Precautions I will be undertaking in my experiment to maintain safety of myself and others include:
- Wearing goggles at all times.
- Wearing an apron at all times.
- Stand up whilst the experiment is conducted.
- No disruptive and dangerous behaviour in the room.
- No deliberate spillage of acids.
- Take care when handling chemicals and equipment.
Prediction
From my preliminary experiment I was able to see that the more concentrated the solution of hydrochloric acid the quicker it takes for the cross to disappear. The results I gained were not great. I feel this was due to the range of the concentrations of acid used. So with this new knowledge I have decided to use more concentrations of acid as to see how the results would continue. I also released that the idea of using the cross gives inaccurate results. As when using the cross method it is prone to human errors. As people may have different sight capabilities or even have different views on when the cross has disappeared. All this problems will affect the overall results of my experiment. I therefore have decided to use a light sensor instead of the cross method. This would work by taking the start light intensity, mixing the solutions, waiting for the light intensity to drop to 10%, and then recording the time in took to do so. I feel that this method would cancel out human errors and therefore gain a truer result.
That results I gained from the preliminary experiment show that the more concentrated the solution of hydrochloric acid the quicker it takes for the cross to disappear. At the concentration of 0.25m it took 36 seconds to disappear, where as at 1.0m it took 32 seconds. Also there was a decrease at 1.5m to 30 seconds for the reaction to complete. These results clearly state that as concentration increases rate of reaction decreases. Concerning the line of best fit I can therefore say that at about 0.8m it would take 33 seconds for the reaction to complete. From the line of best fit I can also see that at about 37 seconds any reaction exceeding this time must contain a hydrochloric acid concentration of less than 0.2m. These results are clearly showing how the more concentrated the solution the quicker a reaction will occur. This is as there are more particles of the concentrated solution in the reaction. This means that the more particles in the solution that more chance of collisions. And the more collisions means the more chances of effective collisions. And these effective collisions are what make and break bond in the mixture to produce sulphur. Also the fact that there are so many particles means less energy is needed to get effective collisions and then this spare energy is also used in the reaction.
I predict that the greater the concentrated solution the quicker it will take for the sensor to lose light. So the less concentrated the solution of hydrochloric acid the slower the thiosulphate solution will precipitate forming sulphur, making it hard for the sensor to absorb light as the sulphur blots attempts to blot out the light rays from reaching the sensor. The undiluted hydrochloric acid will be less reactive as it has a decreased chance of effective collisions; this is because there are less hydrochloric acid particles in the solution decreasing the chances of effective collisions with sodium thiosulphate, as there are not enough particles to produce enough effective collisions for a reaction to occur. It is very important to know the chemical equation of the reaction:
2HCl(aq) + Na2S2O3(aq) = SO2(g) + 2NaCl(aq) + H2O(l) + S(s)
The (aq) stands for a solution, (g) stands for gas, (l) stands for liquid, and finally (s) stands for solid. It is important to realise that sulphur has been used as it is insoluble and will block out the light. That is why it’s been used in this experiment. The reaction at first will be an endothermic reaction as heat is required therefore representing that a bond has been broken, but later the reaction will be exothermic as heat is required to make the new bond to produce sulphur, in this case between sodium thiosulphate and hydrochloric acid to produce sulphur. This loss then gain in heat is why the temperature doesn’t change and is therefore not visible in my table of results. Hydrochloric acid is a molecular compound, whereas sodium thiosulphate is an ionic compound. An ionic compound is a compound whose components are held together by ionic bonding. Ionic bonding occurs when both substances become ions. Ions are charged particles that were once neutral atoms but have either gained or lost their valence electrons to get a full outer shell. The hydrochloric acid and sodium thiosulphate particles will collide with one another. They have the energy to move and collide in the form of activation energy. Activation energy is the minimum energy that the particles of the reactants must have for them to react when they collide. The rate of reaction depends on how many reacting particles have the minimum energy. In many reactions, the particles already have this energy and react straight away. In others, energy has to be supplied for the particles to reach activation energy. When the sets of particles have this energy, which they get from the light turning into heat energy, they use it to collide with one another to break the ionic bond in sodium thiosulphate. Then once the bond is broken the particle collide to make new bonds. When this happens ions from the breaking of the ionic compound found in sodium thiosulpahte are left floating around freely in the solution. The new bond that is made is in fact sulphur and that sulphur is used to blot out the light to the sensor and the 10% decrease in light intensity is used to determine when the reaction is complete.
Equipment
I will be using a number of instruments and solutions to conduct my experiment and they include:
- Sodium thiosulphte
- Hydrochloric acid (0.25, 0.5, 1.0, 1.25, 1.5,2.0)
- Light sensor
- Stop clock
- 2 measuring cylinders
- Thermometer
- Beaker
- Stand with clamps
Method
- Gather all equipment and solution for the experiment.
- Pour 20cm³ of sodium thiosulphate in to a measuring cylinder and then pour 20cm³ of hydrochloric acid (0.25m) in a separate cylinder.
- Set up the light sensor apparatus. Then place the glass beaker directly over it.
- Now place a thermometer in the beaker and make sure the stop clock is set to start.
- Pour both hydrochloric acid (0.25m) and sodium thiosulphate into the beaker at the same time and record the temperature instantly, whilst starting the clock as well. (It is advised to use more than one to conduct this stage.)
- Once the solution turns cloudy and the light sensor takes a definite reading after decreasing by 10% stop the clock and record the time, also check and record the temperature reading, and the light reading as well.
- Then pour the solution away and reset clock and sensor.
- Repeat this experiment for this concentration one more time, so you should have two results; later use these to gain an average.
- Repeat the above steps for concentrations 0.5, 1.0, 1.25, 1.5, 2.0
Analysis
According to my graph and my line of best fit I can say that the greater the concentration of hydrochloric acid, less time it takes for a reaction to complete when mixed with sodium thiosulphate. From my graph I can see that from the concentration 0.25m to 2.0m there is a decrease in time of approximately 13 seconds. At concentration 0.5m my point should be plotted at about 44 seconds but it is found at 37 seconds. Also the same thing occurs at concentration 1.0m when the line of best fit indicates that the point should be plotted at 41 seconds but is actually at 43 seconds. I believe I can class this plotted points as obvious anomalies.
As you can see the slope decreases as the concentrations rises. Therefore I can say again the more concentrated the hydrochloric acid the quicker sulphur would form and block light to the sensor. If I relate back to my prediction I said that the more concentrated the hydrochloric acid the quicker sulphur would form and block light to the sensor. I feel I can therefore say I was correct. Well if you look at my graph you will realise that my results don’t totally comply with my prediction. At first you can see my results fall, but when it reaches the concentration of 1.0 (M) the results steeply rise and then drop again very slowly.
The line of best fit can be explained as the more concentrated the solution the more particles of the concentrated solution there will be, in this case hydrochloric acid. This sulphur is insoluble and therefore clouds the mixture and blots out the light. This light loss is recorded on the light sensor and is then decided when the reaction is complete. What happens is the hydrochloric acid particles and the sodium thiosulphate particles collide with one another over, but occasionally some of the particles will collide effectively, and with such force that they bond. This collision of particles is what bonds the two solutions until they produce a new bond. That new bond in this case is sulphur. Hydrochloric acid is a molecular compound, whereas sodium thiosulphate is an ionic compound. An ionic compound is a compound whose components are held together by ionic bonding. Ionic bonding occurs when both substances become ions. Ions are charged particles that were once neutral atoms but have either gained or lost their valence electrons to get a full outer shell. The hydrochloric acid and sodium thiosulphate particles will collide with one another. When the sets of particles have this energy, which they get from the light turning into heat energy, they use it to collide with one another to break the ionic bond in sodium thiosulphate. Then once the bond is broken the particle collide to make new bonds. When this happens ions from the breaking of the ionic compound found in sodium thiosulpahte are left floating around freely in the solution. The new bond that is made is in fact sulphur and that sulphur is used to blot out the light to the sensor and the 10% decrease in light intensity is used to determine when the reaction is complete. Making the hydrochloric acid more concentrated does is produce more hydrochloric acid particles in the solution, so when the solution is left to react with sodium thiosulphate it is a lot quicker as there is more chances or a sodium thiosulphate particle colliding with a hydrochloric particle as the is simply more hydrochloric particles in the solution. What this means is the ease in colliding means a saving in energy use, and that saved energy, activation energy, is used to break the old bond and make the new one by making the particles have the energy to collide. To break the bond heat is required (endothermic reaction) once it has it the bond is broken, then two sets of particles collide again to make a new bond (exothermic reaction) and heat is given off. The loss of heat from the reaction is therefore equal to that, that is given in and therefore there is no temperature change.
From my results I can calculate rate of reaction between sodium thiosulphate and hydrochloric acid, its calculation looks like this:
One divided by Average time of reaction = rate of reaction
The unit representation for rate of reaction is 1/s. That basically stands for one over a second.
(all results were gained as averages and rate of reaction was calculate to three decimal place.)
As you can see in my graph the higher the concentration of hydrochloric acid the greater the rate of reaction, although there are some obvious anomalies. This is because rate of reaction will increase if the concentartion of one of the reactants is increased, in this case hydrochloric acid. It simply means that having a high concentration means more hydrochloric acid particles to collide with sodium thiosulphate particles. And they do this so quickly, as there is so many acid particles, that the making, breaking, and creating of new bonds is speed up and therefore the rate of reaction is greater. If I realte back to my prediction I said that the greater the concentrated solution the quicker it will take for the sensor to lose light intensity by 10%. So the less concentrated the solution of hydrochloric acid the slower the thiosulphate solution will precipitate forming sulphur, making it hard for the sensor to absorb light as the sulphur blots attempts to blot out the light rays from reaching the sensor. The undiluted hydrochloric acid will be less reactive as it has a decreased chance of effective collisions; this is because there are less hydrochloric acid particles in the solution decreasing the chances of effective collisions with sodium thiosulphate, as there are not enough particles to produce enough effective collisions for a reaction to occur. You can also see from my graph that the results steadily level off. They start to do so at 1.25m and continue to up to the highest concentration I used which was 2.0m. I feel the reason for this is that at a certain concentration the acid can not become anymore concentrated and that no more acid particles could be produce to effect rate of reaction. I can therefore conclude that my overall theory in my prediction was correct and I can prove so with my graphs, table of results, and explanations.
Evaluation
There are two obvious anomalies in my results. The first is found at 0.5m and the second at 1.0m. I feel the reason for these anomalies are for the way in which the group carried out the experiment. At the end of each mixing of the two solutions we never washed the test tubes, but just poured the mixture away. I didn’t realise this mistake until I reached concentration 1.25m and quickly fixed the situation. This means that there would be left over acid and water particles in the bottom of the tube. So when the next concentrated solution is added it will mix with the spare solution at the bottom. How this effects reaction is there is more acid particles now in the solution than there should be so the rate of reaction will be quicker as there are more acid particles to collide with sodium thiosulphate particles. This increase in collisions means the breaking and making of bonds will be quicker and sulphur would form quicker. If the sulphur is formed quicker then the quicker it will take to bloke out the light from the light sensor, and therefore give an inaccurate result. This default may be the reason for the odd results and anomalies.
In doing the experiment again, I would most definitely clean the apparatus and I believe this could dramatically increase the accuracy and quality of my results. I would also think about altering the concentrations of acid. I would make the concentrations closer together as this would show the results more clearer. Doing this would allow me to see the results trend even clearer and therefore produce an even more accurate line of best fit. I would also make the range of concentrations even bigger as to see if the results would continue to level off. I would also think about carrying the experiment out with other reactants to she if the pattern would be the same.
When looking at my results you notice that they are not of a superior quality. There is minor consistency in the repeats and no real pattern in a series of results. I feel if I was to cancel out the problem of not cleaning the apparatus to late in the experiment then my anomalies would be non-existent. The range of my results seem fine as they prove my theory and show that my prediction was correct, although to include more concentrations in the experiment would be a lot better as to see if the results continue to level off.
If I were going to extend the experiment then I would start by introducing different types of acid to the experiment. I would do this as to see if concentrating different acids would affect rate of reaction and also see if concentrating the different acid have an effect. I would also increase the range of concentration and make the concentrations even more close together. I would do this as to see if the line of best fit could be even more accurate and also see if the results would continue to level off. I would also extend the experiment by seeing how heat affects rate of reaction and see if all acids are affected. I would also like to see how rate of reaction would change depending on altering the strength of the light. I would do this as to see how much heat is produced by different strength light bulbs.
Terry Cave
Edenham High School
Chemistry Coursework
Ms. Cheung