- Wear safety goggles
- Mop up any spillages
- Wash chemicals that make contact with the skin immediately with running water
- Put stools under benches
- Put bags under benches
- Always stand when performing the experiment
Variables and constants
The only factor that I will be changing in this experiment will be the concentration of the acid. The concentrations of the acid that I will be using are 0.5m, 1m, 2m, 3m and 4m. 0.5m is the lowest concentration and 4m is the highest concentration.
The factors that I will keep constant in the experiment are:
- Surface area of marble chips
- Temperature – room temperature
- The absence of a catalyst
- Volume of acid
Surface area of marble chips – If the surface area of the marble chips is increased then the rate of reaction will increase because there is more space on the surface of the chips for the hydrochloric acid to react with. I will ensure that this is kept constant by using the same marble chips where possible.
Temperature – If the temperature is increased, then the rate of reaction will increase. ‘An increase in temperature of about 10ºC approximately doubles the rate of reaction. When a mixture of substances is heated, the particles move faster and two things will happen. The particles, because they are moving faster, will collide more often in a given time and they are more likely to be involved in more collisions. As a result of the particles moving at a faster rate a large proportion of the collisions will exceed the activation energy and hence the rate of reaction increases.
The absence of a catalyst – ‘A catalyst is a substance that changes the rate of a chemical reaction, but at the end of the reaction remains chemically unchanged. Catalysts usually speed up reactions’. If a catalyst is added to the reaction then the rate of reaction will increase. This is because catalysts provide an alternative pathway for the reaction. This pathway is one in which a much lower amount of activation energy is needed.’ By the activation energy being lowered more particles have enough energy to react.
Volume of acid – If the volume of acid is changed, then it means that there are more particles in the acid to react with the marble chips. If there are more particles present to react with the marble chips, then the rate of reaction will increase.
Measurements
For this investigation, I will take 20 readings over a range of 10 minutes. This will be done for each concentration of the hydrochloric acid. The readings will be taken at 30-second intervals. I will carry out preliminary work to test whether this range of readings is sufficient or too much.
Accuracy of readings
I will take the accuracy of my readings to 0 decimal places because the 500Ml measuring cylinder only has intervals of 10. It is however possible to get a measurement of x.5Ml (where x is the volume of carbon dioxide produced).
Accuracy of experiment
The clamp stand will hold the measuring cylinder vertical, which makes it hands-free which means that I can concentrate on recording the results. The fact that the experiment is hands-free means that the cylinder will not be slanted and so produce incorrect results.
Method
To perform the preliminary work and actual experiment I will use the following method to conduct the experiment.
First I set up the apparatus by filling up a water trough with water. I then put the clamp stand into the sink, and put the water trough on the base of the stand. I then put took a measuring cylinder and filled it up with water. This measuring cylinder was then placed upturned into the water trough carefully so as not to let any air into the measuring cylinder and so cause inaccuracies. The clamp was then moved down and attached to the measuring cylinder so that it was holding it with the mouth of the measuring cylinder below the water line. I then put the pieces of bent capillary tube in each of the bungs. I then attached the rubber tubing to each of the capillary tubes and attached the short capillary tube to both ends of rubber tubing. The short capillary tube acted as a connecter between the two lengths of rubber tubing.
After setting up the bung I then put the size 45 bung over the mouth of the 500Ml measuring cylinder if it was being used (if it was a different measuring cylinder I used a different size bung). After setting up the main apparatus I measured 3.0g of marble chips on an electronic top pan balance.
To calculate how much acid and water I needed to make the concentration I required, I used the following formula:
m = concentration of acid
t = total volume
v = volume of acid needed
I then put the marble chips into the conical flask and poured in 100Ml of hydrochloric acid of whatever concentration I were using, and immediately put the bung on the conical flask and starting timing the reaction.
The diagram below shows how the equipment was setup.
Preliminary work
Below are the results of the preliminary work.
O.5M
1.0M
2.0M
3.0M
4.0M
Obtaining evidence
The results of conducting the experiment are shown below:
0.5M
1.0M – Conducted using a 100Ml measuring cylinder
2.0M
3.0M
4.0M
Analysing evidence and drawing conclusions
From conducting this experiment I can conclude that as the concentration increases the rate of reaction increases. This is supported by the fact that the results show that in 6 minutes, 315 cm3 of carbon dioxide was produced by a reaction between 4M hydrochloric acid and marble chips. In another experiment using 0.5M hydrochloric acid, 19.0 cm3 of carbon dioxide was produced in 6 minutes. The increase in concentration of the acid caused the rate of reaction to increase because there were more particles in the same volume of acid. Since there were more particles present in the acid this meant that there was a higher chance for successful collisions with the marble chips. I can also conclude that as the reaction time increase the rate of reaction decreases (shown by a curve in a graph showing a reaction). The rate reaction as the reaction time increased because the hydrochloric acid in the reaction was being reacted to form water and carbon dioxide. The water caused the acid to become dilute which decreased the rate of reaction.
From what I can see on the graphs it seems that the graphs seem to be curving in the same places.
I have also proved the second part of my hypothesis. On the 1/t graphs, I can see that the gradients of the lines are starting to level off. The lines start off close to the Y-Axis and as time progresses they curve away from the Y-Axis and towards the X-Axis. This shows that the volume of carbon dioxide produced is not inversely proportional to time. In my hypothesis I stated that the volume of carbon dioxide produced would be not inversely proportional to time and my evidence has proved this.
Unfortunately however, I have disproved the third part of my hypothesis. In my hypothesis I did state that the increase in concentration of the acid and the reaction should be directly proportional. My results have however disproved this. This probably happened because I was using different amounts of marble chips because I cannot use chips that are exactly the same, unless if the chips are specially prepared, in which they were not. Also since the chips had one side covered the surface area of the chip was reduced which meant that that it took longer to produce more carbon dioxide.
It should be noted that the tables in the obtaining evidence section, the time column goes up to 600 seconds, however on the graphs the time scale goes up to 300 seconds. This has been done because one of the experiments only went up to only 390 seconds, so to be safe I chose 360 seconds as a cut-off point. All results on this table are deemed irrelevant.
The graphs are on the next 9 pages from page 19 to page 28.
1/t graphs
Evaluating evidence
From the evidence I can say that my plan has worked well and that there was sufficient evidence to prove parts 1 and 2 of my hypothesis. I can also say that my results were accurate and reliable enough to prove parts of my hypothesis, but not accurate enough to prove the third part of my hypothesis, however that is due to equipment problems such as the marble chips and the use of a 500Ml measuring cylinder. Also I can say that I have a suitable amount of results to prove my hypothesis. In order to prove the third part of my hypothesis I would probably have to use specially prepared marble chips with equal weights and surface area.
I can say that my evidence is fairly solid to support a conclusion that as the concentration of acid increases, so does the rate of reaction. However I must say that my evidence cannot support a conclusion on the proportionality of concentration and reaction time.
There were limitations of the data as a result of the equipment that I used. For the majority of the investigation I used a 500Ml measuring cylinder. The limitation of this equipment was that it did not start at 0Ml; instead it started at 50Ml and had gradations every 10Ml. This would have caused me to obtain inaccurate results at the start of the experiment since the carbon dioxide produced was below 50Ml. Also it required some guess work to work out how much carbon dioxide was produced when it was the reading was between the gradations. This again would have produced inaccurate results which probably lead to the disproving of the third part of the hypothesis. Another limitation of the data was that when I dropped the marble chips into the conical flask. As soon as the marbles came into contact with the acid, a reaction would have occurred and since the bung was being placed on flask some of the carbon dioxide would have escaped out of the flask into the atmosphere and not into the measuring cylinder filled with water. A final limitation of the data was that the marble chips were of the same weight but different surface areas. The change in surface area would have caused the rate of reaction to increase.
To improve the results I would change the method of conducting this experiment. Firstly instead of using an upturned measuring cylinder filled with water to catch the carbon dioxide produced from the reaction, I would use a gas syringe. Finally instead of using marble chips that are found, I would use specially prepared marble chips (if they are available) which have the same surface area. I would also probably repeat the experiment again a second time to see if there was something wrong with my results that I had obtain from conducting the experiment the first time, and then take an average of the results.
Extensions to the investigation
If I was to extend this experiment, then I would want see if the reactions slow down in the same places at higher concentrations. I would also try the experiment with other acids such as nitric acid and sulphuric acid. Also as an extension to the investigation I would want to see how the use of different carbonates such as copper carbonate. Finally I would want to see how surface area affects the rate of reaction and if there is a relationship between rate of reaction and see if there is a relationship between surface area and the volume of carbon dioxide produced.
Bibliography
To write this coursework planning section, I took samples of work from different sources. These samples have been but in quotation marks. The sources that I used were:
- Letts Revise GCSE Chemistry
- EssayBank.co.uk GCSE Chemistry Rates of Reaction coursework.
