Prediction
I predict that the rate of reaction will increase as the temperature increases. As the independent variable (the temperature) increases the dependent variable (the cross) will take less time to disappear. I think this because as solutions are given more heat the particles are supplied with more energy, causing more and harder collisions. Therefore if the temperature doubles, the rate of reaction more than doubles as more successful collisions take place.
In my experiment when the sodium thiosulphate solution is heated up the particles will contain more energy. Thus, when the hydrochloric acid is added to the solution there will be a quicker reaction because the number of collisions possible has increased. The cross disappears because the reaction turns the end product (sulphur) cloudy.
Quantitative prediction
I predict that if I double the temperature, then the rate of reaction will more than double because not only would the particles be moving faster to create more collisions, but they will also have more energy so there will be more successful collisions.
Scientific Explanation:
For a chemical reaction to happen, the reacting particles must bang into, or collide, with each other with enough force or energy (the activation energy) to break bonds. This is collision theory. If the particles do not have this energy, they will just harmlessly bounce off one another. Most reactions go faster at higher temperatures. This is because the reacting particles have more energy and move faster. They are more likely to bump into one another with enough energy to cause a reaction.
Sketch of graph
Preliminary work
For my preliminary work I am going to find the best concentration of sodium thiosulphate and water to use for my investigation.
I am going to keep the temperature at room temperature (21°) and only change the volume of thiosulphate and water. I predict that the solution at which the rate of reaction will be the quickest will be when the solution is 40ml of thiosulphate and 10ml of water.
Method:
Measure out 40cm3 of sodium thiosulphate solution and 10cm3. of water into a flask and place it on the cross. A soon as you put the acid in start the clock. Watch the solution until it goes cloudy and stop the clock when you are unable to see the cross anymore. Repeat this each time for the concentrations shown in the table below.
The apparatus will only consist of:
- Flask containing thiosulphate and acid solution.
- Thermometer
- Paper with cross drawn on
- Measuring cylinders
- Stop Clock
For this is I will not need a Bunsen burner because the temperature of the experiment is staying at room temperature.
Results:
Conclusion:
I have now worked out that the quickest solution for the cross to disappear in adding water is sodium thiosulphate 40ml and water 10ml. These will be the figures I use in my final experiment.
For my final experiment I am going to keep the volume of sodium thiosulphate at 10ml, the volume of water at 40ml and the volume of acid at 5ml. I am going to start at room temperature and go up in tens (e.g. 20°C, 30°C, 40°C etc.) until I get to 60°C where I will finish. I will take each reading three times and then work out the average to make sure my results are as accurate as possible and allowing for any anomalous points.
Reliability
I am going to do the experiment in a range from 20ºc - 60ºc. I will use 10 minute intervals such as 20ºc, 30ºc, 40ºc etc. I will repeat the experiment 3 times and keep all the variables the same and constant. I can then take an average of all my results.
A diagram and list of the apparatus
- Conical flask
- Thermometer
- Sodium thiosulphate
- Water
- Hydrochloric acid
- Gauze
- Heat resistant mat
- Bunsen burner
- Measuring cylinder
- A pencil or pen to mark cross
- Scrap paper no bigger than the conical flask
- Tripod
- Stopwatch
- Tongs
- Safety goggles
Method
In my experiment I will first collect my apparatus: measuring cylinders, tripod, etc (refer to equipment list) and set it up as in the diagram on the previous page.
1). Set up the apparatus
2). Measure out 10cmз of sodium thiosulphate and 40cmз of water. Pour both of these into a conical flask
3). Find the temperature of this mixture (should be around room temperature) and record it.
4). Place your conical flask on the cross.
5). Measure out 5cmз of dilute hydrochloric acid.
6). Pour the acid into the conical flask and swirl the water around for 5 seconds, then start the stopwatch immediately.
7). When the cross is not visible to the naked eye stop the stopwatch and record the time, repeat the experiment 2 more times.
Do the experiment again but increase the temperature by 10ºc
After you have followed the first method proceed with the second one
1). Place the conical flask on the gauze mat, which is placed on a tripod. Place the thermometer carefully on the beaker.
2). Heat up the sodium with a bunsen burner and wait until the temperature is at the right degree for your experiment.
3). Take off your conical flask and add the acid.
Again carry on with the first method
Repeat this experiment 3 times for each temperature. Each time recording your results.
Safety precautions
In my experiment I will try carry out everything as safely as possible. This will include:
Not running –
There will be people walking around with chemicals and hot liquids and glassware. You could fall and hurt yourself and/or others. You could run into or knock someone carrying hot materials or chemicals and as a result they could end up spilling them on you or themselves.
Wipe up spills
– If you spill something it must be cleaned up quickly and appropriately. Someone could slip on chemicals, which had been spilled on the floor and hurt themselves.
Wear goggles
– You must wear goggles at all times when heating chemicals in case they spit or splash out of the test tubes. They may get into your eyes and blind you.
Acids are corrosive
– This is basically the same as wiping up spills but this also involves carrying and handling the chemicals. Be careful not to get it onto your skin.
Report breakages
– You must tell the teacher if you break any apparatus including glassware. Someone may not notice it and cut themselves on it.
Use tongs when carrying hot glassware
– Tongs must be used or else you burn yourself and/or drop some apparatus containing chemicals
Results table
Conclusion
From my results table (above) you can see that as the temperature increased, the reaction got quicker and it took less time for the cross to fade. Raising the temperature by 10°C each time more than doubles the rate of reaction. My graphs were curves rather than linear. Due to giving the particles more energy the sulphur was released quicker and in larger doses, which turned the solution cloudy thus making the cross invisible.
Scientific Knowledge
I have found out that as the temperature increases, so does the rate of reaction. Raising the temperature makes the particles move faster. This means that more particles collide with each other per second. Also, the faster the particles are travelling, the greater is the proportion of them, which will have the required activation energy for a reaction to occur.
My prediction hypothesis was almost exact to the outcome of my experiment. As the temperature got higher, the rate of reaction increased. I predicted that the temperature would more than double and nearly treble. An example of this is at 20°c my rate of reaction was 0.005 then at double the temperature, 40°c the rate of reaction was 0.015, this has exactly trebled.
In taking the readings I have been as accurate as I can. Starting the timer at exactly the same time as the acid mixes with the solution was quite challenging. However, as I was near enough my results did not suffer. Another challenging aspect was when you had to determine if you could see the cross become invisible through the sulphur. As everyone taking part in the experiments has different visions this may have affected the times. Although, as I was the only one having to determine whether the cross was visible or not in my experiment then that would have been a plus to the fair test.
The results that I predicted for my graph were also extremely close. The plotted points on which I had to draw a line of best fit were in a perfect curve shape. I had very little problems with anomalous points in my experiment. My only query would be on the 20°c experiment. Where my results came out as 218, 201, 170. The 170 result was slightly out but I overcame this by taking my average. This may have happened due to a wrong amount of reactants or a wrong judging of time taken. This was the only anomalous point I had during my experiment.
I think I have an ample amount of results to support my hypothesis. The fact that I repeated the experiment 3 times at each temperature should add that extra evidence. I have put together a wide range of scientific explanations and results. My evidence fully supports my prediction and conclusion. Also the fact that my experiment relates with the scientific explanation proves that my method was correct.
To improve my experiment I maybe would have worked in groups. This would help because as soon as I poured in the acid to the solution another member of the group could start the stop clock. Also an extra pair of eyes in deciding when the cross was invisible would have been useful.
My experiment could only be started at room temperature because i did not have any refrigerating equipment to make the solution colder than this temperature. If I wanted to get a bigger range of results then I could have used a refrigerator to cool down the solution and started taking results at 0°C or 10°C and carried on up to 100°C. I did not carry on up to 100°C in my experiment because I did not have enough time to do so. To make my experiment more comprehensive, I could have gone up in fives instead of tens (e.g. 20°C, 25°C, 30°C, 35°C etc.). This would mean I had more results, which is never a bad thing. I do not think it would be necessary to repeat the experiment more than three times at each temperature because the average out of three is pretty accurate.
If I were to repeat my experiment I would have used a wider range to see if the graph eventually levelled off into a horizontal line. Also when getting the solution up to the right temperature I would have used a different method. I would have a bath filled with water; I would get that bath water up to the right temperature and then place the conical flask into it, wait for the solution to get up to temperature and then add the acid. This is because whilst the reaction is taking place with the Bunsen burner method heat can be lost therefore the results are not as accurate.
