Key
S.T = Sodium Thiosulphate solution
Conc. = Concentration
% Conc. = Percentage concentration of sodium thiosulphate
Acid = Hydrochloric Acid
(ml) = volume in millilitres
(g/l) = grams per litre
(mins) = minutes
(Secs) =seconds
I decided to show a graph of my results:
From my graph, I have decided that I will use a concentration on 50%. This is because the reaction speed will be neither too fast nor too slow.
From my textbook, I know that reaction rate approximately doubles for every 10 degrees centigrade it increases in temperature. Therefore, I think that a 50% concentration will be appropriate for this experiment, as I want a good range of results to prove the task.
Now that I have decided to use 50% concentration, I will do my preliminary experiment so that I can test the range of temperatures.
A table to show how the temperature can affect the rate of reaction between sodium thiosulphate and hydrochloric acid.
Table to show how temperature can affect the rate of reaction between sodium thiosulphate and hydrochloric acid
My graph shows that as the temperature increases, the time the diluted sodium thiosulphate takes to react decreases. However, I do not think that I have a wide enough results. Therefore, in my main experiment I will take down more temperatures.
Apparatus
Digital timer Conical flask
Heat proof mat Bunsen burner
Bag of ice Gauze
Sodium thiosulphate Tripod
3 measuring cylinders Water
Laminated cross beaker
Hydrochloric acid goggles
Diagram
Method
Because it is my preliminary experiment, I have decided to take three different temperatures, 20, 40 and 60 degrees Celsius. If there is a constant increase, I will know that my range is enough to use in my final experiment.
Using my measuring cylinders, I will measure out 25ml for each solution. (Water, thiosulphate, and hydrochloric acid). Then I will place my 25ml of sodium thiosulphate with my 25ml of water.
At the same time I will set up my Bunsen burner, tripod, gauze and heat proof mat as I did in “2” in my diagram. Then I will place the hydrochloric acid into the conical flask with a thermometer and heat until I get to the temperature required. As the room temperature is 27 degrees Celsius and one of my temperatures will be 20 degrees Celsius, I will not be able to heat it. I will simply place the beaker with the diluted sodium thiosulphate on top of a bag of ice to cool it down.
In my preliminary experiment I found that it was quite hard to get an exact temperature, so in my final experiment I will record a desired temperature and an actual temperature to make the results more accurate by having an average temperature.
Once I have reached the required temperature, I will place the conical flash containing sodium thiosulphate onto the laminated cross. As soon as I have added the 25 ml of hydrochloric acid at room temperature I will start the timer, and then I will mix them together a by turning the flash so that as many collisions can take place as possible.
As his is happening I shall watch from above and stop the timer at the point when I can no longer see the cross. The results from the timer will be recorded.
Precautions
To prove that my results are correct, I will need to make a couple repeat readings to get an average temperature and an average time and to prove that each set of readings are correct.
Because I am dealing with chemicals, and fire, I must make sure that I wear my laboratory coat and safety goggles, and ensure that my hair is tied back because acids are corrosive and fire will burn.
I must also make sure that my heatproof mat is always under then Bunsen burner as it may get hot and burn the worktop.
Fair Test
To make sure that the test is fair, I must use the same thermometer; as if it is slightly different to another one then it will be constant all through the experiment. I must make sure that I mix the diluted sodium thiosulphate and the hydrochloric acid each time that I add them together.
I must make sure that all of my solutions are measured properly; so that they do not differ from experiment to experiment, otherwise my test will not be fair.
Sources of error
Each time I press the timer, my reaction time may be different, causing errors of a few seconds in the reaction time.
The room temperature could decrease or increase during the experiment, and so the temperature of the acid will vary with the room temperature.
Error could be made whilst measuring the amount of solution, and amounts could be left in the bottom of the measuring cylinder, or amounts could evaporate whilst heating.
O=Obtaining Evidence
Procedure
The experiment was carried out according to the method in the plan, and using all the precautions mentioned in the plan to reduce experimental error.
I changed my range of temperatures from my preliminary experiment, which was 20 – 60, going up in twenties, to the same range but going up in tens, so I had a wider range of results to prove my aim.
In doing the experiment, I made sure that I wore my laboratory coat, safety goggles, and tied my hair back.
When the Bunsen flame was not in use, I made sure that it was on yellow, so that it was easier to see than the clearer, blue flame.
Once I had acquired the desired temperature on heating the sodium thiosulphate, I was careful to make sure that the beaker was cool enough to handle.
Results
Now that I have five sets of readings, and two extra repeat readings, I can see a clear trend.
As the temperature increases,
Analysing.
I found out that in the experiment, as the temperature and concentration is increased, the time taken for the reaction to take place decreases. This means that the rate of reaction increases as it takes less time for a reaction to take collisions take place per second.
In the experiment the time taken for a reaction to take place decreased by roughly 10 to 15 seconds for every 10°C, increase in temperature, with no anomalies.
Therefore, my prediction in my plan was correct;
I proved that when two chemicals react, their molecules have to collide with each other with sufficient energy for the reaction to take place. Using the graphs, with lines of best fit, I can draw a conclusion from my experiment. Firstly I can see that with my graph which I have used with average time against average temperature, the correlation is negative, as well as showing a steep curve. This proves that time decreases as the temperature increases, and because of the constant drop, it is a constant process. Therefore the molecules will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully
Evaluation
My experiment went according to plan but there were flaws in it. For example, I only obtained three sets of results and then worked out an average. I could have done more tests to gain a more reliable average.
I think there is also a human error factor involved when you are measuring liquids and looking for an end point in the reaction. Although the reaction I chose had a fairly definite end point it was still hard to tell whether the whole cross had disappeared or not. Instead of using a cross, a light beam could be used and when the beam goes out that is the end point. A better standard of measuring cylinders and pipettes could be used.
One result was not really an anomaly as it still followed the pattern but was slightly different to the other result of the other test. When the average temperature was 29.5 Degrees Centigrade and the average time was 48 seconds. This could be because of the reaction time it between realising that the cross was covered, and pressing the stop watch, or because I had a partner, and if one watched the cross throughout the experiment, and then the other one of us recorded this result, we could have had different ideas of when the cross was no longer visible. Using my line of best fit I worked out that the result I was aiming for was 44 °C.
If I had used the following equipment then my results may have been more accurate but these factors were beyond my control: A water-bath could have been used to programme the liquids to the desired temperature. This is much more accurate than a thermometer reading. The plastic measuring cylinders we used to measure out our liquids are mass-produced and so are not 100 % accurate. A glass pipette should have been used as they are hand made and much more accurate. In addition, glass is clearer than plastic so you can take a reading of the liquids easier and more accurately. The substances we used were made from stock solutions, which were, made fresh every lesson therefore the concentration of these substances changed slightly from day to day during the experiment. The equipment was rinsed with tap water, which contains many impurities, which may have contaminated or changed the concentration of the liquids. Distilled water should have been used as it is pure and has no impurities but once again, I did not have the time to keep distilling more and more water just to rinse my equipment. I performed my experiment over two different lessons. This may have cause errors such as the rooms temperature may have varied, the exact concentrations of the acid and sodium thiosulphate may have changed, and I used different apparatus, which may have slightly varied if they had been made my different manufacturers.
