There are many variables that I could change in this investigation to see what factors effect the rate of reaction:
- The concentration of the sodium thiosulphate
- The concentration of the hydrochloric acid
- The colours of the paper on which the cross is drawn and the colour of the cross
- The surface area of the reactants
- The temperature of the reactants
I could also use a catalyst in this investigation as catalysts work by lowering the activation energy. More collisions would have more energy than the minimum needed to react and therefore the reaction would go faster.
The variable that I have chosen to test is the concentration of the sodium thiosulphate. I will see how this affects the time taken for the cross to disappear as this will show how the concentration of sodium thiosulphate affects the rate of reaction.
I predict that ‘as the concentration of sodium thiosulphate increases, the time taken for the cross to disappear will decrease. The higher the concentration of sodium thiosulphate, the faster the reaction will take place.’
Reactions can only take place when the reactant particles collide but most collisions are not successful in forming product molecules, despite the high rate of collisions. Particles have a wide range of kinetic energy but only a small fraction of particles have enough kinetic energy to break bonds and bring about chemical change. The minimum kinetic energy required for reaction is known as the activation energy.
The collisions between particles which do produce a chemical change are called ‘fruitful collisions’. The reactants collide with enough kinetic energy to break the original bonds and form new bonds in the product molecules.
Increasing the concentration increases the probability of a collision between reactant particles because there are more of them in the same volume and so this increases the chance of a ‘fruitful collision’ forming products. When the concentration of sodium thiosulphate is increased, there will be more sodium thiosulphate particles in the same volume and this will increase the chance of more successful collisions with the hydrochloric acid particles.
Increased concentration of sodium
thiosulphate particles
If there are more successful collisions, the products will be formed quicker meaning that the sulphur will be produced quicker to blot out the cross and the time taken for the cross to disappear will decrease.
I will make the investigation a fair test by:
- Washing measuring equipment with distilled water before use and between measurements
- Using the same volume of reaction mixture each time
- Keeping a constant temperature of the solution so that the kinetic energy of the particles is the same throughout
- Looking at the cross from the same angle each time- from above the conical flask
- Starting the stopwatch when all of the acid has gone into the conical flask every time.
Apparatus
- Conical flask
- 50ml measuring cylinder
- 10ml measuring cylinder
- 20ml measuring cylinder
- Stopwatch
- Hydrochloric acid
- Sodium thiosulphate
- Distilled water
- White paper with a black cross
- Safety goggles
Method
I will first collect all the apparatus that I have listed above. I will wash out the measuring equipment with water so that I know they are clean and don’t contain any other previous substances that could affect my results. I will then measure out the correct amount of sodium thiosulphate, hydrochloric acid and water I need
e.g. Sodium thiosulphate- 10cm³
Water- 40cm³
Hydrochloric acid- 10cm³
I will pour the sodium thiosulphate and the water into the conical flask so that I will have the concentration of sodium thiosulphate I need. I will then pour in the hydrochloric acid and the two reactants will start to react together. I will start the stopwatch when the last drop of acid has been poured in each time I do the test. I will watch the cross from above the conical flask and when it has disappeared and I can no longer see it, I will stop the stopwatch and record my result.
I will do the same thing for five different concentrations of sodium thiosulphate so that I can see how the concentration of sodium thiosulphate affects the time taken for the cross to disappear. Here are the measurements I will use:
I will ensure my results are accurate by using the smallest possible measuring cylinder to measure the solutions. The gaps between the numbers are bigger on the smaller cylinders so it is easier to read and will therefore be more accurate. When calculating the average time taken for the cross to disappear, I will record my results to 1dp so that it is more accurate than rounding to a whole number. When calculating the rate of reaction I will record my results to 3dp as the change will be very small.
I will ensure my results are reliable by repeating the whole experiment 3 times and then finding an average of the time taken for the cross to disappear for the different concentrations of sodium thiosulphate. I will also make sure that my experiment is a fair test.
Obtaining evidence
Safety precautions- In my experiment I had to take safety precautions so that neither me or any of my classmates were in danger.
- I wore safety goggles to protect my eyes from harmful chemicals.
- When using a Bunsen burner in my preliminary experiment, I made sure that it was on the safety flame when we weren’t heating anything.
- I kept my stool under the bench at all times. The stool would obstruct the pathway and this could be dangerous. If this was not carried out, a person passing by could have tripped over the stool whilst holding a piece of equipment, which may endanger themselves or people around.
- I kept my bag under the desk and hung my coat up so that they weren’t potential hazards
- I made sure that I didn’t spill any solution on the floor so that no one walking past slipped and fell.
Analysing evidence
To work out the concentration in percentages of the sodium thiosulphate, I divided the volume of the sodium sulphate by 50 (as this was the maximum volume of sodium thiosulphate I used) and then multiplied the number by 100 to change it into a percentage. For example:
10 =0.2
50
0.2 x 100= 20%
To find the averages of the 3 results for each concentration of solution, I used the calculation:
Result 1 + result 2 + result 3
3
An example of this is:
20% concentration of sodium thiosulphate= 134+133+137
3
= 134.7 seconds
This allowed me to plot the average points on my first graph and draw a line of best fit.
To work out the rate of reaction I used the calculation:
Rate= 1 which can also be written as s
Time, s
An example of this is:
20% concentration of sodium thiosulphate= 1
134.7
= 0.007 per second
This allowed me to plot the points for the rate of reaction on my second graph and draw a line of best fit.
I can now analyse both my graphs and come to a conclusion on whether my prediction was correct.
From looking at my first graph I can see that my results do follow a pattern. My line of best fit starts at the top left and curves down to the bottom right, showing that as one thing increases, the other decreases. In this case, the time taken for the cross to disappear decreases as the concentration of the sodium thiosulphate increases. I can see that the graph is steepest when the concentration of sodium thiosulphate changes from 20% to 40%. My results show that when the concentration of sodium thiosulphate was 20%, it took an average of 134.7 seconds for the cross to disappear and that when the concentration was 40%, the time taken more than halved to an average of 60 seconds.
From looking at my second graph I can see that my results show positive correlation and that as one thing increases, the other also increases. As the concentration of sodium thiosulphate increases so does the rate of reaction. The higher the concentration, the faster the rate of reaction and this is why the time taken for the cross to disappear decreases as the concentration of sodium thiosulphate increases.
To conclude, I can see that my results and my graphs support my prediction and I can now see that it was correct- ‘as the concentration of sodium thiosulphate increases, the time taken for the cross to disappear will decrease. The higher the concentration of sodium thiosulphate, the faster the reaction will take place’. My first graph shows that as the concentration increases, the time taken for the cross to disappear decreases and my second graph shows that as the concentration increases, the rate of reaction increases and the reaction occurs faster. The reason for this is because increasing the concentration increases the probability of a collision between reactant particles because there are more of them in the same volume. This increases the chance of a ‘fruitful collision’. When the concentration of sodium thiosulphate is increased, there are more sodium thiosulphate particles in the same volume and this increases the chance of more successful collisions with the hydrochloric acid particles. As there are more successful collisions, the rate of reaction increases and the products are formed quicker. The sulphur is produced quicker and so the solution turns yellow and cloudy quicker and the time taken for the cross to disappear decreases.
Evaluating evidence
Even though that my results do follow a pattern, I can see that on my first graph I do have 1 slight anomalous result that does not quite fit in with the general trend of the graph. I have circled this on my graph in pink pen and I can see that it occurred when the concentration of sodium thiosulphate was 40%. The average time taken for the cross to disappear was 60 seconds but from looking at my line of best fit I can see that it should have taken 66 seconds. If I look at my results table I can see that when the concentration was 40% I got an odd result which was a lot lower than the other two. The first time we did it, the time taken was 65 seconds and the third time we did it the time taken was 67 seconds but the second time we did it the time taken was 48 seconds. I have also circled this in pink pen because it is this result that has caused the average time taken when the concentration was 40% to be pulled down and be lower than it should. I could have got this anomalous result from not checking that the cross had fully disappeared and from just taking a quick glance and thinking that it wasn’t still visible. Also, the solutions might not have been measured out correctly and might have been a bit higher concentration than needed, therefore decreasing the time taken for the cross to disappear. Also, the temperature could have increased as we didn’t check it with a thermometer and this would make the cross disappear faster than it should because of the particles moving faster and colliding more.
We made sure that our results were accurate by:
- Measuring from the bottom of the concave meniscus and at eye level
- Using 3 measuring cylinders of different sizes. The gaps between the numbers are bigger on the smaller cylinders so it is easier to read. I always used the smallest measuring cylinder possible.
- Using the same person to time how long it took for the cross to disappear each time. Different people have different reaction times and if we used different people, one person might stop the stopwatch really quickly and another person might stop the stopwatch slowly.
- Using the same person to watch the cross and determine when it disappeared. Whenever that person decided they couldn’t see the cross anymore, it would be the same every time we did the experiment as they have the same vision.
- We cleaned the equipment out with distilled water each time something had been in it. The excess water would have diluted the concentration of the sodium thiosulphate and therefore would have affected the results because it would have taken longer then usual for the cross to disappear.
We made sure that our results were reliable by repeating the experiment 3 times and then finding an average.
If I was to do the experiment again I could improve it by using a light sensor to detect when the cross had disappeared. Even though we used the same person to determine when it had disappeared, it would still be more accurate to use a light sensor as you know for certain the cross has definitely disappeared. I would also check the sensitivity of the stopwatches we used. Some stopwatches are more sensitive than others and we should have checked all of the stopwatches to make sure that we had a sensitive one as this would be more accurate. Finally, I would use a thermometer to measure the temperature of the solution so that I could make sure it was kept constant so that it wouldn’t affect the time taken for the cross to disappear, as this could be the reason I got my anomalous result.
I believe that even though my results could have been more accurate by doing the points above, the reliability of my evidence is still sufficient enough to support my conclusion. By doing the experiment 3 times and finding the average it eliminated any human errors that I may have made as it took all of the data into consideration, and didn’t just rely on one result for each concentration of sodium thiosulphate.
I could have done further work to provide additional relevant evidence for my investigation. I could have repeated the whole experiment but changed a different variable. For example I could have changed the temperature of the reactants; I could have changed the concentration of the hydrochloric acid; I could have changed the surface area of the reactants or I could have used a catalyst. I could then draw another graph and see if the different variable would have the same affect on the time taken for the cross to disappear. I could compare this to my other graph and then analyse the differences or similarities between them.