My Plan for Investigating the Effect of Concentration Change
I shall use 50 cm3 of sodium thiosulphate solution in each experiment and mix it with 10 cm3 of diluted hydrochloric acid. I will change the concentration of the sodium thiosulphate solution by diluting the given solution with a known amount of deionised water. I checked possible concentrations of sodium thiosulphate by doing a preliminary experiment to see how long each test would take. I first used 50 cm3 of undiluted sodium thiosulphate solution. It took 22 seconds – long enough to measure accurately. I then tried mixing 10 cm3 of the original sodium thiosulphate solution with 40 cm3 of deionised water (to make my 50 cm3 of sodium thiosulphate solution). It took 150 seconds – not too long, so I could dilute the solution this much.
Apparatus
- Conical flask
- 3 beakers
- 3 burettes
- Stopwatch
- Paper marked with black cross
Safety Precautions
- Put on safety glasses before anyone starts an experiment.
- Sulphur dioxide will be given off during the reaction. This gas is poisonous so do not to breathe it in.
- Open the laboratory windows before conducting the experiment
- Do not run in the laboratory.
- Do not eat or drink in the laboratory.
Method
We drew up a raw results table like this:
We collected some sodium thiosulphate, hydrochloric acid and deionised water in our three beakers. We then laid out the cross on the bench and put our conical flask on top of it. For the first experiment, we measured (by burette for accuracy) 50cm3 of the original sodium thiosulphate solution. Then we measured out 10 cm3 of hydrochloric acid. We put these liquids into the flask making sure to add the hydrochloric acid last. Before we put in the hydrochloric acid we checked the temperature using a thermometer. As soon as we inserted the hydrochloric acid, we pressed the start button on our stopwatches. We looked through the liquid and waited until the cross underneath was no longer visible. As soon as this happened, we pressed the stop button and recorded our times. We rinsed our flask and then repeated the process two more times. We then went on to get results for the other concentrations of sodium thiosulphate.
Variable 2: Temperature
I shall investigate the effect of changing the concentration of sodium thiosulphate. I shall keep the volume of the original solution of sodium thiosulphate constant (10 cm3) and I shall use the same concentration of acid in each test. The reason I shall keep the volume of the original solution of sodium thiosulphate at 10 cm3 is I need the times to be long enough to measure.
I expect that if I increase the temperature I will increase the rate. I also expect that increasing the temperature by 10°C will double the rate (halve the reaction time).
This should be the case because the reaction can only occur if the sodium thiosulphate particles collide with the hydrochloric acid particles with enough energy to react. As the temperature of the reaction is increased, the particles have more energy and so move about faster so there are more collisions per second. The major way that temperature affects the rate of reaction is by giving the particles more energy so that when they do collide they have more chance of colliding with enough force to react. This increases the number of successful collisions and so increases the rate of reaction.
Keeping the concentration of the sodium thiosulphate solution constant is very constant is very important because the concentration has a big effect upon rate. Increasing the concentration of the sodium thiosulphate means there are more sodium thiosulphate particles so there more collisions per second. This increases the number of successful collisions and so increases the rate. Because the concentration of the sodium thiosulphate is so important, we shall measure the volume of the sodium thiosulphate solution very carefully using a burette.
My Plan for Investigating the Effect of Temperature
I shall use 50 cm3 of sodium thiosulphate solution in each experiment and mix it with 10 cm3 of diluted hydrochloric acid. I will change the temperature of the reaction by conducting the experiment in a thermostat. To make sure the thermostat is working properly I will check the temperature from time to time with a thermometer. I will use 10 cm3 of undiluted sodium thiosulphate solution and 40 cm3 of water to give times that are long enough to measure.
Apparatus
- Conical flask
- 3 beakers
- 3 burettes
- Stopwatch
- Paper marked with black cross
- Thermometer
- Thermostat
Safety Precautions
- Put on safety glasses before anyone starts an experiment.
- Sulphur dioxide will be given off during the reaction. This gas is poisonous so do not to breathe it in.
- Open the laboratory windows before conducting the experiment
- Do not run in the laboratory.
- Do not eat or drink in the laboratory.
- Make sure you do not spill any of the water from the thermostat, as it may be dangerously hot.
Method
We drew up a results table like this:
We will collect some sodium thiosulphate, hydrochloric acid and deionised water in our three beakers. We will then lay out the cross on the bench and put our conical flask on top of it. For the first experiment, we will conduct it at room temperature. We measured out 10 cm3 of the sodium thiosulphate solution, 40 cm3 of deionised water and 10 cm3 of hydrochloric acid. We put these liquids into the flask making sure to add the hydrochloric acid last. Before we added the hydrochloric acid we checked the temperature of the solution using a thermometer. As soon as we inserted the hydrochloric acid, we pressed the start button on our stopwatches. We looked through the liquid and waited until the cross underneath was no longer visible. As soon as this happened, we pressed the stop button and recorded our times. We rinsed our flask and then repeated the process two more times. We then went on to get results for the other temperatures using a thermostat.
Results
Analysing
My graph has a very strong positive correlation. This shows that concentrated sodium thiosulphate reacts faster than dilute sodium thiosulphate. I can also see that doubling the concentration of sodium thiosulphate doubles the rate of reaction. As the line of best fit goes roughly through the origin, I can also state that the rate is directly proportional to the concentration of sodium thiosulphate. This reinforces what I stated in my planning section that increasing the concentration of the sodium thiosulphate should increase the rate of reaction because the reaction can only occur if the sodium thiosulphate particles collide with the hydrochloric acid particles with enough energy to react. As the concentration of sodium thiosulphate is increased, there are more sodium thiosulphate particles present in the solution so there are more collisions per second. This increases the number of successful collisions and so increases the rate of reaction. This conclusion supports both my predictions well.
Evaluating
This experiment seemed to work quite well to me. It was quite simple to conduct and it gave results that were easy to measure. From my graph, I can confidently state that I had no anomalous results. I think that one way we could have improved the procedure was to have taken five results for each concentration of sodium thiosulphate instead of three. This would have reduced the effect of anomalous results considerably. We could also have done the experiments in a thermostat. This way we could easily regulate the temperature accurately. Even if the dial were not set precisely on 20ºC, (for example it may have been set at 21ºC) all the experiments would have been conducted at the same temperature. Another way of improving the experiment would have been to shine a constant light through the conical flask and set up a light sensor at the other. Then we could set up a circuit so that when we start a timer it stops when there is no more light coming through the solution (it has turned opaque). This would give results that are more accurate and would give uniform times from student to student whereas when judging by eye people have different vision.
