Before the experiment can take place, preliminary work has to be done to decide what concentration of Sodium thiosulphate is to e used in the investigation.
In the preliminary work I am going to use 1/3/5/7 and 10g of Sodium thiosulphate in 100cm³ water. This will be added into 10cm³ hydrochloric acid.
To do this experiment I am going to use the following apparatus:
- Sodium thiosulphate
- Hydrochloric acid
- 2X measuring cylinder
- Beaker
- Conical flask
- Stopwatch
- A ‘X’ marked on a piece of paper
To carry out the experiment add the Sodium thiosulphate into 100cm³ water and stir until dissolved. Measure out 50cm³ of the solution and pour into a conical flask. Add 10cm³ hydrochloric acid into the solution, stir and wait until the X disappears.
For safety I will be wearing safety goggles when doing the experiment and making sure all stools and bags are out of the way.
After carrying out the experiment these are the results.
Looking at these results I have decided to use the concentration of 5g/100cm³ as it is has a good reaction time that wouldn’t bee very slow when diluted and wouldn’t be too fast when concentrated at 50g/l.
To do the investigation I am going to use the following apparatus;
- Sodium thiosulphate
- Hydrochloric acid (10cm³)
- 2X Teat pipette
- 2X beakers
- Conical flask
- 10cm³ measuring cylinder
- 50cm³ measuring cylinder
- ‘X’ marked on a piece of paper
To keep it a fair test I am only going to change one factor – the concentration of sodium thiosulphate. Factors such as temperature, stirring, and the volume of hydrochloric acid will be kept the same as they would affect to reliability of the results.
To make the readings as accurate as possible I am going to use the teat pipettes to get the Sodium thiosulphate and hydrochloric acid as accurate to the measurements as possible.
The same safety precautions are going to be used as in the preliminary work.
To start the experiment take 50cm³ Sodium thiosulphate and add to the conical flask. Add 10cm³ hydrochloric acid and pour into the conical flask and stir once. Start timing the experiment and watch the solution to turn colour until the X underneath the conical flask and stop the stopwatch.
I then repeated the same method using the ratio sodium thiosulphate : water for:
40:10
30:20
20:30
10:40
The results for the investigation are below.
Looking at my results, I can see that my prediction was right and there is a clear pattern seen between the time taken and the ratio between the sodium thiosulphate. As the sodium thiosulphate decreases in concentration the time taken increases. My quantitative prediction was also true as the time taken also doubles when the concentration halves. For example, the 20:30 and the 40:10 double from 0.021 to 0.042. Even though there are a couple of anomalies there can still be a pattern seen in both graphs.
The rate of reaction increased as the concentration of the sodium thiosulphate increased because there were more sodium thiosulphate particles for the hydrochloric acid to collide with. As there were more particles to collide together, the frequency of collisions increase which makes the chance of a successful collision greater.
The rate of reaction doubled as the concentration doubled because there were twice as many sodium thiosulphate particles to collide with the same amount of hydrochloric acid. As there are double the amounts of collisions happening, the number of successful collisions will also double.
Looking at my results on the graph, I can see that nearly my results for the rate of reaction are quite accurate and a pattern can be seen even though they are not on the line of best fit. All the results are near to the line of best fit except for the result for the 50:0 concentration on the 1/t graph. To keep the results as accurate I measured the hydrochloric acid and the sodium thiosulphate by using the meniscus as a measuring guide. To make the measuring more accurate I used a teat pipette when the liquid was close to the measurement needed. The technique I also tried to keep accurate but there were some areas that could not be. The stirring of the solution could have been different in all or some of them. This could have been more accurate by using a magnetic stirrer. The time taken for the ‘X’ to disappear could also have been more accurately measured by using not just our eye sight. In my experiment I can see that there is a couple of anomalous points, especially that 50:0 concentration on the 1/t graph. Nearly all other points are correct or slightly wrong. The experiment used is an easy experiment to use for the investigation although it could be made more accurately by using more advanced techniques. Out of all the factors that had to be kept the same there were two that were very difficult to keep the same all the way through the experiment. The hardest was keeping the temperature the same as it was carried out in room temperature. Also, stopping the stopwatch at the exact time is hard to do as there is a reaction time before you press the button.
Some of the results had to be repeated a 3rd time. This could be because of many different factors such as timing incorrectly, spilling acid or a slight increase/decrease of acid. These factors could also be applicable to the anomalous results in my experiment. In my opinion using the average of 2 results only gives a rough idea of the time taken. To make the results more reliable, I think 5 readings would be more accurate and up to 10 concentration.
To make the timing as accurate as possible I would use a light probe to measure the amount of brightness passing through the solution. When the light passing through gets dark enough, that is when the timing of the experiment stops. To keep it a fair test we would stop when an exact amount of brightness is lost through the solution. To further advance the investigation I would then do another experiment changing a different factor such as increasing/decreasing the temperature. If the temperature is increased the rate of reaction would also increase as the particles would have more energy to collide with each other, in turn increasing the successful collision rate.
