashes.
- Stand up to conduct the experiment, therefore reducing the risk of tripping and spilling chemicals.
- Taking care when handling chemicals, particularly Hydrochloric acid and Sodium Thiosulphate because they are irritants. I will not touch my eyes or mouth until I have thoroughly washed my hands.
- Mop up any spills
- Put bags under desk to prevent tripping over.
Variable Control
To make this experiment a fair test I will only vary one thing - the concentration of the Sodium Thiosulphate solution. I will conduct all the tests at room temperature because temperature has an effect on the rate of the reaction. The measures of Hydrochloric acid will all be the same (10cm). The person timing the experiment will look for the disappearance of the cross, otherwise there would be a time lapse between seeing the cross disappear and telling the other person to stop the clock and then eventually stopping the clock.
Method
- I will pour 30mls of Sodium Thiosulphate or Na S O into 10cm of Hydrochloric Acid or HCL in a conical flask.
- The concentration of the sodium Thiosulphate is 40mls and the concentration of the Hydrochloric Acid is 2 molars.
- The flask will then be placed on a piece of paper with a black cross drawn on it.
- Start the stopwatch when the sodium thiosulphate and hydrochloric acid are combined.
- Stop the stopwatch when the solution is cloudy enough to prevent me seeing the cross.
- I will repeat this procedure with the measurements in the table below.
- Then to gain an average I will repeat all the tests that give a more accurate result.
Collecting Results
I will record my results in a table and then work out an average afterwards to gain a more accurate result.
Analysis
I have observed that the more concentrated the solution, the quicker the rate of reaction time will be.
This is because of several reasons. Firstly, my results give conclusive evidence that as the amount of Sodium Thiosulphate decreases and the amount of water in the solution there are less atoms to collide and therefore less successful collisions causing chemical change so the reaction rate is slower. In a more concentrated solution, there are more atoms to collide so the reaction time is quicker.
My results support the prediction I made because I said 'the greater the concentration of Sodium Thiosulphate the faster the rate of reaction time.' I believe I was correct and a secondary source states that the reaction time will be faster with a more concentrated solution because,' the more molecules there are, the frequency of successful collisions is greater and therefore the reaction rate is speeded up.
Evaluation
The experiment roughly went according to plan but there were problems. For example, I only obtained two sets of results and then worked out an average. I could have done three tests to gain a more reliable average.
There was also a human error involved when I was measuring liquids and looking for an end point in the reaction. Although the reaction I chose had a fairly definite end point it was still hard to tell whether the whole cross had disappeared or not. Instead of using a cross a light beam could be used and when the beam goes out that is the end point. A better standard of measuring cylinders and pipettes could be used.
However, there was one result that was not really an anomaly as it still followed the pattern but was slightly different to the other result of the other test. When the amount of Sodium Thiosulphate was 10 cm, the time of the first test was 3043 compared to 287 in the second test. This could be human error, which is the reason for doing more tests.
