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Surface area: if the surface areas of the marble chips are increased, then this increases the rate of reaction. The surface area is the measure of how much surface is exposed, therefore smaller chips have a larger surface area than larger chips, this means that rate of the small chips will be quicker than the bigger ones. Hence I will keep the size of the marble chips the same in each experiment, so that they all have the same surface area.
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Concentration: I intend to use different concentrations of acid This changes the rate of reaction, because if I have more acid than the water in a fixed volume then the reaction will take place quicker because there is a higher concentration of acid to react with the marble chips. Therefore there is greater chance of acid particles hitting and colliding, with particles on the surface of the marble. However, if I add more water, the concentration will be less so the rate of reaction will take longer because there is a lower concentration of acid to react with the marble chips. Therefore, I will change the concentration of nitric acid so that there will be 5 different concentrations and I will repeat each experiment 3 times.
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Catalyst: the use of a catalyst helps speed up the reaction, as it gives the particles more energy, hence more collisions can take place. However, I will not be using a catalyst within this experiment.
Other factors that I will keep the same:
I need to carry out a fair test; therefore I have to control some other aspects within the experiment. These are:
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Volume of acid solution: If there were a bigger volume of acid solution, then there would be less chance for the particles to collide one another. I will keep the volume of acid at 50 cm3 each time.
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Same Gas syringe: To make the experiment a fair test, the gas syringe will always be the same starting at 0cm3.
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Conical flask: I will use the same flask for each experiment. Therefore the same amount of space will be available for the particles to react. I will not shake or stir the flask when the reaction is taking place. This is because, if I do this, then it will give the acid particles more kinetic energy to collide with the particles on the surface of the marble chips. Therefore, this may speed up the rate of the reaction. If I use a bigger flask then the amount of space for the particles will increase, hence fewer collisions are likely to occur.
What I will vary:
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Concentration of hydrochloric acid - This is so that I can see how fast 50 cm3 of carbon dioxide is produced, at the different concentrations.
Table to show, how I made the different concentrations of nitric acid by adding water to 2M nitric acid:
My teacher gave me 2M of nitric acid in a beaker, which I diluted with water to make 5 different concentrations. This is shown below:
- To make 2.0M of concentration I did not add any water as it already made 40cm3 of acid.
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To make 1.5M of Concentration I added 10cm3 of water to 30cm3of acid. Therefore, the total volume was 40cm3 of acid solution.
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To make 1M of concentration I added 20cm3 of water to 20cm3 acid together and got the total volume to 40cm3 of acid solution
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To make 0.75M of concentration I added 25 cm3 water to 15 cm3 acid and got the total volume to 40cm3 of acid solution
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To make 0.5M of concentration I added 30cm3 of water to 10cm3 acid and got the total volume to 40cm3of acid solution
Number Of readings I intend to take:
I intend to take each measurement of carbon dioxide three times. Therefore I can find the average of the results. Here I am also able to see the anomalies of the experiment. If there are incorrect or odd results, then I can repeat the experiment again.
What is the Independent variable?
The independent variable that I will investigate will be the concentration of Nitric Acid. The more concentrated a solution is then the greater its molarity (M). A 2M solution is twice as strong as 1M.
What is the dependent variable?
The dependent variable, which I will investigate, will be the time it takes to produce 50.0 cm3 of carbon dioxide. However, this relies upon the different concentrations of the Nitric Acid.
How will I keep everything accurate?
To gain the best results I can, I will have to keep everything within the experiment accurate as possible. To do this I will:
- I will start the timer as soon as the bung is back in the flask.
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As I stated before, I will also always make sure the gas syringe at starts at 0cm3.
We found that 1M acid produced a reasonable rate of reaction with medium size chips. I intend to use a range of concentrations from 0.5M to 2M.
I will use five different concentrations of Nitric acid, as this will enable me to produce a graph with a reasonable set of points. Therefore, I will then be able to analysis my results to see if my prediction is correct.
Prediction
I predict that as I increase the concentration of Nitric acid i.e. 1.0M – 1.5M the rate of reaction will also increase so to collect 50cm3 of carbon dioxide will be in a shorter period of time. This is because there are more acid particles in a higher concentration to react with the marble chips; hence more collisions are likely to occur.
Lastly I also predict that if I double the concentration of Nitric acid then the rate of reaction will double and the time taken to produce carbon dioxide will be halved. This is because higher acid concentration means more particles in the same volume and more frequent collisions, or double the amount of collisions. Therefore, the time taken will be quicker and halved.
I can explain this through the collision theory shown below:
1 2
As you can see, shown above in Box 1 there are 6 acid particles and 2.0 grams of marble chips. In box 2 there are 12 acid particles and the same amount of marble chips as box 1.
Box 1 contains 1.0M of concentrated acid particles and Box 2 contains 2.0 M of concentrated acid particles.
The speed taken for the acid particles and marble chips to react is slower in box 1.This is because box 2 has a higher concentration of nitric acid particles, therefore more collisions are likely to occur in a shorter period of time.
Also Box 2 has double the particles so the acid particles will collide more frequently with the marble chips to produce carbon dioxide.
Shown below are the predicted graphs for:
Concentration and time:
Concentration
(M)
Time (sec)
Concentration and rate:
Rate (1/t)
Concentration (M)
Method
- Using an electric balance I measured 2.00g of marble chips. However, I made sure that they were all the similar size (medium).
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Using a measuring cylinder measured 40cm3 of 2M of Nitric acid.
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I then set up the apparatus and a water bath, which was set at 250 C using a thermometer. (See diagram)
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The marble chips were placed in to the flask and added the 40cm3 of Nitric acid quickly. This had to be a very quick procedure so as little gas as possible was lost.
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The stop watch was started and I then measured how long it took for the carbon dioxide to reach 50.0cm3.
- I then repeated the experiment for two more runs.
- I then repeated this whole procedure for the other concentrations; this is shown on the results table.
Obtaining evidence / results
The table below shows the results from all of my experiments. As you can see there are five different concentrations of Nitric acid, which I changed each time I did the experiment. It shows the time taken for 50.0cm3 of carbon dioxide to be produced each time.
The table also shows that I repeated the experiment for each concentration of Nitric acid, three times, to gain accurate and reliable results. Therefore I would be able to produce good graphs. In addition, I did a repeat run, for any unusual results that I obtained. The table also shows the average time it took for each concentration to produce 50.0cm3 of carbon dioxide. To do this I added each time and divided it by the number three. Lastly it also shows that I worked out the rate of each concentration, to do this I divided one by the average time for each concentration
Analysis of graphs
In general, I found that if the concentration of acid increases the rate of reaction will also increase. This is what I predicted, which was if I increase the concentration of nitric the rate of reaction will also increase, therefore it would take a shorter period of time to collect 50.0cm3 of carbon dioxide.
In graph one at 1M, it took an average of 86 seconds for the syringe to collect 50.0cm3 of carbon dioxide, whereas at 1.5M it took 62 seconds. The rate of reaction has increased; therefore the time taken to collect carbon dioxide is shorter at 1.5M.This is because the concentration has been increased. This supports my prediction, as I predicted that if the concentration of acid increases then the time would decrease. This is because there would be more collisions occurring in a higher concentration, as there are more acid particles in the same volume colliding with the marble chips and producing carbon dioxide, hence the reaction would be quicker.
In graph two it shows the plot of concentration of acid against the rate of reaction. This means that if the concentration increases the rate of reaction will also increase, this is shown on the graph. At 1M the rate was 0.0116 S-1, whereas at 2M S-1 the rate was 0.0240 which is about double. From this I can see that the rate for 2M was faster, as there is double the concentration in the same volume. Therefore, more particles are likely to collide with the marble chips and react, at a faster rate. When the concentration, is lower, the rate of reaction is slower, as there are fewer particles in the same volume, therefore there are likely to be less collisions. I can explain my results using the collision theory. As I increase the concentration of acid the number of particles in the same volume will be increased. This means that the number of collision occurring with the acid and marble chips will also increase.
This graph is a straight line and it passes through most of the points. It also nearly passes through the origin, if there was no concentration, then there would be no reaction, hence no rate of reaction. This straight line means that the rate of this chemical reaction is directly proportional to the concentration of acid. This tells me that doubling the concentrations, doubles the rate of reaction. This again agrees with my prediction that if I double the concentration of Nitric acid then the rate of reaction will double and the time taken to produce carbon dioxide will be halved
Evaluation
I think that my results are accurate, since I used a gas syringe, which is a precise piece of equipment, to collect substances. Here, I measured for plunger to read 50.0cm3 of carbon dioxide. In addition, I made this even more accurate by attaching weights to the ends of the syringe to allow the plunger to move out evenly as the gas collected
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The graphs for my results are good as most of the points fitted the line of best fit. Since I had repeated each experiment three times I can say that the results were accurate. However, when I had an anomaly I did this run again.
While conducting these experiments, I found some aspects difficult. For example, it was difficult to place the bung in to the conical flask and start the stopwatch at the same time. Also I may have stopped the stopwatch to early or too late, therefore this could have affected the time values. The problem could have been my eyesight or my reaction time, which could have altered the results of the experiments. To solve this problem, I could have used some automatic method like the light gate that I used for measuring speed.
Another problem I had while performing the experiment was controlling the surface area of the marble chips. They were not exactly the same, which meant that the experiment did not give the same results. Also the acid was constantly reacting with the marble chips and dissolving parts off them, which meant that the surface area had change over time. To solve this problem I could have used powered calcium carbonate instead, therefore the size of the chips, would be relatively the same. However, I did not use this because the acid would have reacted too fast with the powdered calcium carbonate and produced carbon dioxide at a very fast rate, pushing the gas syringe out totally. Therefore, I would not have been able to plot a graph.
While conducting the experiment I noticed the conical flask warmed up when the nitric acid and marble chips were reacting. I know that this is known as an exothermic reaction, where the reactants, in this case acid and marble chips, react together to give off heat so the conical flask warms up. To control this I used a water bath. Therefore, this keeps the temperature constant so that the particles have the same energy through out the experiments. To make sure that the temperature stays constant, I used a thermometer. However, I could have also measured the temperature at the beginning and at the end of the experiments, to make sure that they were the same. Therefore, the results would be more accurate and reliable.
As a conclusion, if I were to do the experiment again I would use a bigger range of concentrations to make the results more accurate and to make the graphs more reliable.
Chemistry coursework – Rate of reaction