3. , the surface area, is the main problem area. The surface area of the chips will have a big effect because for example you may have one large chip weighing 3g and many tiny chips collectively weighing 3g. The larger the surface area of the 3g of chips, the more area the hydrochloric acid has to react on. In this case it would mean that the large chip would react very slowly and the small chips would react very quickly. This is almost like the experiment I carried out between marble chips and chalk dust (see ‘Information from secondary sources/preliminary work’) where I observed a completely different rate of reaction between the two substances despite the fact that they both are constituted mostly from the same element (calcium). The difference between the surface area of the large chip and the surface area of all the small chips would be similar to the difference in surface area between the small chips used in the other experiment and the chalk dust. This will mean that I should choose chips with roughly similar surface areas. 4. may alter very slightly depending on the atmospheric pressure on the day, but probably not enough to make a significant difference to the rate of reaction.
Diagram
List of equipment
100ml beaker
3g marble chips
10ml hydrochloric acid (various concentrations)
Syringe
Pylon type thing
Pylon clamp
50ml glass measuring cylinder
Conical flask
Rubber and glass tubing with bung attached
Stopwatch
Pencil
Paper
Margarine tub full of water
Weighing scales
Fair test
To ensure that my investigation is a fair test I will:
- Carry out all the readings at room temperature.
- Use exactly 10ml of hydrochloric acid for each measurement.
- Read the measuring cylinder at eye level for all readings.
- Carry out a preliminary reading for one concentration to check that the test is fair.
Section 3
This section was not necessary since I only have one independent variable
Section 4
Information from secondary sources/preliminary work
Unfortunately I have no secondary sources for this investigation but i carried out some preliminary work at the start to see whether or not one of my results was accurate. If it turns out that the two results are similar it will suggest that the method that I am using produces similar results each time. I also carried out a similar experiment but the first time I used marble chips and the second time chalk dust. This may give reasons for any anomalies because surface area will be a factor to consider.
OBTAINING EVIDENCE
Results table
Detailed method
Having set up the apparatus, i weighed 3 g of marble chips (to the nearest 0.1g) and placed them in the conical flask. I then squirted the hydrochloric acid into the flask and quickly put the bung in. I then measured the amount of gas produced every minute and added each total to the one before I then washed out the conical flak and repeated the process.
ANALYSING EVIDENCE
Section 1
Graphs
Section 2
Process data and evidence
Generally, all the graphs support my first prediction. This can be noticed from the fact that the graphs for the higher concentrations of hydrochloric acid are both higher up on the graph, and steeper. Most of them have a curve that gets less steep. The graphs showing the higher concentrations of hydrochloric acid have a more curved curve, which shows that they are slowing down at a greater rate than the reactions involving lower concentrations of hydrochloric acid.
Section 3
Conclusion based on section 1 & 2 above
From my results I conclude that as the concentration of the hydrochloric acid is increased, the rate at which the marble chips react increases and that the anomalies are due to differences in surface area. I have also noticed that the reactions involving the higher concentrations of hydrochloric acid slow down at a higher rate.
Relate conclusion to detailed scientific knowledge
When the concentration of the hydrochloric acid is increased, there is more ‘pure acid’ to react with the marble chips because the rest of the solution is water (and the hydrochloric acid is dilute in the first place). This means that with a higher concentration of hydrochloric acid, more gas is produced from the reaction within the 3-minute time. The surface area is the main problem area. The surface area of the chips has a big effect because for example you may have one large chip weighing 3g and many tiny chips collectively weighing 3g. The larger the surface area of the 3g of chips, the more area the hydrochloric acid has to react on. In this case it would mean that the large chip would react very slowly and the small chips would react very quickly. This is almost like the experiment I carried out between marble chips and chalk dust (see ‘Information from secondary sources/preliminary work’) where I observed a completely different rate of reaction between the two substances despite the fact that they both are constituted mostly from the same element (calcium). The difference between the surface area of the large chip and the surface area of all the small chips would be similar to the difference in surface area between the small chips used in the other experiment and the chalk dust. The fact that the reactions involving higher concentrations of hydrochloric acid slow down at a higher rate is due to the fact that the reaction rate is so fast that there is not enough marble left towards the end for the reaction to reach its full potential. If I tried the same experiment for the same amount time but put in more marble chips, I would expect there to be less of a curve. The fact that the reactions involving lower concentrations of hydrochloric acid hardly slow down is due to the fact that the hydrochloric acid is a lot weaker so it can’t react as fast as the stronger concentrations and there is easily enough marble for it to work on at its full potential.
Relate to original prediction
This supports my original prediction that as the concentration of the hydrochloric acid is increased the amount of gas produced in the given time will increase.
EVALUATION
Accuracy of evidence
My results were not terribly accurate. Generally, the results don’t follow a pattern e.g. in the ‘3rd minute’ column you might expect the difference between the results to be either: +3, +4, +5, +6,etc, +3, +3, +3, +3 etc or +6, +5, +4, +3, etc but they weren’t. some of the differences were: +3, +3.5, +0, -2.5 which shows general inaccuracy. Despite this however, the graphs do support my prediction and they show correlation in their curves. The main reason for this inaccuracy is due to the surface area of the chips. The surface area of one 3g of chips can be a lot less or more than another, so it will have a bigger influence even the concentration of the acid. Other smaller factors such as putting the bung in too slowly after the acid was added to the chips would have affected the final outcome since gas would have escaped into the atmosphere rather than into the measuring cylinder where it could be measured. Also, at the end of each set of three readings for each concentration of acid, we washed out the conical flask so there was no acid left in it that would increase the concentration of the next 10ml of acid. However, in doing so, some water remained from the washing and this would have diluted the next bit of acid further and slowed down the rate of reaction. Another possibility, (although I am not so sure about this one), is that when there was either a lot of water in the ice cream tub, or not much of it the pressure was either higher or lower. Because the measuring cylinder is within the water of the tub the pressure would have been the same so it would have pushed down on the air trying to escape up the measuring cylinder and slowed the reaction down or even speeded it up because then the pressure in the conical flask would have been higher increasing the rate of reaction. One thing that I did not expect was the upward curve on some of the graphs of the lower concentrations of acid. I put this down to reused chips. Because some of the chips we used had been used before, they will have had a coating of something on them which means that in our reaction, when the acid has broken through this layer, the reaction will speed up. The reason that we notice this trait more on the graphs of the lower concentrations of acid was because the higher concentrations of acid broke through the layer very quickly and there fore no difference is noticed on the graph.
Improvement in accuracy
One way of improving the accuracy of this investigation would be to find a way of measuring the surface area and then work out the gas produced per square centimetre or use chips that are all exactly the same size and have been pre-cut into spheres. Another alternative would be to use a totally different substance such as magnesium, which is cut into identical strips, all with the same surface area. This would still enable us to investigate the same factor within rates of reaction but we would no longer have the problem of surface area. Curing the problem of putting the bung in too slowly is easy to solve-just put it in quicker and comical flasks should be dried out before carrying out a new set of readings.
Reliability of evidence
I think that the evidence is fairly reliable since the preliminary result that I carried out before the investigation for a concentration of 0.5 moles was almost exactly the same as the proper result. However, one result may not be enough evidence to determine the true reliability of the evidence.
Anomalous results
There were a number of anomalous results but the most easily recognisable was the one for 0.6 moles. This was actually slower to react than 0.5 moles. The reason that it was so anomalous was because it may have been a large chip therefore with a smaller surface area than multiple small chips.
Support for a firm conclusion
Looking at the graphs, I think that I have support for a firm conclusion in that the curves of the graphs follow a pattern and they support my prediction. However I am not sure how true my results will be outside of the range within which I carried out my investigation. On the graphs it is noticeable that the curves on them get less and less steep until they become upwards sloping. If I carried on the experiment within concentrations down to say, 0.01 moles I may find that there is a point when the marble just doesn’t react at all. At the other extreme, if I used an acid that was about 1000 moles it would probably produce less gas than 20 moles so if I had continued the experiment upwards I would have found the concentration of acid that produces the most gas.
Improvements to the method
To improve my range of results I could have done about 10 readings in a space of about 5 minutes so I will get a more accurate curve for my graphs and I would find out what would happen in a longer time span.
Additional evidence
To give extra support for my results I could carry out the same experiment but with a different substance such as magnesium. This would both improve the accuracy (see accuracy section) and give extra support for my results.
