I have done some preliminary work for the investigation – I carried out an experiment to find how the concentration affects the rate of reaction. That helped me to decide how I would carry out this experiment and how to structure my work.
. In this experiment I will be changing the temperature and investigating how this affects the rate of reaction. The rate is a measure of the change that happens in a unit of time. For example how fast or slow Sodium Thiosulphate ( Na2S2O3) reacts with Hydrochloric acid (HCl). The reaction causes a fine yellow substance to be formed called Sulphur, along with Sodium Chloride, Water and Sulphur Dioxide. This can be written as an equation:
Sodium + Hydrochloric Sodium + Sulphur + Sulphur + Water
Thiosulphate Acid Chloride Dioxide
The balanced, chemical equation can be represented like this;
Na2S2O3 (aq) + HCl (l) 2NaCl (aq) + S (s) + SO2 (g) + H2O (l)
My prediction is that the higher the temperature, the quicker the rate of reaction will be.
I think this because in every day life there are examples of how temperature affects the rate of reaction for example in an oven the higher the temperature the quicker the food cooks, and if milk is left on a bench instead of in a refrigerator, the warmer it is the quicker the milk will turn sour.
Variables and constants.
In my experiment I will be keeping the same amount of Sodium Thiosulphate and Hydrochloric acid, I will use the same beaker each time and the same measuring cylinders. I will then rinse these out after each time to ensure nothing is contaminated.
I will be changing the temperature each time, as this is what I am doing my investigation on and it will enable me to see how the temperature and the rate of reaction are proportional to each other.
In the experiment I will be measuring the time taken for the ‘X’ to disappear.
I will make sure that the thermometer isn’t touching the glass because otherwise I would be finding the temperature of the glass and not the solution. During my investigation I will need to consider safety. I will be using a bench mat and gauze, I will use tongs when handling the beaker if it gets too hot and I will use goggles to protect my eyes.
Apparatus: 250 cm ³ Na2S2O3, 50 cm ³ HCl, beaker, thermometer, paper, measuring cylinder, stopwatch, tongs, gauze, tripod, Bunsen burner, safety goggles, stirring rod, bench mat.
Method.
1) Collect your equipment (as shown above) and set it up as shown in the diagram.
2) Use 30 cm ³ of Na2S2O3 and place the beaker on top of the paper with the ‘X’ marked on it. Add the 5 cm ³ of HCl and time how long it takes for the ‘X’ to disappear. Record the time and the accurate temperature.
3) Heat 30 cm ³ of Na2S2O3 until it reaches your next target temperature (approx.) then place the beaker on the paper and add another 5 cm ³ of HCl.
4) Time how long it takes for the ‘X’ to disappear, remember to take the accurate temperature and record your results.
5) Keep repeating this until you get a full set of results, making sure you keep the volume of acid and thio the same.
My results.
Conclusion.
My results show me that as the temperature increases, the rate of reaction increases too. This is shown by the line of best fit, which is a curve, on my graph. On my graph I have rounded the results up to the nearest whole number because the graph paper is not accurate enough to plot decimal places.
At the beginning of my experiment there was a dramatic decrease in the time taken for the ‘x’ to disappear. Then as the temperature increased further, the time taken decreased by less. From the results I can see that there must be a pattern because the graph is a curve however I cannot find a mathematical pattern:
When the temperature is 26 the time taken is 21 seconds, the temperature is then doubled to 52 and the time taken is 6 seconds. This means as the temperature is doubled from 26 to 52 the time taken is almost 1/3 ( 6 is nearly 1/3 of 21). I have looked at my results to see if this pattern is visable in any of the other results however it is not.
The results back up my background information because it proves that the temperature is one of the factors, which affects the rate of reaction. This is because if the reactants are heated they will have more energy, the particles will move faster and collide more so more collisions will be successful. Other factors which affect the rate of reaction are the effect of a catalyst, the surface area of solid reactants, the pressure of gases and the concentration of dissolved reactants (this is what I based my preliminary investigation on).
From my results I have drawn dotted lines up to the curve to find the results I should have according to the curved line. These are also shown in a table, doing this will let me to see what my results should be and if I have any errors in my experiment.
My results match my prediction – my graph shows that as I predicted, the temperature affects the rate of reaction. This means that the 2 are proportional to each other; I can also calculate that there is strong negative correlation between my points on the graph.
Evaluation.
I think that my experiment was quite a good one because we used the same variable throughout the experiment, I used the same sized beaker and measuring cylinder and these were rinsed out each time. I made sure the thermometer wasn’t touching the glass so that we were taking the temperature of the liquid and not the beaker. I think my experiment was carried out safely, using safety equipment such as goggles and a bench mat.
Most of my results were accurate and fitted my line of best fit on the graph , my result for 37° was 10 and should have been 10.5 according to the line of best fit, so the point was only 0.5 out, this occurred for my 52 °result too. There are many reasons why this point was out – such as human error. If the stopwatch wasn’t used correctly, this may cause an error in the result. If the beaker wasn’t rinsed out thoroughly enough this may have caused contamination and lead to a mistake in my results. As I was using a measuring cylinder with a capacity of 25 ml and I needed 30 ml of sodium thiosulphate – I had to measure it out twice and this could easily have made my point slightly of the graph. I think my method was a good but I could improve it by making the instructions slightly clearer although I found it easy to follow.
If I were to do the experiment again I would improve it by taking more results to ensure it was accurate, I would do the experiment twice and then find my average results which would make the experiment better. I would also use a larger measuring cylinder to avoid mistakes whilst measuring out the substances.
I think my experiment was good enough to give the conclusion that temperature affects the rate of reaction between Sodium Thiosulphate and Hydrochloric Acid. This is because my experiment was as fair as possible and I didn’t make many errors.
There are other experiments, which could be done to support my conclusions and to extend the investigation. The temperature, concentration, surface area, pressure of gases and the effect of a catalyst are all other factors which affect the rate of reaction so I could base another experiment on one of these to obtain more evidence and make the investigation more accurate.
