Measuring volume of gas produced Measuring precipitation (because sulphur is produced) e.g. look for X to disappear Measure mass lost

Transcription

1 Introduction My investigation is about the rate of reaction. A rate of reaction is defined as how fast or slow a reaction takes place. For example, the oxidation of iron under the atmosphere is a slow reaction which can take many years, but the combustion of butane in a fire is a reaction that takes place in fractions of a second. For a reaction to take place particles must collide with sufficient energy; this is called activation energy. When temperature is increased the particles move faster and with more energy, therefore the collisions happen more often and with more force so the reaction is faster. Four Factors which affect Rates of Reaction: Surface Area - Surface area is the measure of how much exposed area a solid object has, expressed in square units. Temperature - Heating chemical substances can make the molecules separate and move around faster. This can increase the speed with which they mix with surrounding molecules from other chemical substances, thus speeding up a reaction. Concentration If solution contains high concentration of reactants, this means that more collisions would take place because there is more of a chance that the reactants would collide. On the other hand if a solution has low concentration, the chances of the reactants colliding is less, this increases the rate of reaction. Use of Catalyst If a catalyst is present then the reaction rate will increase due to the fact it replaces the amount of energy needed for the collision, allowing for more collisions to take place. Aim: We will be investigating the rates of reaction when we will mix together sodium thiosulphate and hydrochloric acid. I will carry out trial runs to decide which of the four factors that affect rates of reaction I will measure in my final run. I could do different experiments. The experiments I could do are: Measuring volume of gas produced Measuring precipitation (because sulphur is produced) e.g. look for X to disappear Measure mass lost The reaction of the experiment happens with this formula: Na 2 S 2 O 3 + HCl = 2NaCl + H 2 O + SO 2 + S The importance of running trial runs is to see whether any mistakes are made and to prevent them from happening in the final run. We carried out 2 trial runs, this is because we wanted to compare both the methods to find out which was most accurate out of both of them. Trial run 1 was most accurate. This is because there were more data recorded. Trial run 1 had a wider range however trial run 2 hadn t. 1

2 TRIAL RUN 1 How concentration of sodium thiosulphate affects rate In this experiment we did a trial run to familiarise ourselves with the experiment and the equipment. It also helps us what kind of patterns we expect to get. The concentration of sodium thiosulphate was changed by adding water. The solution was tested to see if it goes cloudy by observing an X. Fair Test: To make the investigation a fair test we should make sure: Use the same person to observe the reaction because different people have different eyesight so keeping the same person to observe the X disappear is better. Use the same X as a darker or bigger X can increase the time it takes for it to disappear. The temperature is kept the same. Same volume of solution total volumes need to be the same. Accuracy: Timing is accurate e.g. start and stop clock on time/quickly Wash equipment before next reading to avoid volume error Measure volumes ate eye level Use large measuring cylinders for larger volumes Distance X viewed from Pipette accurate to nearest drop What to change: Concentration Equipment used: Conical Flask To put the solution of sodium thiosulphate, water and acid into. Three Measuring Cylinders (two 10ml)(one 25ml) Each used to accurately measure how much sodium thiosulphate, water and acid are used in the experiment. Pipette Used to put small amounts of acid into a measuring cylinder because it holds small amounts of acid in it so I can receive an accurate measurement. Digital Stopclock Used to time how long it took for the solution to turn cloudy and it s a more precise reading than an analogue clock (mili seconds). They re to the mearest hundred to the second. Paper marked with X Used to see whether the solution is cloudy enough to make the X not visible. Two Beakers To hold acid and sodium thiosulphate in while we carried out the experiment. 2

3 Safety: To prevent accidents from occurring we took some safety procedures for this experiment: Fumes are dangerous so be careful with the reaction. To prevent anything from happening we could open the windows. Liquids are corrosive so do not touch them. If liquid goes on hand then wash then thoroughly. Safety goggles must be worn at all times during the experiment. Experiment should be handled carefully and slowly. Method: In trial run 1 we used a solution called sodium thiosulphate to measure how long it takes to turn cloudy when you mix a certain amount of water and acid to it. The way we measured how the solution has turned cloudy is by: Draw an X on a piece of paper and place under a conical flask. Measure volumes of sodium thiosulphate and water using results table Add 3ml of HCL into the flask. Immediately time how long it took until the X was no longer visible, how long it took for the solution to turn cloudy The person observing when the X disappeared then recorded the time. Results of trial run 1: Concentration (%) Volume of sodium thiosulphate (ml) Volume of H2O (cm3) Volume of acid (ml) Time taken for X to disappear (s) 3

4 Evaluation: One of the weaknesses of the trial run was that the person recording the time changed and this could of lead to slight deviation in the result. However, other factors like the room temperature and the amount of acid in the solution were kept controlled to keep the test as fair as possible. Different people watching when the X disappeared meant that there are more likely to be errors and could make our results less reliable. Our results show a clear patter as the concentration of sodium thiosulphate increased, the time taken for the X to disappear decreased; this proves the collision theory. However, as we tripled the concentration, the time did not triple. This may be because of our lack of confidence and also it was our first measurement and we were definitely going to make mistakes. The changes we will make are to increase the concentration by 10% each time rather than tripling it. We proceeded with our trial run once and in my opinion made our results less reliable. This is because we do not have enough data to prove that concentration affects the rate of reaction. We will make sure the same person observes the X and records the time. Prediction: Using my trial run I can come to make a prediction. There is a pattern when the concentration is higher it takes less time for the reaction to happen. When we raised the concentration to 60% it takes less time for the reaction to be complete, also when we raised the concentration to 100% the reaction happened very fast; this compares and suggests to me that when the concentration is risen the time it takes for the reaction to be complete is less the rate of reaction is obviously different. 4

5 TRIAL RUN 2 How surface area affects rate Here the surface area of solid sodium thiosulphate was changed. Equipment used: Pestle and Mortar used to crush the sodium thiosulphate pellets to increase the surface area Conical Flask used to put in hydrochloric acid into and mix with the crushed sodium Digital Stopclock used to time how long the crushed sodium mixed with the hydrochloric acid took to turn cloudy Safety Goggles to protect our eyes from anything going in them Safety: To prevent accidents from occurring we took some safety procedures for this experiment: Fumes are dangerous so be careful with the reaction by opening windows. Liquids are corrosive so do not touch them and wash hands if touched. Safety goggles must be worn at all times during the experiment. Experiment should be handled carefully and slowly. Method: The method we used in this trial was to time how long it took for the small amounts of solid sodium thiosulphate pellets to react with the acid. We measured 20g of sodium thiosulphate pellets We measured another 20g of sodium thiosulphate pellets however we crushed it with the pestle and mortar to turn it into a powder We added the sodium thiosulphate pellets into a conical flask containing 10ml of hydrochloric acid and recorded the time taken for it to fully react with the acid. Then a group member recorded the time. We added the powder thiosulphate to the acid and recorded the time for it to react. The person inspecting the sodiumthiosulphate remained the same; this was done so that the test was fair. Other variables were also kept controlled. 5

6 Results of trial run 2: Surface area of sodium thiosulphate Mass (g) Volume of Hydrochloric acid (ml) Sodium thiosulphate Pellets Powder Time taken to react (s) This trial run proves the collision theory, it says that increased surface area increases rate of reaction Evaluation: The results of this trial aren t very accurate because enough data was not collected. This is because it is difficult to measure surface area of tiny grains of sodium. Due to this, we managed to acquire only 2 results, powder and solid states of sodium thiosulphate. In addition, one other reason why the results were not that reliable was because we did not repeat this trial run more than once. We should have repeated the trial run more times to make the results more reliable and to see whether surface area does affect the rate of reaction. I compared my graph to another group s to see the differences and to see if I had made any errors. It also would help me to draw the reasons for my anomalous results for example when the mass of sodium pellets was 20, my time was 60 seconds, but the other group s time was 65 seconds; this helped me to detect any errors. On the whole this is a very subjective method e.g. crushing method is not very accurate. Also the reaction was completed when the pellets or powder had completely reacted, but because the solution got cloudy it became difficult to see. I will therefore to my trial run 1 as my final experiment. We decided to use the method which we used in trial run 1. We decided to use the trial run 1 method because we found the experiment easier to do rather than trial run 2 also; we found the measurement in trial run 1 to be more accurate than trial run 2. So, for our final run we agreed on measuring more levels of concentration; we started with 100% concentration and then reduced the concentration by 10% each time, this lead us to 10 levels of concentration. Each time while doing the experiment we reduced the amount of sodium thiosulphate on the other hand we increased the amount of water. We made sure the level of solution was level and we also kept the hydrochloric acid always the same. We made sure it was the same by measuring it with a measuring beaker. 6

7 FINAL RUN Due to the fact that we were measuring how concentration affects the rate of reaction, the main factors which affect the rate of reaction were controlled, for example: Temperature we did this by making sure all windows and doors were closed and all 3 final runs were done on the same day so whether was not an issue and did not affect the results Use of catalyst a catalyst was not used in any of the experiments Surface area the same volumes of hydrochloric acid was used and also the volume of sodium thiosulphate and water always added up to 30ml To make our experiment as fair as possible we made sure the following variables were controlled: We used the same tap so the temperature of the water was kept the same. The same equipment was used and washed thoroughly after each experiment to not affect volumes. The same person adding the solution was kept the same and also the person observing the X was kept the same, this is because different people have different eyesight and so the observation of the X when it disappeared was recalled by the same person. The hydrochloric chemical we used was kept at a constant concentration. We used 3ml of it although we could have kept the concentration of sodium thiosulphate the same too. The solution was kept at a constant volume throughout the experiment. If the volume was different then it could have given us a different result. We must also make sure the water and hydrochloric acid were placed into the beaker at precisely the same time or it could affect the result of our experiment. The colour and the size of the X were kept the same. We used a pencil and very accurately made sure the X was drawn the same each time. Accuracy: Pipette Nearest drop Measure volumes at eye level Same lighting of room Same equipment Two different measuring cylinders 7

8 Equipment used: 50ml Conical Flask to add the solution of water, sodium thiosulphate and hydrochloric acid One 10ml Measuring Cylinder to measure small amounts of hydrochloric acid into because we only used 3ml of hydrochloric acid, it was easier to use the 10ml as you could see clearly how much hydrochloric acid we was measuring Two 30ml Measuring Cylinders we used each cylinder to measure out sodium thiosulphate and water. Pipette used to insert small amount of acid into a measuring cylinder Digital Stopclock used to time how long it would take for the solution to turn cloudy, less vulnerable to human errors because it is more precise Paper marked with X used to see whether the solution has turned cloudy and made the X not visible Two Beakers to hold hydrochloric acid and sodium thiosulphate into while carrying out the experiment Method: Safety Pour solutions into beakers. Draw X and put under conical flask. Measure volumes of acid, water and sodium thiosulphate into measuring cylinder using results table. Put sodium thiosulphate and water into the conical flask. Add the acid and start the stopclock immediately. 8

9 Concentration (%) Volume of sodium thiosulphate (ml) Volume of H2O (ml) Volume of HCL (ml) Time 1 (s) Time 2 (s) Time 3 (s) Time 4 (s) Average time (s) Rate

10 CONCLUSION In conclusion I have found out that that the concentration does affect the rate of reaction. I believe that my results were accurate. For a reaction to take place particles must collide with sufficient energy; this is called activation energy. When concentration increases the number of particles increases in the same amount of space resulting in less space for particle movement and more frequent collisions. When temperature is increased the particles move faster and with more kinetic energy, resulting in reaching activation energy level quicker. Because of this the collisions happen more often and with more force so the reaction is faster. The diagram below illustrates the effect of temperature change. Surface area also plays a part in the collision theory. When surface area of a solid increases the time for the reaction to occur decreases. As I can see from trial run 2 this is evident. My results can be described using the collision theory. The collision theory is when the temperature is increased the atoms conduct the heat and vibrate; this causes the atoms to gain kinetic energy to move, this be because heat energy is transferred to kinetic energy for the atoms. Because there is more kinetic energy the atoms move faster with more energy. The extra energy enables the atoms to move and collide with other atoms and particles; because of this the collisions happen more often and with more force so the reaction is faster. When this reaction proceeds, the process of the ionic bond (a chemical reaction losing and gaining electrons so the reaction is complete between the two reactants to produce a new product) happens faster because the collisions happen faster, therefore there are more collisions with sufficient energy and this are called activation energy. Activation energy can be thought of as the height of the potential barrier (sometimes called the energy barrier) separating two minima of potential energy (of the reactants and products of a reaction). For a chemical reaction to proceed at a reasonable rate there should exist an appreciable number of molecules with energy equal to or greater than the activation energy. Average Time Graph The general trend on my graph was that as the concentration increases the time taken for the reaction to occur decreases. At first the line of best fit was very steep however as the concentration increased the line of best fit became less steep. For example: at 20% concentration it took 164 seconds, then at 40% concentration it took 74 seconds and finally at 60% concentration it took 49 seconds according to the line of best fit. There is a pattern, as 10

11 the concentration increases, the time taken decreases. Evidence of this can be seen by the gradients. I calculated the gradients concentrations 10, 30 and 50%. At 10% the gradient was 17.33, at 30% the gradient was 3.73 and finally at 50% the gradient was I could see that as the concentration goes up the gradient becomes smaller so, this agrees with the fact that as concentration goes up, time goes down. I believe that this pattern shows enough credible evidence to support my prediction. The result of why time decreases as concentration increases is because more particles are moving about in the same amount of space causing them to collide more often (more frequent collisions). When the concentration is low the time increases. This can be explained using the collision theory. Rate Graph From my graph I can see that the there is a positive correlation. When the concentration is high the rate is high. This means that higher the concentration, the faster the rate of reaction will be. This causes the atoms to gain more kinetic energy, and this is because there are more particles in the same amount of space so they collide more often and therefore create activation energy. Evidence of this can be described from the graph: at 20% the rate was 0.58, at 40% the rate was 1.14 and finally at 60% the rate was Therefore it is clear the concentration is directly proportional to the rate. 11

12 EVALUATION I compared my graphs to another group s to see the differences and to see if I had made any errors. It also would help me to draw the reasons for my anomalous results for example when the concentration was 10 %, my average time was seconds, but the other group s time was seconds; this helped me to detect any errors. Concentration (%) My Time (s) Their Time (s) Average Time (s) After I compared another group s graph and mine, I could see that the results were very similar, but not exact. From this I was sure that my results were on the right path and that my prediction was correct. Reasons for any anomalous results will be explained below. In my results table I circled any outliers. When I found any outliers I repeated the experiment one more time taking extra care. This allowed me to receive reliable and accurate readings. The error bars of some points at a lower concentration have a larger range compared to the points which have a higher concentration. This shows that these points are not as reliable examples are: at 10% the range is 36.00, at 20% the range is and finally at 30% the range is The reasons why I have obtained outliers in my results table and big ranges on the error bars could be due to many factors for example: The experiment was done over two different days hence there may have been a change in room temperature. There was a window open which may have affected the temperature of the solution. Tiredness affecting eye judgment. Contaminated equipment e.g. pipettes, beakers etc. To improve the experiment in the future we could use such equipment that would give an accurate amount of the reactants like a burette. It would give the solution a precise amount of concentration. Another way to have been accurate is if the range for the concentration was in 5 s for example: 5, 10, 15, but in my experiment the concentration went up in 10 s for example: 10, 20, 30, this way my results would have been really accurate and would have been unlikely to have any anomalous results. If we increased the solution of sodium thiosulphate in 1 s rather than 3 s the experiment would be a whole lot slower, therefore we 12

13 would have more time to get accurate results because the reaction time would increase and limit the number of inconsistencies made. We may obtain better results if we change the variable to temperature however, an inconsistency could be when heating the solution; the temperature would decrease a bit while putting the beaker on the cross, adding the acid and starting the stopwatch. In future these points should be taken as advice so experiments are accurate and inconsistency free. To improve my experiment even further, there are certain things I could have used or done to make my experiment better for example: Light sensor do not turn off the light if they are turned on and or do not turn on the lights if they are turned off while doing the experiment. Increase the concentration in 5 s rather 10 s. Measure the temperature by placing the solutions in a water bath so the temperature does not change instead it is at a constant temperature. Use a thermometer to measure the temperature. Do the experiment all in one day so our results don t vary. Use a burette so our volumes are measured to the nearest ml. Use a glass pipette because it is clearer to see what is in it. An alternative method for future experiments could be using a light detector because it would judge the end point of the reaction accurately. As soon as a certain amount of light does not pass through, it shows that the reaction is complete so it is more accurate than using the human eye. Overall I support my conclusion that concentration does affect the rate of reaction because my graph is how I predicted it to be; the science backs up my results. Also after repeating my experiment for the 4 th time the range of the error bars became closer together. My data was reliable, this is because I done my experiment 3 times and a further 4 th time if there were any outliers. I made the increments of the concentration by 10% each time which showed a clear pattern. My method was reliable because I made sure the same person observed the X disappear, hence the same eye judgement was used for accuracy. My equipment was reliable for example, I used a pipette to accurately measure the volume of sodium thiosulphate inserted into the beaker. I made sure the X was kept the same and I also washed all the equipment I used to ensure minimisation of errors. Next time to be more confident in my conclusion I would compare my results and my graphs or maybe use CaCO 3 instead of Na 2 S 2 O 3 and carry out more trial runs. 13

14 REFERENCES: shfiles/stoichiometry/acid_base.html Cambridge Advanced Series endorsed by OCR, Chemistry 1 AQA Additional Science 14