e.) pressure
A higher pressure results in a faster rate of reaction. This is because an increase in pressure ultimately results in an increase of concentration, resulting in a greater chance of successful collisions. This does not apply to my experiment as I am not considering any gas.
For my investigation I decided to investigate the way in which different concentration levels of sulphuric acid affect the rate of reaction, and in the next section I will make some predictions.
The reason I chose to investigate this topic for my coursework is because I have already studied and read about it as part of my G.C.S.E syllabus and so I am knowledgeable and dextrous with all areas of the investigation. It is also a practical investigation and a topic which I find relatively interesting.
Section 3 – prediction
Prediction1:
I also predict that as you increase the concentration of sulphuric acid the rate of reaction will also increase.
Prediction2:
I further predict that the rate of reaction will be proportional to the concentration of acid. For any given quantity and surface area of zinc, doubling the concentration level of sulphuric acid will double the rate of reaction. Therefore the time taken to produce any given volume of hydrogen will be halved.
From my theory above, I saw that with a higher concentration of acid will result in a faster rate of reaction because there are more sulphuric acid particles which leads to a greater chance of them reacting with the zinc. With a lower concentration the particles of sulphuric acid are more spaced out with more water present which reduces the chance of them reacting with the zinc.
Prediction3:
For any given concentration of sulphuric acid and given amount of zinc, the rate of reaction will be fastest at the beginning but will slow down overtime.
At the beginning of the process there are many zinc atoms and acid particles, however as the reaction continues both the zinc atoms and acid particles are used up and after some time there are few zinc atoms and the acid solution becomes more dilute. For this reason the rate of reaction will be fastest at the beginning and slow down over time. This is shown in the diagram below.
Fig1 Expected Rate of Reaction
Section4 Method and Fair Test and Apparatus used
Measuring the rate of reaction.
To measure the rate of reaction I can either measure
a.) the amount of the reactants used up per unit of time
b.)the amount of the product produced per unit of time
in the case of the reaction between zinc and sulphuric acid, I could either measure
- the amount of zinc used up per unit of time(per min)
- the amount of sulphuric acid used up per unit of time
- the amount of zinc sulphate produce per unit of time
- the amount of hydrogen produced per unit of time.
In my investigation I plan to measure the amount of hydrogen gas produced per unit of time as it the most practical of all my options.
Apparatus:
Zinc in units of 2g
Sulphuric acid 20cc of 0.5M, 1M, 2M, 3M, 4M
Test tube – to hold the zinc and sulphuric acid while reacting
Bung- to stop any hydrogen from escaping
Delivery tube- to carry the hydrogen into the measuring cylinder
Beakers- …ml - to hold the sulphuric acid
Clamp stand – to hold the test tube
Measuring cylinder-…ml- measure the amount of hydrogen produced
Matches and splint- to test for hydrogen
Thermometer- to measure the temperature of acid before and after
Basin with water – to ensure no other gas is in the cylinder
Electronic top pan scale- to measure the mass of zinc
Scissors- to cut the zinc into appropriate size
Stopwatch – to time how long it takes to produce 50cc of hydrogen
Sand paper- to clean zinc
Test and result sheet- to record my results
Method
Description of methodology for test
The purpose of my experiment is to investigate the affect of different concentration levels of sulphuric acid have on the rate of reaction when mixed with zinc. To see how concentration affects the rate of reaction I will have to keep all other factors constant. In particular the volume of acid, the mass of zinc, the surface area of the zinc and temperature of the acid at the start. A diagram of the apparatus is shown below.
First the apparatus is set up; I set up the clamp stand, test tube and delivery tube. I then filled basin with water and inside the basin filled measuring cylinder up with water until there are no air bubbles in it. Then I used another measuring cylinder to measure a set volume of sulphuric acid, yet to be decided, at a specified concentration, I then add the sulphuric acid to the test tube. Next I measure the temperature of the sulphuric acid and recorded it on my test sheet. The zinc is then weighed to correspond to a weight of 2.00g, it is then cleaned with sand paper and placed inside the test tube along with the sulphuric acid, the bung is then placed on the top of the test tube to ensure that none of the hydrogen yield escapes into the atmosphere, as this is done, simultaneously a colleague of mine presses the start button on the stopwatch in order for me to record what volume of hydrogen is produced
I then pull the hydrogen filled cylinder out of the water and put my palm over it, I then position a lit splint in at over the opening of the cylinder and remove my hand. This is to test whether or not the gas yield is hydrogen. I then measure the temperature of the test tube and recorded it on my test sheet. I then separate the zinc from the zinc sulphate and measure the weight of the zinc. I repeat this three times for each concentration and took the average in order to eliminate bias or human error.
For the first experiment, where I was trying to find out how long it took to produce 50cm3 of hydrogen, I shouted out when 50cm3 of hydrogen had been produced for each concentration of sulphuric acid and my friend recorded the time.
In the second experiment, where I was trying to find out the rate of reaction for each 30second time interval. A friend shouted out after each 30seconds had past and I recorded how much hydrogen had been produced.
Fig2 Apparatus used.
Section 5 Preliminary Experiment
Before I carry out the proper investigation, I need to undertake a preliminary experiment in order to highlight any inefficiencies in my methodology so that I could amend them for the real experiment to produce a more accurate set of results.
I used the preliminary experiment:
1. To confirm the amount of zinc to use and concentration of sulphuric acid.
2.To establish the induction period, how long it takes for first bubble to appear in order to start timing.
3. To establish how much hydrogen is produced for the first experiment.
4 To establish the most suitable intervals of time or hydrogen, and decide whether to record time taken to produce given amounts of hydrogen or hydrogen produced in given slots of time.
From my preliminary experiment I learned that 2grams of zinc and 20cm3 of sulphuric acid was suitable. For my first experiment I chose to use all five concentrations available, whereas for my second experiment I chose to use only 2M and 4M. I also established that 50cm3 was a suitable amount of hydrogen yield, as it wouldn’t take too long. For my second experiment I allowed for 100cm3 to be produced. I chose to measure hydrogen yield against allocated time slots of 30seconds.
Section 6, Results
Obtaining Evidence. The results of my experiments are shown below;
Table 1 Reaction times of different concentration level to produce 50 cm3 of Hydrogen
Table 2 Average amount of zinc used. to produce 50 cm3 of Hydrogen
Table 3 – Reaction rates for producing hydrogen 2M conc of sulphuric acid.
I only used the averages of test one and 2 because test3’s temperature distorts the results.
Table 4 – Reaction rates for producing hydrogen 4M conc of sulphuric acid.
Section 7, Analysing Evidence and Drawing Conclusions
Fig3 – Time taken to produce 50 cm3 of Hydrogen at different conc levels. sulphuric acid.
I predicted that the higher the concentration level of sulphuric acid, the faster will be the time taken to produce a given quantity of hydrogen gas. In my experiment the given amount of hydrogen was 50cm3
The results of the experiment are shown in Tables 1 and figure 3 above. At a concentration level of 0.5M, the average time taken was 22.67mins to produce 50cm3 of hydrogen. When I used a concentration level of 1M the time taken was 11.39mins. At 2M the time taken reduced to 4.13mins and at 4M the time taken fell further to 1.69. The results support my prediction that an increase in concentration will reduce the time taken to produce 50cm3 of hydrogen. This is shown in Fig 3.
I also predicted that the time taken will be proportional to the concentration levels. If you double the concentration you will halve the time taken to produce a given amount of hydrogen. From my results in table 1 we can see that when the concentration doubled from 0.5M to 1M the time taken to produce 50cm3 fell from 22.67mins to 11.39mins which is a reduction of 50%. We can also see that as 1M was doubled to 2M there was a decrease from 11.39mins to 4.13mins which is a reduction of 39%. Finally as 2M was doubled to 4M there was a reduction from 4.13mins to 1.69mins. This was a decrease of 41%. My conclusion is that the results recorded roughly support my prediction that rate of reaction is proportional to concentration of acid, though there seems to be some deviation
I also calculated the reaction rates in table1. At a concentration of 0.5M the reaction rate was 2.2cm3/mins as I double the concentration to 1M the rate increases proportionally to 4.4. I recorded 2M which produced a rate of 12.1cm3/min which is 3 times the rate of 1M. As I doubled the concentration from 2M to 4M the rate increases by of approximately 2.4.
Fig4 – Mass of zinc used up in reaction
Referring to the table2 and figure4 we can see that the amount of zinc used up in my experiments to produce 50cm3 of hydrogen was approximately 0.30g.
Fig5 – Rate of production of hydrogen
I also predict that for any concentration level the reaction will be fastest at the beginning and then slow down after some time as the zinc atoms and sulphuric acid particles are used up. I carried out my experiments with two concentration levels (2M and 4M.) My results are shown in table 4 and fig5.
From table 4 we can see that for the 2M concentration the first 30second produced 1cm3 of hydrogen from 0.5 to 1min (rate of 0.03cm3/second) 3.5cm3 of hydrogen was produced in the following 30seconds (0.12cm3/second) the next 30seconds produced 4.5 cm3 (0.15cm/second) and then 6cm3 (0.2cm3/second) and the highest which was 16cm3 produced from 3.5 to 4mins(0.53cm3/second). There seemed to be a problem in the production of gas because only 6.5cm3 was produced between 3 and 3.5mins.
For the 4M solution, the first 30 seconds produced a greater amount of hydrogen at 6cm3 (0.2cm3/second) in the following 30seconds 12cm3 was produced (0.4cm3/second) in the next 30 seconds between 1 and 1.5mins 22cm3 was produced (0.73cm3/second). Between 2 and 2.5mins 27cm3 was produced (0.9cm3/second.) After 2.5mins I could no longer take any readings because the measuring cylinder had been filled with hydrogen.
The results show that for both 2M and 4M the production starts off slowly and then increases in rate. This does not support my prediction. I will need to explain why this is in my evaluation. The graph also shows that the 4M solution produces more hydrogen and at a faster rate than the 2M this is shown by the slope and height of the graphs for the two solution.
Section 8, Evaluation.
In order to evaluate my work, I looked at the methods and techniques I used, the quality of data I collected and the results I obtained.
There were a number of issues which affected the accuracy of my experiments. I was investigating concentration and tried to keep all other factors constant. Unfortunately this was not always possible.
I kept the mass of zinc constant at 2grams but I could not keep the surface area the same for each test. This would affect the results. Another factor with the zinc was the impurities formed on the surface which would delay the reaction with the sulphuric acid. I tried to clean the zinc with sandpaper, but it was not always available.
Another factor that I had to keep constant was the temperature; again there were problems, as my results tables show. In my first experiment, the temperature at the beginning ranged from 19ºC - 21ºC and this would have affected the time taken to produce 50cm. in the second experiment, to see how the rate of reaction changes, I had to discount the third reading for 2M because the temperature was significantly low (21 ºC). Even though I ignored the high temperature, there were still differences in temperatures at the start of the experiment and this would have affected the rates of reaction and results I obtained.
I used a measuring cylinder to ensure I used 20 cm3 of sulphuric acid, I am confident that they were relatively accurate, however there may have been one or two cases where the amounts were wrong.
There was also the possibility of human error, when looking at the amount of hydrogen produced and the time taken. My friend called out the 30second intervals while I observed the hydrogen yielded.
I tried to reduce the bias and error by taking 3 measurements for each concentration.
The first prediction I made was confirmed by my results, with the higher concentration producing 50cm3 of hydrogen at the fastest rate. The second prediction about the proportional relationship was not fully supported by my results. The reason for this could be some of the problems I raised above in particular temperature and surface area of the zinc. For the third prediction I would have expected the rate of reaction to be fastest at the beginning and then to slow down as more and more zinc atoms and acid particles were used up. My results did not confirm this because the rates were slowest at the beginning and then grew faster. This was probably because of the induction period at the beginning and the impurities on the zinc surface which could have slowed the reaction down at the beginning. I was not able to see a reduction in the rate of reaction as I expected because hydrogen was still being produced when I stopped my experiment and had failed the 100cm3 cylinder I was using, thus I would have needed more time and a bigger cylinder or a smaller amount of the zinc and sulphuric acid.
How could I improve the experiments?
To achieve better results I could make sure that the surface area of the zinc was the same for each experiment by using a machine to cut out the zinc. I could also ensure that the zinc was free of impurities. I could also make sure that the experiments temperature was kept constant for each test. I could also try to minimise the human error factor of measurements of hydrogen and time by having an alarm clock to beep at specified times, and a dry board marker to mark on the levels of hydrogen produced at each time interval. I could also use a bigger cylinder to ensure that I can carry out the experiment to the end, or alternatively I could reduce the amount of zinc used.
What further tests could I do?
I could carry out a test that would enable me to measure the rate of reaction until the end when the total amount of hydrogen had been produced. This would allow me to observe the falling rates of reaction as the acid and zinc were being used up. I could also see what impact additional grams of zinc would have on reaction time.
I could also see how the other related factors affect the rate of reaction. For example for surface area, I would keep the mass of zinc constant but in different forms up to powder form and see what impact this has on reaction rates. I could also look at temperature and carry out tests to see how a degrees increase impacted the rates of reaction. I could also carry out a test to investigate what impact a catalyst would have on a rate of reaction.