To investigate the effect of concentration on rates of reaction.

        I had to keep the temperature constant during the experiment because the heat energy from the temperature is converted to kinetic energy making the particles move faster and therefore there would be more collisions and they would be more likely to reach their activation energy. If I had increased the temperature half way through my results would have been anomalous.

        The concentration does not affect the energy of the particles and therefore does not make the activation energy any more or less likely to be reached.

Apparatus:

• 100cm3 beaker
• Two measuring cylinders 10cm3
• Stopwatch
• Black cross
• Burette 50cm3
• Funnel
• Distilled water
• Sodium thiosulphate 0.2M
• Hydrochloric acid 1.0M

Method:

1. Fill one measuring cylinder with 10cm3 of sodium thiosulphate using a burette.
2. Fill the other measuring cylinder with the correct amount of acid and water depending on the concentration required (table below) using two separate burettes. I will use five different concentrations.
3. Label each cylinder acid or not acid so that there is no cross

        contamination at later concentrations.

4. Take temperature of chemicals.
5. Set up the apparatus as shown in diagram.
6. Measure time taken from the time you pour in the chemicals to when the black cross can no longer be seen. Write down the time.
7. Repeat experiment 3 times for each dilution for better reliability of results. Also repeat the experiment if you get an anomalous result so it wont be included in your average.

Diagram:

Precise and reliable data:

        My data will be precise because I will make all volume measurements accurately with a burette, the most accurate measuring device available to me. I will watch my experiment all the time and stop the stopwatch as quickly as my reactions will allow when I think the black cross has gone.

        My data will be reliable because I will control or monitor the other variables temperature; by measuring with a thermometer and then adjusting, surface area by using liquids and I will not let any catalysts near my experiment. I will do each concentration 3 times and find the average and redo any anomalous results. I will do it three times because the more data you process the more accurate your average.

        I will follow my fair test guidelines as closely as possible.

Changes to the plan:

        Although I made changes during the preliminary work I could not see any obvious improvements or changes to be made during the practical work.

Results:

Conclusion:

A higher concentration = higher rate of reaction

Explanation of conclusion:

         I came to this conclusion because on the ‘graph to show the effect of concentration on rates of reaction’ the line slopes downwards showing that as the concentration of the acid increases the time taken decreases and therefore the rate increases. The line gradually became less steep this shows that as the concentration of the acid got to 0.8M and 1.0M it stopped having such a dramatic affect on the time the reaction took.

I used a rate graph because my other graph does not give a measure of rate. You can see from my other graph that as the concentration of acid is increased the rate of reaction also increases. Rate is inversely proportional to time therefore rate is proportional to the inverse of time. So:

        RATE = 1/TIME

This graph clearly shows that rate is proportional to a rise in concentration because there is a straight line with a positive correlation.

This completely supports my prediction that ‘the higher the concentration of the acid the faster the rate of reaction’. It can clearly be seen from this graph that this is correct.

This happened because: The collision theory states that for a reaction to occur it is necessary for the particles to collide and that they must collide with sufficient activation energy.

        High concentration had a faster rate of reaction because an increase in concentration means there are more reacting ions. More reacting ions means there were more collisions because they were more probable to collide. This is because you have more reacting particles in the same volume therefore they are closer together and more likely to collide. There was a higher probability of collisions so there were more collisions in a given time.

        The concentration does not affect the energy of the particles and therefore does not make the activation energy any more or less likely to be reached.

        I got one anomalous result it is highlighted red in my results table I knew it was anomalous because it did not fit the pattern of the other two results for that concentration furthermore it did not fit the pattern of the rest of my results. I did not include this result when calculating my averages and I did a fourth test for that concentration to compensate for the anomalous result.         

The result could have been caused in a number of ways one being a parallax error when measuring the acid this is human error and should not have occurred, water or thiosulphate this would have caused the volume of the liquid to change, if the depth is deeper which it probably was the black cross would have disappeared more quickly because more sulphur would have been produced and it is more difficult to see things when they are at a greater depth. I may have used the wrong volume of acid this would not only change the overall concentration of the solution but also the depth, for the result I got I would have used to much acid as this would have increased the rate and therefore lowered the time taken although it would also have increased the depth I do not think this would have had a larger affect than the increase in concentration.

The result could also have been caused by a change in temperature although I think this unlikely as I monitored the temperature at intervals throughout the experiment and it only changed by roughly 0.5o this small change in temperature could not change the time taken by 4 seconds. More proof that temperature was not the cause was that I did the retest nearly straight after the original error and I didn’t get an anomalous result, this shows that the temperature would probably not have had time to go back to normal. For the temperature to have affected my result it would have had to rise dramatically as that test went much faster than all of the others and when the temperature rises the time taken decreases.

I could have got the result through cross contamination his is possible because the reaction was faster than it should have been. Cross contamination causes the reaction to start to early and therefore decrease the time taken.

Comments on the plan:

        My plan was not as precise as it could have been since the instrument that measured when the black cross-had disappeared was not incredibly accurate this is because my eye is not 100% accurate and it is likely that I thought the cross was gone at a different point every time. This is a small error and would not drastically affect my results

        Also the temperature was only monitored not controlled this may have affected my results as there may have been variations in the temperature between my measurements of it.

        My measuring cylinders may not have been properly cleaned between tests and may have contained traces of water or chemicals. This would have affected the precision of my results by changing the concentration of the whole solution and starting the experiment early.

        I think the plan worked adequately for me to say without doubt that my results were reliable enough to prove my conclusion and prediction right. My evidence definitely is reliable enough to support my conclusion. I got a very clear pattern on my rate graph that showed reliable results, also there is an easy to see pattern in my average times and I only received one anomalous result, which I have fully accounted for.

Improvements:

        There are ways that I could improve the reliability and precision of my results. I could do the experiment in a lab where the temperature was monitored and controlled so that the energy of the particles would not change and the activation energy would not be more or less likely to be reached. The temperature would not only be controlled but also monitored constantly to make sure it didn’t change unlike in my experiment where it could change between checks. To do this I would use a thermostatically controlled water bath.

        This experiment could be done without a black cross by a colorimeter this would measure the change in colour of the solution and when the reaction reached a certain colour the reaction would stop.

My measuring cylinders and the rest of my equipment would be new between each test to stop cross contamination.

        I could do some further work to make sure that my conclusion was universally true; I need to find out if my conclusion applies to all reactions not just this one. For example I could use marble chips and Hydrochloric acid then measured the loss in weight over I certain time when I increased the conc. of acid this would show how much gas was given out and when more gas is given out in a certain time the rate would be faster. I could also measure how long it took to collect a certain amount of gas from an experiment; this would also show how rate increased with higher conc. of acid, when the time taken to collect it was lower the rate would be higher.

References:

• Help from teachers
• Encarta 98 – collision theory
• Work in book