If the acid solution is made more concentrated it means there are more particles of reactant between the water molecules that makes collisions between the important particles more likely. Therefore, in my experiment, for each pan of chips (small, medium and large) I must ensure that the concentration of acid remains constant.
When the temperature is increased, the particles have more energy and therefore move quicker. If they’re moving quickly they are not only going to have more collisions but the collisions they have will be more forceful. In my experiment, the temperature will be the same all the way through.
A catalyst, which is a substance that speeds up the rate of reaction without being used up, could be used in the experiment. I’m not going to use a catalyst because I’m timing how long it takes to produce a gas, and if it was used, the time taken may be too quick for the reaction with small chips.
A solid in a solution can only react when particles collide with the surface.
The bigger the area of the solid surface, the more particles can collide with it per second, and the faster the reaction rate is. So in my preliminary work, before I can start my experiment, I need to find the right chip size to use for accurate results.
Apparatus I’m going to use.
A conical flask – to carry the gas from the flask into the cylinder of water.
A 50cm3 measuring cylinder – to hold the water in
A beehive – to hold the delivery tube in place and for the measuring cylinder to rest on.
A large pot filled with water to put the beehive in. This is where the method comes from, collecting a gas in the cylinder over a pot of water.
A stopwatch – To time the reaction
An electronic balance – to weigh 5g of marble chips
The apparatus is shown in the diagram below:-
Preliminary Work
Before I started my experiment, I needed to do some preliminary work to determine the best size of chips to use because, as stated earlier, the surface area of the chips has an effect on the reaction rate.
I put 5g of each chip size (large then medium then small) into six different concentrations of acid and recorded the amount of time it took for the water to be replaced by gas. This method is called: “collecting a gas over water.”
The equation for the reaction of Hydrochloric acid and marble chips is shown below: -
Hydrochloric acid + calcium carbonate = calcium chloride + carbon dioxide + water
2HCl(aq) + CaCO3(s) = CaCl2(aq) + CO2(g) + H2O(l)
The results from my preliminary work is shown in a table below:-
I decided to use medium marble chips because the reaction wouldn’t be too fast. Finding the right chip size to use helped me because if I’d chosen large or small chips I wouldn’t have been able to do the experiment properly because the reaction between the chips would have been either too slow or too fast.
The reaction can be followed by plotting the produced against time.
For the reaction with small marble chips, the reaction will be faster. Because of its large surface area, the slope will be steeper.
Note that the same mass of calcium carbonate (marble chips) will
give the same mass of carbon dioxide whether the chips are large or small.
The smaller chips will just do it more quickly.
Graph to show figure 1 results
Prediction
Using the collision theory, I predict that the lower the concentration, the more time the marble chips will take to react with the acid and produce gas in replacement of the water in the cylinder.
I predict this because, if the concentration is low, there are fewer particles to collide with each other. This means that there will be fewer collisions so the rate of reaction will be lower.
Method
Having determined the chip size to use during my preliminary work, I decided on the other factors that had to be kept the same to ensure that the test would be fair.
I kept the amount of acid the same by measuring it out into a 50cm3 measuring cylinder.
The marble chips were weighed using a digital balance because these are more accurate than analogue balances.
The time was measured using a stopwatch that I stopped when the gas completely replaced the water in the cylinder. I bent down to the level of the cylinder so that I could stop the stopwatch at exactly the right time.
I used six different concentrations of acid. These I put into six different beakers. The concentrations were all quite a distance apart to obtain reliable results.
To begin, I set up the apparatus (as shown in the diagram). I then weighed out 5g of medium marble chips and collected the six beakers of acid. I put the 5g of marble chips and the first concentration of acid into the conical flask and put the bung into the neck of the flask. Then the marble chips and acid started to react. I could tell this because there was effervescence in the flask. As soon as the acid was in, I started the stopwatch. I then watched the measuring cylinder carefully to see how long it took for the 50cm of water to be replaced by C02. I recorded my results and then repeated the experiment for the remaining five beakers of acid.
To ensure the most accurate results were obtained, I repeated the complete test again and averaged the results.
Results
Figure 2 - Table showing the time taken to collect CO2
Evaluation
In my plan I mentioned that my experiment would follow the collision theory. This is because we know that for a reaction to happen, the molecules have to collide. In my experiment, the marble chips’ molecules are colliding with those of the hydrochloric acid to produce a reaction.
The results I took were quite accurate but limited to the six concentrations I used. I measured the marble chips and acid accurately and recorded the exact time that the gas replaced the water.
If you look back at my graph above on this page you can see that the results I got were very accurate. This meant that I did the experiment right both times and made it a fair test. The graph’s shape is a positive skew, this shows that the higher the concentration, the lower the time taken for CO2 to collect in the cylinder.
The results for the dilution between 0.25 and 0.5 molar showed a steep drop from over 300 seconds at 0.25 molars for the cylinder to fill with gas, to 80 seconds at 0.5 molars. I didn’t expect quite a big a drop as that - I expected it to go down quite steadily. This could be an anomaly or it could suggest that the marble chips had more resistance to the lower concentrations of acid.
At 0.75 molars, the time taken levelled out to a more gradual decline indicating a more even reaction rate, which is what I would have expected to start with.
The experiment went very well, although there were some things that I could have improved upon if I did it again.
These are the areas I would change:
a) When I increased the concentration from 0.25 molers to 0.5 molers there was a steep drop in the time taken to collect CO2. Next time therefore, I would add more concentrations between 0.25 and 0.5 molar so that I could investigate this steep drop further, perhaps 0.3, 0.4 etc.
b) In addition I would also add more concentrations after 2 molar to see if they continued along the line shown on my graph or whether there was a rise. Due to the uncertainty around the first three readings, these would need to be excluded when drawing conclusions from my results. This therefore only leaves four readings from which you cannot draw definite conclusions.
c) I would change the mass. Instead of having 5g of marble chips I would have 10g. This means that I would also have to increase the amount of acid in the same proportion to make it a fair test; otherwise I would be changing the ratio of marble chip particles to acid particles.
d) I finally would change the time at which I measured the CO2 in the cylinder. I.e. when the cylinder is quarter, half or three-quarters full I would stop the stopwatch. I would do this to see whether the reaction started off slowly and increased as it went on, or whether it stayed the same speed throughout.
I would be interested to see other experiments like this one, because I could see if those results were similar to my results. I could then do a comparison of them and come up with a better set of results and an even better conclusion.
Conclusion
I found that when I took the moler concentration higher the time taken to collect the gas got smaller.
The conclusion is my results strongly support my prediction of the lower the concentration, the more time taken to collect the gas.
