The Effect of Concentration on Rates of Reaction

The Effect of Concentration on Rates of Reaction
Aim: To investigate the effect of concentration on rates of reaction. Sodium thiosulphate + hydrochloric acid à Sodium chloride + sulphur + sulphur dioxide + water Na2S2O3 +2HCl à 2NaCl + SO2 + S + H2O (aq) (aq) (s) (g) (s) (l) Risk assessment: ? I will wear goggles at all times to prevent chemicals going in my eyes especially when pouring acid in to the burette. ? I will make sure that all bags are under tables so I wont fall over them whilst doing my experiment. ? I will not run around the room as I may knock somebody over if I did. ? I will be careful when using chemicals so I don?t get them on my clothes or skin. ? When I have done the experiment I will tip it straight down the sink whilst the tap is running because the gas given off at the end of the experiment (sulphur dioxide) is poisonous and can be dangerous to asthmatics. ? I will not sit down during the experiment because it makes your face closer to the chemicals. ? I will wipe up spills quickly so that nobody is exposed to them. Preliminary work: I tried to find the optimum amount of hydrochloric acid and sodium thiosulphate to use in my experiment. The acid needed to be a suitable amount to dilute easily and the sodium thiosulphate needed to be in a proportion to the acid so that the highest concentration of acid would not have a very low or too fast time.
I used the method described later to find the highest and lowest concentration of my experiment, I chose the highest and lowest because they cover the entire range of results and nothing is going to be outside those times if the experiment is done correctly. I decided to use 10cm3 for both of my chemicals. I chose these amounts because using them the experiment did not go to fast or to slow. The one change I made due to my preliminary work was the black cross since it did not disappear for a long time as it was too dark and it affected my results The fact that it was to dark made my experiment take much longer since the black cross took longer to disappear. This was not practical because I had a limited amount of time to work in and I needed the experiment to take a shorter time. Results in preliminary Concentration (M) Time (seconds) 1 40 0.2 56 Prediction: The higher the concentration of the acid the faster the rate of reaction. Explanation of prediction: 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. I think the high concentration will have a faster rate of reaction because an increase in concentration means there are more reacting ions. More reacting ions means there will be more collisions because they are 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 will be a higher probability of collisions so there will be more collisions in a given time. All of this should increase the rate 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. [image] 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. Concentration of acid (M) Volume of 1.0M HCl Acid cm3 Volume of distilled water cm3 Volume of 0.2M Sodium thiosulphate cm3 1.0 10 0 10 0.8 8 2 10 0.6 6 4 10 0.4 4 6 10 0.2 2 8 10 Diagram: [image] Fair test: Decision made Reason Monitor the temperature throughout the experiment. Different temperatures change the rate of reaction, changing the results. Keep volume of sodium thiosulphate the same all the way through. I am testing the concentration of acid not sodium thiosulphate. If I change the amount of sodium thiosulphate the rate of reaction will remain the same but the amount of sulphur produced will be smaller therefore the cross would take long to disappear. Also if I used different amounts of thiosulphate it would change the effective concentration of the acid because the total volume of liquid involved is changed. So there would be the same amount of particles in a larger volume. Thus changing the results. The depth of the liquids would also change making it harder o see the cross and therefore increasing the experiment time. Use same measuring cylinder each time Stops cross contamination of acid and sodium thiosulphate. So the reaction will not start until both chemicals are poured in to the beaker. Use the same cross each time. I used the ?black cross method? because it allows you to see the amount of sulphur produced in a certain amount of time. The sulphur is a solid and stops the cross being seen through it so when enough sulphur is produced the cross will be obscured. It will take the same amount of sulphur each time to obscure the cross. If I change the cross it will take a different amount of sulphur to obscure it this would alter the time recorded. Keep the volume of dilute acid the same all the way through If I used a different amount it would change the depth therefore making the cross easier or harder to make disappear. Keep the concentration of thiosulphate at 0.2M throughout the experiment. If I used a different concentration it would change the rate of reaction since there would be more or less particles to react with. When measuring make sure eyes are level with meniscus. So I will not make parallax errors when measuring. 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: Concentration of acid (M) Time 1 (seconds) Time 2 (seconds) Time 3 (seconds) Time 4 (seconds) (incase of anomalous results) Average Time (seconds) (2sf) 1.0 41 41 42 41 0.8 42 41 38 42 42 0.6 45 45 46 45 0.4 50 50 51 50 0.2 56 56 56 56 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. [image] Anomalous Results: 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.

The Effect of Concentration on Rates of Reaction 7.4 of 10 on the basis of 1819 Review.