I have identified as important variables in this experiment. An independent variable is an input variable which means it is something that will affect the outcome of the experiment. A dependant variable is an output variable, an outcome of the experiment. The dependant variable I am going to monitor is the ‘rate of reaction’. Concentration of acid I am going to change the concentration of acid whilst the other variables will be kept constant throughout the experiment. As the concentration of acid increases so does the concentration of hydrogen ions, which then results in an increase in the rate of reaction.
This is because an acid is a donor of hydrogen ions and the strength of an acid is a measure of it ability to ‘release’ hydrogen ions Temperature I need to control the temperature of the reactants this is due to several reasons. As heat is transferred to the particles, they gain energy as heat and convert it to kinetic energy which means that they will move much faster thus resulting in more collisions between the particles and an increased rate of reaction. The particles will react more frequently and are then more likely to react.
The cooler the reactant is the less movement there is and it is then less likely that a reaction will take place quickly. Surface area of marbles Another variable which I will have to control is the surface area of the marble. The larger the surface area of the marble the more collisions there will be between the hydrogen ions, this will result in an increase in the rate of reaction. I have a choice of various sizes of marble that I can use, I can use large chips, small chips or powdered marble. I have to control this by using the same one each time or it will affect the results of the experiment.
Preliminary work Acid As part of my planning I had to do some preliminary tests so that I could find out what the best conditions are to get the best results out of my experiment. I did an experiment to find out what the best acid to use for my experiment was. The acids that I used for my preliminary work are as follows: Sulphuric acid – H2SO4 Nitric acid – HNO3 Carbonic acid – HCO3 The tables below show the results which I obtained from my preliminary experiments. To make my experiments fair, I made sure that I kept all of the other factors of the experiment the same throughout the experiment.
I used 20cm3 of 2 molar acid and 2 grams of small sized marble chips. I monitored each one for 4 minutes. Sulphuric acid Nitric acid Time/minutes Weight/grams Time/minutes Weight/grams 0Carbonic acid Time/minutes Weight/gramsSulphuric acid is the strongest acid used but it isn’t suitable for my experiment as the range of change in mass was small (0. 02), HCO3 was the weakest acid I used and gave the smallest range (0. 01) of results and so is the least suitable for my experiment.
Hno3 gave the largest range (0. 03) and so is most suitable for this experiment: 2HNO3 + CaCO3 –> Ca(NO3)2 +CO2+H2O Marble As surface area has an effect on the rate of reaction I am going to do some preliminary tests on this to find out what the best type of marble is for my experiment. I am testing small marble chips, large marble chips and medium marble chips.
Again I had to ensure that this was a fair test so I kept other factors constant throughout the experiment. I used 20cm3 of 2 molar nitric acid and 2 grams of marble chips and then I monitored the change in weight for 4 minutes.
The results are shown below. Small marble chips Medium marble chips Time/minutes Weight/grams 0Time/minutes Weight/grams 58 Large marble chips Time/minutes Weight/grams I have decided to use small marble chips as they have given the largest range difference thus will be the best choice for my experiment. The small marble chips give a larger range difference for my results then the medium and large marble chips. Prediction I predict that if were to increase the concentration of acid the rate of reaction would also increase in proportion to the concentration of acid, I also predict that the rate of reaction will gradually decrease as there will be less marble chips for the acid to react with.
The concentration of acid is the variable that I am going to be changing; all other variables will be controlled and kept the same. If I were to decrease the concentration of acid, the acid would be more dilute, there would be a higher ratio of water particles to acid particles, as there are less acid particles they would not react with the marble as quickly thus a slower rate of reaction.
I believe this as when I double the concentration of the acid the numbers of hydrogen ions will also double this will also lead to an increase in the probability of collisions, the hydrogen ions will not always collide with the marble they may sometimes collide with each other or with water molecules, this is demonstrated in the diagram below. Notice how in the diagram with the higher concentration of acid there are far more hydrogen ions to collide with the marble. KEY: – Water particle – Marble particle – Hydrogen ion Plan Equipment Burette Conical flask Funnel Top pan balance Spatula Stopwatch Paper towel Safety goggles 20cm3 of 2 molar nitric acid 2g of small marble chips Plan 1. Collect all of the equipment stated above.
2. Weigh out 2 grams of the small marble chips using the top pan balance and place these chips on the paper towel 3. Put the weighed marble chips to one side and pour any amount of acid into the burette making sure the tap is closed afterwards 4. Place the empty flask on the top pan balance and tare it so that you do not have the weight of the flask included 5. Place the burette over the conical flask and open the tap. Carefully and accurately allow 20cm3 into the conical flask, when the acid is poured record the weight. 6.
Place the 2 grams of marble chips into the flask containing the acid, as soon as this is done start the stopwatch. 7. Record the weight at intervals of 30 seconds for 5 minutes. 8. Repeat steps 1-7 2 more times to get three sets of results for increased reliability. 9. Now repeat steps 1-8 for the following concentrations – 1. 5M, 1M, 0. 75M, 0. 5M, 0. 25M Diagram Detailed scientific knowledge There are many factors which affect the rate of a chemical reaction Temperature is one of the factors which affect the rate of the reaction. For a reaction to take place, the particles of the substances that are reacting have to collide.
If they collide with enough energy then they will react. The minimum amount of kinetic (movement) energy that two particles need if they are going to react when they collide is called the activation energy, that is the collision theory. By heating the chemicals you raise the energy levels of the molecules involved in the reaction thus causing the molecules to move much faster and collide with greater energy resulting in a greater rate of reaction. Concentration is another factor that affects the rate of the reaction this is also the factor that my experiment is based around.
Increasing the concentration of the reactants will increase how frequent the collisions are as there will be more particles to react. Precision When choosing the apparatus to use I have to consider the precision of the experiment. I am using equipment which will ensure that my experiment is as precise as I can make it. I have used two pieces of equipment to ensure precision in my experiment these are the burette and the digital top pan balance. The burette is a piece of equipment which is used to measure volumes of liquid such as acid or water, this piece of equipment is much more accurate then an ordinary measuring cylinder.
This is because it has a tap on it which allows you to administer one drop at a time. When using a measuring cylinder the acid would just pour out and you would not be able to let it out as slowly as you could with the burette. A meniscus is formed when a liquid is poured into a measuring cylinder; it is a small dip from which you would take readings from the bottom. The diagram below demonstrates this. Meniscus The digital top pan balance is another piece of equipment which I chose to ensure precision in my experiment.
This piece of equipment is far more accurate then an analogue balance which displays weight by a needle, the readings must be read off the scale this can result in human errors for example if the reading is not read at eye level it may not be correct. The top pan balance shows a digital display of the weight to 2 decimal places this gives further precision. Results The tables below show all of the results which I obtained from my main experiment. There are 6 tables each of which shows a different concentration ranging from 2 molar down to 0. 5 molar. The tables show three repeats of each one and the differences between each 30 seconds.
I have also calculated and stated the averages of the differences. I have shown anomalous results in red text. 2 Molar Nitric Acid ;——————-Weight/grams——————–> Time/Minutes Set 1 Difference 1 Set 2 Difference 2 Set 3 Difference 3 Conclusions drawn from graph My first graph shows the results I plotted when comparing the average weight difference with time. The average weight difference was measured in grams and the time was measured in minutes.
I got the results for the graph by working out the average weight differences from my results tables. I took the three weight differences, added them together and then divided them by three. An example is shown belo The formula for this is as follows: a = difference 1 b = difference 2 c = difference 3 If I came across an anomalous result I would omit the result and then when I worked out the average I would ignore the anomalous result and take an average of the two remaining results. I used this method to work out the averages for all concentrations through all 5 minutes.
I was left with 10 points to plot for each of my 6 concentrations, with 6 lines on my graph I had to come up with a way to distinguish between these lines, I have done this by simply labelling beside each line which concentration it is, you can see this shown on my graph. I can see from my graph that the higher the concentration of acid, the faster the rate of reaction. My graph shows that the steepest slope or highest gradient came from the start of the graph (the point nearest to the origin) and then the slope gradually eased up. I can see that this was common for all 6 of the acid concentrations.
During the start of the experiment in the first 2 minutes or so there are a lot of reactants to react and the reaction is at its most vigorous point, the rate of reaction is high. Then from about 2 minutes to about 3 and a half minutes I can see that it becomes a gentler slope so there is now a slower rate of reaction, reactions have not stopped as all of the reactants have not been used up but collisions between molecules are not as useful as before as some of the reactants have been used up. Then from about 3 and a half minutes to 5 minutes you can see that the reaction has slowed down a great deal for all of the acid concentrations.
The slope starts to become much gentler and the graph starts to become straighter, this is because most of the reactants have been used up and now the collisions between molecules are now much less common as there are less marble chips for the acid to react with, eventually the reaction would come to a complete stop but my experiment didn’t reach that point as I only took results for 5 minutes. The line showing the 2 molar concentration of acid had the highest average weight difference; this was because the acid was much stronger and reacted more vigorously with the marble causing much of it to be given off as gasses in the air.
The line showing 0. 5 molars had the lowest average weight difference as the acid was not very strong and did not react as much with the marble chips. The 2 molar line had the steepest gradient; this was because the reaction was much more vigorous and occurred much faster, the rate of reaction was much quicker. The line showing 0. 5 had the lowest gradient as the rate of reaction was fairly slow. I also noticed that the line for 2 molar acid had a gradient which was near enough double that of the 1 molar line and that the gradient of the 1 molar line was almost half that of the 2 molar line.
Prediction I predicted that if I were to increase the concentration of acid, the rate of reaction would increase in proportion to this. I can see from my graphs that this is true as the peak for my 2 molar line is 0. 48g and the peak for my 1 molar graph is 0. 26 which is nearly half, only 0. 02 g off in fact. I predicted this as when you increase the concentration you increase the number of hydrogen ions. The more hydrogen ions there are the more likely they are to collide with the marble chips and react.
If I were to double the concentration of acid this does not necessarily mean that I would double the rate of reaction as the hydrogen ions will not always collide with marble particles, they may sometimes collide with water particles. Accuracy of observations Looking at my results and the graph which I had drawn from them, I am quite happy with the overall result. On my graph I had 6 lines each of which was a fairly good curve but there were quite a few results which looked slightly out of place, they did not fit in properly with the lines I had drawn. I had a few anomalous results which were quite far off from my lines of best fit.
I can see that my graphs showed me some good results as they showed me what I was expecting to see. I was most pleased with my curve for 1. 5 molar acid as it had a fairly clean curve with most of the points sitting on or very near to the curve. My worst curve I would say was my one for 0. 25 molar acid as it did not give me a proper curve as there was not enough correlation between the results and the line was pretty flat, not steep enough. I would say that about 50% of my results sat on the curve but even so there ware a few results which were far off.
All of the lines on my graph showed a positive correlation as the line stretched away from the origin upwards and sideward and most of the results were near to or sitting on the line. Anomalous results I had quite a few anomalous results in my results tables and I also identified a few on my graph. I decided to point out these results by colouring them red in my results tables and I circled them and labelled them ‘anomalous result’ on my graph, this is to ensure that they are noticed and taken into account as anomalous results.
I decided on which results were anomalous by doing the following. For the anomalous results in my tables I just looked at the result preceding it and the result following it and if it was not in between the two I would class it as an anomalous result. For my graph I just drew a line of best fit for each concentration and any results which were noticeably away from the line I would class as anomalous. I came up with 8 anomalous results in my initial results tables and I came up with 4 anomalous results on my graphs, one on each line except the 2 molar line and the 0.
5 molar line. When I was calculating averages from my initial results tables I selected and highlighted anomalous results by changing the text colour to red. I then omitted these results and calculated the average from the remaining results. For me an anomalous result was anything that was not in between the result preceding it and the result following it. Suitability of procedure I think that my final experiment was good overall. The choices of equipment which I used I’d say were fairly good as they gave me a good set of results.
I think that my experiment was quite good as I tried to use the equipment accurately. I ensured that it was a good test by using accurate equipment such as the burette to accurately measure my volumes of acid and the electrical top pan balance to weigh my solution. I believe that although I did a lot to try to ensure that my experiment was fair I could not control all of the variables, below I have stated and written about two variables which I believe had effect on my experiment and were not controlled perhaps as well as they should have been. Temperature
This is the first of the two variables which I believe was not controlled properly. I have discovered that the temperature is something that needs to be kept constant as if the temperature keeps changing it can have a fairly big effect on the results of the experiment. As temperature increases heat is transferred to the particles causing them to move much faster then they would normally would thus leading to many more collisions which in turn leads to an increase in the rate of reaction, as particles gain energy they collide more actively and are more likely to react.
If the temperature decreased the reactant would be a lot cooler therefore there would be a lot less movement and less chance of a reaction taking place quickly. In my opinion the best temperature to do the experiment in is room temperature. The table below shows how the temperature varies over a five day period in our science lab. Day Room temperature (oC) 1 20. 4 2 21. 1 3 20. 7 4 21. 3 5 21. 5 The information above shows how the temperature can vary, as you can see it varies a fair bit, it may not seem like a lot at first but if you take into account the affect that it can have on the results of the experiment it is quite a lot.
The room temperature can always be changing this can be due to many reasons. For example, windows can be left open, the door could be left open. Temperature could be high one day due to the heating system, it could be low another day again due to the heating system. Surface area of marble The other factor which I believe was not controlled as well as it should have been is the surface area of the marble chips. Every single marble was different in shape or size not one of them identical to another. If the marble chips kept changing in shape and size this would very much affect the results of my experiment.
The smaller the pieces of marble the greater the area of contact. The larger the surface area, the more collisions there will be with the hydrogen ions thus leading to an increase in the rate of reaction. All of the marble chips are irregular; this affects the amount of collisions, the rate of reaction and my end results. Suggested improvements If I were to repeat my experiment there are a few aspects of it which I could change to significantly improve the experiment. Some changes I would like to make are stated below.
The main aspect which would need to be revised is temperature control. I would need to control the temperature; I stated earlier how it affects my experiment and can decrease the reliability of my results. The temperature is a factor that is somewhat difficult to control but I can control it, I can do this by using a water bath. A water bath is a heated sink which can be quite big, big enough to store this whole experiment in the water at once. You can adjust the temperature inside the water bath; this is usually done by the turn of a dial.
The good thing about a water bath is that the temperature inside will not be affected by the temperature of the surroundings. Surface area of marble Another aspect which again would need to be revised were I to carry this experiment out again is the surface area of the marble chips. I previously stated how the surface area can affect the accuracy of my results as none of the chips were identical; they were all different in shape and size, all irregular. When surface area changes it affects the amount of collisions taking place between the molecules and the rate of reaction.
The marble chips which I used were irregular, I could fix this problem by using regular marble chips, for example they could be in the form of small cubes, tablets, spheres. Powdered marble is another solution as the surface area would always be the same. Extension of enquiry Extension of an enquiry is making our small experiment into a much larger one, by extending it I would make it more detailed and on a larger scale. Here are a few ways in which I could extend my enquiry. One way is to see if acid rain affects marble in the same way, this could relate to how acid rain wears away buildings.
I would want to do this as an extension as it would attract a much greater audience as this is an issue that is recognised by many. Another way is to investigate in more detail different types of marble, I could do this by collecting marble which comes from different places around the world. I would do this to see how the results differ from the marble I have used already, this could show me the differences between marble which is found in this country and marble from other places in the world.
Another extension which I could do is to experiment using all 3 acids which I had initially considered for my experiment. These three acids are nitric acid, sulphuric acid and carbonic acid. I could investigate the affects of different acid on the same marble, this could again attract a wider audience and make the experiment appealing to others. Show preview only The above preview is unformatted text This student written piece of work is one of many that can be found in our GCSE Patterns of Behaviour section.