I believe that the graph will look like this:
Time (secs)
Concentration of acid (cm³)
The graph will be a downward curve. This is because the lower the concentration of acid, the longer it will take. Also, I believe the higher the concentration, the shorter the time. If I plot the points in accordance to my hypothesis, it will form the aforementioned downward curve.
Plan: The apparatus I will use for this experiment will be as follows
- Test Tube.
- Sodium thiosulphate (constant concentration).
- Hydrochloric acid (different concentrations).
- Water.
- Stop watch.
- Paper with black cross on it.
- Three measuring cylinders for the thiosulphate, water and hydrochloric acid.
- Test tube rack.
Firstly, I will put 10cm³ of hydrochloric acid in a test tube (measuring the hydrochloric acid in a measuring cylinder), and then will put the test tube with the black mark on a test tube rack. I will add 20cm³ of sodium thiosulphate to the hydrochloric acid, and at this time I will not need to add any water. Immediately, I will start the stopwatch.
I will wait for the black cross to be completely obscured by the mixture. When this happens I stop the stopwatch. By this time the mixture will be completely cloudy. Then I will record what the time was.
I will empty out the contents of the test tube, and clean it out, and dry it will great care. Next, I will add the 9cm³ of hydrochloric acid; I will keep the sodium thiosulphate acid constantly at 20cm³. Now, because I have to keep the volume the same for it to be a fair test, I will add 1cm³ of water. (I have to add the hydrochloric acid and the water at the same time for this to be a fair test also). I will record the time for the cross to become completely obscured.
I will repeat this many times, adding different amounts (for exact measurements, see the table later) of hydrochloric acid and water each time, and recording the time taken each time. I will repeat the test twice more so I can take the average reading and eject any results that I deem unfair or just plain wrong, making the results more credible.
Diagram:
Fair Test: To make this experiment a fair test, I need to make sure I do a number of things.
In this experiment we are trying to find the rate of reaction using concentration as a factor, so there is a number of things we need to make sure we do to keep it a fair test.
- I will need to measure the liquids very accurately as a slight difference in a measurement of liquid could make a big difference to the results I obtain.
- I need to keep a chemical at a constant concentration. So, in this experiment I have chose to keep sodium thiosulphate at a constant concentration. We could have, however, used hydrochloric acid as a constant, but we had chose to use sodium thiosulphate.
- I must make sure that the solution is kept at a constant volume throughout the experiment. If the volume is different, then it could give different results to if it was at a constant volume.
- I will also make sure that we add both the water and the hydrochloric acid at exactly the same time (into the test tube with the sodium thiosulphate in it), or it could affect the results of the experiment.
- I must start the stopwatch at the exact time as we put the water and the sodium thiosulphate into the conical flask. To do this it is a lot easier if there are two people doing the experiment, so one person can put the two substances in the sodium thiosulphate, and one person can start the stopwatch.
- I must do is to make sure that the test tube is completely clean and free of any water or any other substance before we attempt to start the next experiment because the test tube if not cleaned, may contain more acid and will either increase or decrease the concentration of the solution, making the results inaccurate and unreliable.
- Temperature affects the rate of reaction, so I will conduct the experiment at room temperature. If I did the experiment outside, the heat will cause the reaction to go faster, and will not be because of the concentration.
- I will be the one timing the experiments because if we kept changing the timekeeper, there will be differences in reaction times, opinions on how obscure the black cross is and general concentration. If these points are not taken into account, results will be come very inconsistent and I would have to deem the results inaccurate.
Safety Precautions: There are a lot of safety issues I must abide by in this experiment also.
- I must remember that the substances that we use in this experiment can be very harmful if used the wrong way.
- When I do this experiment, it may be necessary to wear safety goggles, as things are very unpredictable, and even though it is very unlikely that the solution would come out of the test tube during the experiment, one must still be cautious of spills.
- I must make sure that coats and bags are all out of the way while doing the experiment. Ties and hair should be tucked out of the way, so they do not make contact with any of the chemicals.
- We should also try our best not to spill any chemicals, as these substances can stain clothing and other upholstery
- I must not eat or drink in the lab while dealing with these harmful chemicals, if I handle chemicals without washing my hand afterwards, and then handle food, I could inadvertently swallow some of the acid on my hand, and I might fall ill.
- Plasters should cover any open wounds that I may have, as any chemical spillage on the wound can cause me immense pain and possible infection of it.
-
If any chemical is spilt onto any part of the body, most notably the hands, I must make sure that I clean them afterwards thoroughly as any contact with acid-covered hands to my eyes or my mouth will cause great and extensive injury.
- Sulphur dioxide is an irritant, so I will have to make sure that the work area is well ventilated.
I will use about five different concentrations:
I think these concentrations will give some interesting results I can interpret into a graph. I have not chosen to do totally dilute liquids, as the reaction will be either too quick, or too slow to time. I will repeat the experiment because this will add credibility to my results and I can overrule any results that I consider faulty.
I will now test some preliminary results to help me define the concentrations I will use, and whether they produce good enough results for my graph.
Preliminary Results: Below are the table of the results I obtained from the preliminaries, and the results from the repeat experiment. I will state below whether I will think about my procedure method
As you can see, there is one very noticeable anomalous result, which I think was my fault in that I didn’t clean the test tube as well as I did before, and there were a few drops of water, which diluted the liquid enough to produce an awkward result. Both sets of the results followed my hypothesis that the more concentrated the result, the quicker the black cross will disappear. For the real experiment, I will up the concentrations from five to ten, to give a more accurate reading and hopefully add extra backing to my hypothesis.
In order to create a reliable graph for the real thing, I will need to calculate the average of the two results for each concentration. I will use the mean average to calculate this, as the mode requires two or more results to be the same, and there needs to be more three or more results to be able to find out the median. In any case of a result having more than two significant figures, I’ll round them up to the nearest tenth. This will happen when the third significant figure is between five and ten inclusive, if not, I’ll round down. On the axis, I will convert the minutes to seconds, so that it will be more convenient when plotting the points.
Results
Conclusion: With increasing concentration, the rate of a chemical reaction progressively increases.
The time taken for the cross to disappear relies on the concentration of the hydrochloric acid.
This means that the acid to water ratio is bigger. If there is more acid to water in the solution then this means that there are more hydrochloric molecules in it.
The fact that there are more hydrochloric acid molecules colliding with water molecules means that collisions between the sodium thiosulphate molecules are more likely to occur.
My results link in very well with my prediction because I predicted that when I increased the concentration hydrochloric acid into the flask with the sodium thiosulphate, the cross will disappear a lot faster than if I added more water. By looking at my results on the graph I believe that my results do agree with my prediction, as all of my results help to form a downward curve.
Evaluation: I think my results were reasonably accurate as I did follow closely the fair test instructions, except for the once which was an isolated incident. All precautions were taken to ensure it was a safe test and I doubt very highly that my experiment was contaminated in any other way. On my graph, the points on it were quite close to the line of best fit, with one exception, but it didn’t ruin the graph, as it was not way off the mark.
My results were reliable because, as I said above, I followed each and every step for a fair test, and the safety precautions. To make my theory credible, I tested them again and they were very close with each other, showing that I do indeed have credible, reliable and above all, accurate results.
There was one anomalous result, which I explained was an isolated incident. The anomalous result was because I was rushing when cleaning the test tube because I was running out of time. As I didn’t dry it thoroughly, there was a few drops left over, and this diluted the impending solution, which I think, contributed to the surprising result. I re-tested that concentration after drying the test tube very thoroughly, and it produced a very accurate result, which I used for the graphs instead of the anomalous result. When I figured out that this was a dodgy result, it made me take extra care when cleaning the apparatus, and this helped me produce some very plausible results.
I think I did have more than enough evidence to reach my firm conclusion. My first test and the retest were very closely linked result-wise; the points on my graph were fairly close to the line of best fit and this proved my conclusion had enough to back it up.
To improve my method, I would most definitely get a second person to help me out. Although I explained the pitfalls of having another person helping you, it would have made the experiment so much easier, and not as rushed (the rushing contributed to my anomalous result). Also, I think having a second person would be helpful in the timing aspect of the experiment, as I found it difficult to look at the test tube while fumbling for the ‘stop’ button on the stopwatch. I would have also prepared five different test tubes to use, instead of frantically cleaning one each time after use. In the science lab I was using, the sodium thiosulphate and the hydrochloric acid were constantly being spilt, and in some cases, were in contact with each other, this meant that the pair reacted before entering the solution. I would have set them very far apart i.e. one at each end of the classroom. Finally, I would have asked for a white scientist’s coat, or something like that to wear, because fibres from my clothes might have contaminated the solution. Though unlikely, and didn’t affect my experiment, it could ruin someone else’s experiment.
Next time, I would also make sure that the artificial light maintains the same intensity, as the higher the intensity, the hotter the air around it become. Since temperature affects the rate of reaction, it could affect my experiment. I will also make sure that my eyes are the same distance from the test tube and the black cross, because the closer you are to the black cross, the better you can see it. For each experiment, if I keep changing the distance between the black cross, and myself the times aren’t going to be consistent. I will stay at a constant distance so that my observations of the black cross are constant.
For further work, I could have made the solution more concentrated. For instance I could have the very concentrated solution, where there is no water in the solution. In the other extreme, I could make the solution totally diluted, but I think that could have tested my patience. I think that I could have diluted the mixture next, by adding more water each time than hydrochloric acid. This will test anyone’s prediction and could lead to some interesting results.
