Key factors:
The key factors, which can be changed or controlled to aid my experiment, are:
TEMPERATURE: if the temperature is increased, the particles of Sodium Thiosulphate move more quickly, therefore, there is a larger chance of a collision-taking place. A raise in temperature also gives the particles more energy (making them move more quickly) so there is a higher chance of reactions taking place, because the energy level is more likely to be over that of the activation energy level
CONCENTRATION: the higher the concentration of Sodium Thiosulphate, the lower the concentration of water molecules, so therefore, there is a larger chance of a collision between Sodium Thiosulphate particles, and those of Hydrochloric acid. In a reaction where one or both reactants are in high concentrations, there are more particles, closer together, so therefore they are more likely to collide. This is the variable we are changing because it is the easiest to control.
SURFACE AREA: the larger the surface area of a reactant (irrelevant to this experiment, as we are not using solids), the increased opportunity for there to be a reaction between the solution, and this reactant, because of an increased are of active site, leading to more collisions. A small particle has a large surface area in relation to its volume so more particles are able to collide.
CATALYSTS: if a catalyst is present, it provides a surface for the reacting particles to attach to, producing an area where they can collide, therefore, increasing the number of collisions. A catalyst lowers the amount of energy required for a successful reaction; so more collisions are successful, making the reaction faster. A catalyst enables the reactants to reach their activation energy level faster.
PRESSURE: As we increase the pressure on a gas, we push the particles closer together, making them reach there activation energy level faster, and increasing the change for a reaction to take place.
In this experiment, I will be controlling factors of temperature, surface area, pressure and catalysts. I will keep these constant, by not using a catalyst, or using any solids (in liquids, surface area is not important), and control temperature by carrying out the experiment over one day at room temperature. I will keep the pressure constant by carrying out the experiment on the same day, and room pressure. I will vary the concentration of Sodium Thiosulphate, by adding water, but control the concentration of hydrochloric acid, by using a constant concentration of 1.0M.
Prediction (and explanation):
I think that in the experiment, the higher the concentration of Sodium Thiosulphate, the faster the reaction time between the Sodium Thiosulphate, and Hydrochloric acid. Also in this reaction, the higher the concentration of Sodium Thiosulphate, the faster time in which Sulphur will be formed. This is because the higher the concentration of Sodium Thiosulphate, the lower the concentration of water molecules. A chemical reaction will only occur between particles when they collide, there is a minimum amount of energy required in order to react with each another. Yet if there isn’t enough energy for the reaction to take place, the particles don’t react, and just bounce off one another. This minimum energy level is called the activation energy. This is once a reaction has started – and therefore enough energy has been created in the collision to break the covalent bonds – and energy is therefore released from bond formation (in the products). As bond forming is exothermic, releases heat energy this energy then helps more collisions to become successful. Increasing the concentration of the Sodium Thiosulphate solution also increases the number of particles per dm3 of substance, this therefore leads to (seeing as there are more particles) more collisions between Sodium Thiosulphate particles and Hydrochloric acid particles, making the reaction faster. I will keep the practical a fair test by only changing one variable, and keeping the others constant.
Fair test
To make the practical a fair test, I am only varying the molarity of Sodium Thiosulphate solution, and keeping the molarity of Hydrochloric acid the same. I must also use the same volume of Hydrochloric acid as Sodium thiosulphate (25 ml). In the experiment, I must also control the use of temperature, surface area (in experiment pressure), and the use of catalysts. I will control this by only varying the concentration of one reactant (sodium thiosulphate) and by controlling everything else. I will control temperature by carrying out the experiment over one day, at room temperature. Catalysts will be controlled by not using any. Carrying out the experiment on the same day will control the pressure of the liquids. If these weren’t controlled, they would make the experiment un-reliable. The experiment would become un-reliable because each of the variables would be acting differently with each other, thus, therefore, making the results inaccurate.
Safety
To keep the practical safe, we must wear goggles, tie long hair back, move bags and other personal belongings out of the way, along with putting stools under the table; personal belongings need to be out of the way so we have a clear, and safe working area. We need to also be careful of spillage’s, as we are working with acids, which can be corrosive.
We must wear safety goggles in case there are any splashes from the beakers when we are mixing the two reactants, because of working with acid’s and alkalis.
Equipment:
Method:
- Check Safety.
- Draw a black cross onto the white tile with a bold marker pen and put the glass beaker on top so the cross is visible through the base.
- Carefully measure out the concentrations to make up 25 ml of sodium thiosulphate using a pipette and put it into a measuring cylinder. Read from the bottom of the meniscus when measuring every time so that it is a fair test. If we read from different points on the meniscus we will get a different volume each time. Keep the same person measuring during every experiment because different people's opinions on where to read the measurement will differ.
- Carefully measure out 25 ml of hydrochloric acid into a different measuring cylinder (so the two reactants don't react together) and put it into the glass beaker.
- With the stopwatch checked and ready to start, pour the sodium thiosulphate into the beaker.
- As soon as the two reactants touch, start the stopwatch and time how long it takes for the cross on the tile to be no longer visible through the base of the beaker. Keep the same person timing every experiment, as different people will have different reaction times, this would therefore make the experiment un-fair (see fair test).
- Record the results in the appropriate part of the table.
- Clean the equipment for the next results; make sure that they are washed thoroughly as there could be remnants left in it that affect the next set of results. This is because it would mean that the concentration would be changed in the following experiment, this would lead to a concentration increase/ decrease depending on the remaining substance (if it was water it would decrease the concentration, if it was Sodium thiosulphate, it would increase the concentration).
- Do the experiment again with different concentrations of sodium thiosulphate. These concentrations are:
- Repeat each concentration 3 times for accuracy, if you find an anomalous result redo that test again. We can tell if a result is anomalous because it would stand out from the other results as being a lot higher or a lot less than the other results in that series.
Results:
Anomalous results:
In the practical, we didn’t have any anomalous results. If we had have done, we would have retaken the results, and if this produced similar results, I would have checked the equipment. It would have been obvious if I had have achieve an anomalous result, because it would have been either much larger, or smaller then the previous set of results, or of that in that set of results.
Accuracy:
I used accurate equipment such as pipettes, measuring cylinders, and stopwatches. The pipettes and measuring cylinders were accurate to 0.5 ml, whilst the stopwatch was accurate to one hundredth of a second. For all of my results, I had too round up or down to the nearest tenth of a second because the reaction times are not accurate enough (too slow) to include hundredths of a second. Not including hundredths of seconds makes my results easier to plot on my graph. When we were reading results, one person read the meniscuses so the margin of human error was similar each time, and at the same point (meniscus – the bottom of the curve). The other timed using the stopwatch, making the results reliable because this made the margin of human error similar each time. We timed each result from first contact of the sodium thiosulphate and Hydrochloric acid solutions until the cross was no longer visible through the base of the beaker.
What the graph shows us:
From my graph, I can see that as we increased the concentration of Sodium Thiosulphate, we found that the time for the cross on the tile not to be visible through the base of the beaker decreased. The graph shows us that the higher the concentration of Sodium thiosulphate, the faster the reaction time between the 2 solutions. This is because the higher the concentration of Sodium Thiosulphate, the larger amounts of particles in which there are, leading to more collisions. The higher the concentration of sodium thiosulphate, the quicker time which it took for the cross not to be visible through the base of the beaker, and for the reaction to have taken place. As the concentration of sodium thiosulphate increased, the time for cross to not be visible through the base of the breaker became quicker. This was because of the higher concentration of Sodium Thiosulphate was, the more likely it was for there to be a collision between particles.
From my results, I have found out that my prediction was correct, and that my theory behind it was also correct. I have found that as I increase the concentration of sodium thiosulphate, the time for the cross to become non-viewable decreases. This happens because the solutions contain more particles, and more collisions occur, which are needed for the reaction to work (energy also has to be over that of the activation energy level). The energy level has too be over the activation energy because otherwise, there will be less frequent collisions, and the reaction will happen slower
Evaluation:
I feel that the method, which I used to collect my results, was a good one due to the fact that my results didn’t have any anomalies. If I had have had any anomalies, it would have been obvious, because it would have been either much larger, or smaller then the previous set of results, or of that in that set of results. Other evidence in which I have to say that my practical was a success, is the table of percentage differences, they are below the 18% difference which we expected. The method, which I used, was easy to follow, with exact directions, which I feel lead to my results being accurate. I think that due to my good method, I also achieved good results, which are reliable. I achieved these reliable results by following my method exactly, and measuring everything correctly e.g. reading meniscuses from the bottom of the curve, and by using accurate measuring equipment such as measuring cylinders accurate to 0.5 ml, also by the same people reading different pieces of equipment. Accurate results were also achieved by averaging the results for each of the concentrations, so as to give us a mean result, and so a better line of best fit could be produced.
If I could change anything in the future, I would have let the practical take place in a temperature-controlled environment, so, as this had no effect on the reaction between the Sodium Thiosulphate particles and those of hydrochloric acid. I feel that my results are accurate because my graph has produced a smooth curve, which describes an accurate reaction. I made sure the practical was accurate by only varying the molarity of Sodium Thiosulphate solution, and keeping the molarity of Hydrochloric acid the same. I also used the same volume of Hydrochloric acid as Sodium thiosulphate (25 ml). In the experiment, I controlled the use of temperature, surface area (in experiment pressure), and the use of catalysts. I controlled this by only varying the concentration of one reactant (sodium thiosulphate) and by controlling everything else. Carrying out the experiment over one day, at room temperature controlled the temperature. I controlled catalysts by not using any. As I carried out the experiment on the same day I successfully controlled the pressure of the liquids. We calculated averages when we had collected each set of results because this gave us a mean point to plot on our graph, and a better line of best fit, and reduces inaccuracies. If I was to complete this experiment again, I would use the same method, but use more accurate measuring equipment, for example, titration to make and check the accuracy of concentrations of Sodium Thiosulphate.
If this experiment was completed mechanically, it would make it more accurate. The solutions of Sodium Thiosulphate could be mixed more accurately, and as soon as the angle of a measuring cylinder containing the hydrochloric acid was over a certain angle, a stopwatch would begin timing. A light detector ray could be passed through the glass beaker onto a white background, if the ray wasn’t detectable by the computer, the stopwatch would be immediately stopped. If this happened, we could use hundredths of seconds because the level of accuracy would be higher, thus making the practical be more accurate and giving us better results.
