I have chosen to do concentration of acid because surface area is difficult to measure, pressure is hard to do in a school lab, and temperature could be dangerous e.g. if acid boils it could spit.
Prediction:
I predict that as I strengthen the acid (increase molarity) the reaction will take place faster. This is because there will be more acid molecules in the same amount of liquid, so they are more likely to collide with the chalk particles, therefore causing a reaction. The results graphs will show completely straight lines, not curves like in Biology experiments. This is because the chemicals in this experiment are not living cells, so they aren’t denatured. (This also means this experiment can be carried out at almost any temperature.)
Method:
I’ll have the chalk in one test-tube and pour the measured amount of acid into it. The test-tube will be a special one with a delivery tube at the side so a measuring syringe can be attached. Once the acid has been poured in, I’ll put a stopper in the tube and switch on a stopwatch. Every 30 seconds I’ll take a reading off the syringe to see how much hydrogen gas has been collected.
Safety:
Because this is a strong acid, goggles and overalls must be worn, no running in the lab is allowed, and there are eye-washing tubes in the lab in case anyone gets any in their eyes. All stools and bags are under the tables and everyone must stand up as they do their experiments so they can run off if there’s any danger.
Fair test:
The amounts of acid and chalk stay the same – only molarity changes. It will always be the same chemicals used, and the experiment will always be at room temperature. The chalk will always be pieces of around the same size so there is always a similar surface area. If I shake the tube once, I’ll have to shake it every time, as this moves the chemicals around so some unreacted acid could come into contact with a part of the surface of the chalk which hasn’t reacted with anything yet, for example.
Diagram:
I chose this apparatus because once the rubber stopper has been put in, little or no gas can escape, so it has to go into the syringe, and pushes it out, giving a reading. I’m quite confident with using this apparatus for my work as I’ve used it before and it worked well. To make sure readings are accurate I’ll wash the test-tube each time in case there’s any chemicals left, and do at least 3 readings for each molarity. I’ll also do some trial runs first to make sure, but I won’t use these results in my final analysis.
Alternative equipment:
I chose not to use this because it would be hard to mix the two, put the stopper in, and the tube in – I’d have to be very fast and in the time lost some carbon dioxide could escape.
When the experiment starts it is tipped so the acid mixes with the chalk. I didn’t use one of these because I couldn’t find one.
Preliminary work:
I decided to take readings every 30 seconds because that is what I’ve done before in similar experiments, so it seems like enough. The chalk had been sorted into sizes 1-6, and I used size 3. The experiment worked well but I think I’ll use size 4, which is slightly larger, because it would have a lower surface area, and so take slightly longer, even if it is the same amount. Then it would be slightly easier to time the reaction, still taking readings every 30 seconds.
The hydrochloric acid available was 2M, so when I wanted something different I diluted it to get weaker strengths (lower molarities). For example, to get 40cm3 of 1M HCl, I mixed 20cm3 of 2M HCl with 20cm3 of water, or if I wanted 40cm3 of 0.5M HCl I mixed 10cm3 of 2M HCl with 30cm3 of water.
Results:
Gas produced:
Rates of reaction:
Conclusion
These results show that my prediction that the rate of reaction would increase as molarity of acid increased was right.
Evaluation
If I had to improve this experiment so it was more accurate I would do it with a larger size of chalk pieces so that it would take longer (smaller surface area) so my results would be more accurate, but if I wanted it to be quicker I could use smaller pieces. I could also use a wider range of molarities.
