Prediction
I evaluated each of the factors and I have chosen to pursue the experiment by investigating the effect of a concentration on the rate of reaction. I chose to investigate the concentration because this experiment is easier to pursue than the others because it takes less time, resources and is easier to keep the variables controlled than in experiments investigating the other factors, which will hopefully give more accurate results.
I think that as the concentration increases the rate of reaction will also increase. I think this because as the concentration increases, it means there are more particles in the acid, meaning there is a higher amount of collisions taking place with the powder, so there is a higher rate of reaction, as the reaction is taking place more quickly.
The concentration is directly proportional to the rate of reaction, so as one is doubled, so is the other. This is because as the concentration is doubled, there are twice as many particles to collide with the other reactant, meaning the product is made in twice the amount of time. This is shown in Chemistry for You by Lawrie Ryan, which shows a picture of the two experiments and how they react in the same amount of time.
Fair testing
To ensure that my experiment is fair the only variable I will change will be the concentration of the solvent. This is the independent variable.
The variable I will measure is the time it takes to react. I will do this by using a stop watch to measure the amount of time it takes for the powdered marble chips to totally react with the hydrochloric acid solution. This is known as the dependent variable.
The variables I will keep the same are the equipment that I use, the amount of acid and amount of powdered marble chips I use. I will make a large stock of the powder at the beginning of the experiment to ensure that the powder is all of the same surface area. I will use a quarter of a spatula of powder for each experiment as any more than this will take to long to react, thus increasing the error in the experiment.
Preliminary work
I decided to use 25ml of acid solution in each test tube; this is so I could make a stock of 100ml for each concentration so I could have some spare if I needed it.
I investigated to find the amount of marble chips I would need. I found that the reactions with a couple of marble chips, even when one experiment contained powdered chips, still took too long. I found that powder reacts more quickly with the acid, so I have chosen to use powder as it takes less time. I investigated how much powder to use and found that I only needed a very small amount, so I chose to use ¼ spatula full. We will measure this by filling the grove in the bottom of the spatula which we know fills a ¼ of the spatula.
I investigated to find which concentrations of acid I should use and I have chosen to use 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95 and 1 molar solutions. This was because I found that anything below 0.5 did not react very quickly, and time was an important factor in the experiment. I decided to choose 11 values of 0.5 molar steps as this would give a much more accurate results.
Diagram
Precision and Reliability
The key variable which I will be testing is the concentration of the acid; I will use 11 different concentrations in this experiment. They are: 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95 and 1 molar solutions.
I will be measuring the time it takes for the reaction to take place, this is the rate. I will then find the average rate of reaction at each concentration of acid and plot it on a graph to see if my hypothesis; that rate of reaction increases with a greater concentration, is correct.
By using the same equipment in the same environment, it will produce fair results as the conditions would be the same.
It is important that I find more than one rate of reaction at each concentration so that I can be sure that it is not anomalous due to human error and so that I can be sure that the average rate of reaction I calculate from the readings is accurate. This is why I have chosen to have 3 results for each concentration.
Method
- I will make up 100ml of each concentration using the correct amount of hydrochloric acid and water. I will find this by calculating the percentages of each needed in the concentration.
- I will label each of the stocks of solution.
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I will use a pestle and mortar to grind up the marble chips (CaCO3) into powder
- I will pour 25ml of a concentration into a boiling tube, I will have 3 lots of boiling tubes for each concentration, so I have 33 boiling tubes in total.
- I will start with the 3 boiling tubes for 0.5 molar solution and add ¼ spatula of powder to each
- I will use a stop watch to time how long it takes to finish reacting from the moment I put the powder in the boiling tube
- I will record my results in a table
- I will repeat this for the rest if the concentrations
- I will use my results to find the average for each concentration by multiplying each result by 60 to put them into seconds, then adding the 3 results for each concentration together and dividing by 3.
- I will then use my average results to plot a graph to show the change in the rate of reaction.
I predict that the graph will look something like the sketch below; I think that the results will be in a straight line.
Safety
I will make the experiment safe by considering the dangers and finding ways to prevent them. I will make sure that no one touches the reactants with their hands, as the acid could be corrosive or an irritant, I will use tongs to move the boiling tubes incase there has been some acid spilt on them. I will make sure I keep the concentrations in well labeled containers incase they are mistaken for the water. I will also wear goggles to protect our eyes and wash our hand thoroughly afterwards.
Obtaining evidence
During the experiment I noticed that bubbles of hydrogen gas were given off, quickly at first, but then more slowly, as the chance of the two products colliding gets less as they get used up.
I have included the average time (to two decimal places) at each concentration as this will help me later in this investigation to analyse the data. I have also high-lighted the anomalous results in red.
We found that there were too any anomalous results to prove a firm conclusion, so we retested the values for which we had anomalous results.
Here are the average times against their concentrations, this is the data I will use to draw my graph.
Analysing evidence and drawing conclusions
I was able to calculate the results of the average times by converting the times into seconds. I did this by multiplying them by 60.
E.g: For 0.5 molar solutions, the readings were 4.18, 7 and 9.58 minutes
Multiplying these by 60 gave 251, 420 and 575 seconds
I then added the 3 results together to get 1246
I then divided this by 3 to get the average of 415.33 to 2 decimal places
I found quite a few anomalous results in the readings. I felt that they would make quite a difference to the graph if I left them in the calculations of the average times. I decided to repeat the experiment to replace each anomalous result with a more accurate reading to ensure that my results were as accurate as possible.
Conclusion
From the graph I can see that there is a decrease in the amount of time it takes for the experiment to complete as the concentration increases, this shows the rate of reaction is increasing. From this I can see that the concentration does affect the rate of reaction. Just as I predicted, as the concentration increases, so does the rate of reaction. This is because as the concentration increases, there are more acid particles, this means that the acid particles collide with the CaCO3 particles and react, as there are more acid particles in higher concentrations, they finish colliding more quickly.
I found that the reaction that takes place is:
2HCl + CaCO3 → Ca(Cl2)+ H2O + CO2
Hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide
Although the graph does match my prediction, it is only a small section of the entire pattern of how concentration effects reaction as I only tested a small range.
Evaluating evidence
I found that it took a long time setting up the experiment as it took a long time to ensure that the concentrations were all accurate, and the amount of powder was the same, inaccuracies in the experiments set up would lead to a greater inaccuracy in the results so I wanted to be as accurate as possible.
It was had to tell exactly when the experiment had finished, as the bubbles were often hard to see, and as soon as we moved the experiment to check, we found that it was still going. I think this would have affected the quality of the results and may have been the cause of the anomalous results.
I found that the experiment worked well, but it was hard to control the variables. I think this would have affected the quality of the results. To improve the experiment, I would try to be more accurate when controlling the variables, and when timing the experiment. The variables I had to control were:
- The equipment I used
- The amount of Calcium Carbonate
- The amount of acid solution
- The temperature of the experiment
- The surface area of the powder
Alternatively I could use another method, such as using a syringe to collect the gas produced. It would b easy to see when the reaction had finished as the syringe would have stopped moving. I could have also put the experiment on a set of scales; this would show me the decrease in mass as the time went on, when the mass stopped decreasing the experiment would have finished.
When comparing the 3 results at each concentration, I found that some were not as close to the same value as others. They show that the results were not reliable enough to base a firm conclusion; this means that although the average results fit the pattern, I still have to allow for the fact that the initial results could be quite inaccurate and this could have created a larger margin of error in my results.
I found that most of the points lie quite far from the line of best fit, this shows me that the results are not that reliable. The graph does not look like the one I predicted, and due to scientific reasoning it should. I think if I was to repeat the experiment again I would do more tests for each concentration. Despite the inaccuracies, the trend of the line does support my prediction, so the rate of reaction is affected by the concentration. The results are reliable to show me the trend, but not to show if there is a quantitative trend. The graph I have only shows a small part of the big picture, so the results may not be as inaccurate as they seem to be.
Further work I could do is to use a larger range of concentrations, this would enable me to get a clearer, more accurate understanding of the effect of the concentration, and it will hopefully enable me to determine the end point.