The values of my variable (concentration), to make it a fair test will be:-
Acid Water
10 mls 40 mls
20 mls 30 mls
30 mls 20 mls
40 mls 10 mls
50 mls 0 mls
We have chosen to use 50mls of solution as that is a reasonable amount to use to react with the marble chips. We decided to have a difference of 10mls each time to compare the concentrations against one another; it gives us a big enough difference to get accurate results and come up with a more defined conclusion. We will do each concentration twice so that we can compare the results and work out an average. It also makes our experiment more accurate as it gives us the opportunity to pick out any anomalous results and it makes our final results more precise.
We will make our experiment a fair test by using the same person to start and stop the stop clock as each person’s reaction time may vary. Also we use the same person to pour the solution into the conical flask as people have different speeds and that may affect our results. We made sure that we had exactly 50mls of solution in the conical flask – to do this we used a pipette to measure the concentration of both acid and water accurately. We checked that the water we used was at the same temperature for each experiment as this could have affected our results by causing the water particles (if the water was warmer) to have more energy making them move about more and increasing the chance of gas particles bumping into them and slowing down the time it took for the gas particles to reach the surface.
During our experiment, we could only change one thing. We chose to change the concentration as we thought that this was the best way to test the rate of reaction. There are 2 more ways of testing a reaction rate, 1) the temperature and 2) the size of the solid particles (surface area). As we chose the concentration, we had to keep the other two the same so that the concentration could be tested fairly on the same size of marble chips and the same temperature. If we didn’t keep the other two the same, it wouldn’t be an accurate test as nothing could be proven and the results for each concentration couldn’t be compared solely on the effect that the concentration had on the reaction time.
Preliminary Results
Before we started our experiment we carried out some preliminary results, the first being ‘how many marble chips we should use’- this was our calculation to decide how many we needed to react with 50mls of solution.
CaC03 + 2HCl → CaCl2 + CO2 + H2O
(Marble chips + Hydrochloric acid → Calcium chloride + carbon dioxide + water)
2 moles of HCl means that there are 2 moles of HCl in one litre
HCl = 1+35.5 = 36.5 so 2 moles = 36.5 x 2gm = 73gms/litre
If we use 50mls of acid, it will contain:
7 ÷ 1000 x 50 = 3.65gm
Moles = mass ÷ Mr = 3.6 ÷ 36.5 = 0.1moles
From the equation: 1 mole of CaCO3 reacts with 2 moles of HCl, so
0.5 “ of CaCO3 reacts with 0.1 moles of HCl
Mr of CaCO3 = (40 + 12 + 16 + 16 + 16 = 100)
Mass of CaCO3 = 0.5 x 100 = 5gms
5gms of marble chips would react exactly with 50mls of acid but to make that all the acid reacts, I would use 6gms of CaCO3 as this is excess.
So once we had calculated the amount of marble chips we were going to use, we then had to decide upon what size marble chip. We tested the large and small chips to see which one gave us more consistent results.
We only used the highest and lowest concentrations to compare the results. The small chips have a larger surface area than the large chips, creating more surface area for collisions to occur. This is why we chose to use the smaller chips so that the acid particles had more area on the marble chips to react with and give more accurate results. Also, the more collisions that occur, the faster the rate of reaction, the more spread out and the more defined the results are. (Another bit of detail that we found out from doing our preliminary results was that reaction was exothermic and gave off energy).
The preliminary results helped us plan our final experiment as it clarified the reaction between the marble chips and the acid so we knew what to expect and gave us an idea of the speed in which it would take for the small marble chips to react with the acid.
RESULTS
Volume of gas collected
Volume of gas ÷ time = rate
This was another calculation I did to reach a conclusion. It measures the rate of the reaction between the different solutions after 3 minutes. I drew up a graph so that I could identify any other patterns in the results.
ANALYSIS
The graph that shows the average volume of gas made by the marble chips and hydrochloric acid, for each concentration it steadily increases as the time increases eg 1 minute = 20mls, 2 minutes = 26mls and 3 minutes = 32mls etc. This is because more gas is being made every minute so therefore the greater the time, the greater the volume of gas made. Also, the more concentrated the solution the steeper the line of average gas made. For instance, 10-40mls = 14mls of gas made on average, 20-30mls = 86mls/gas and 30-20mls = 150.5mls/gas. This is because a higher concentration contains more acid particles, in the same volume, that will collide with the marble chip particles. With there being more collisions it will increase the number of successful collisions, and the more successful collisions then the more powerful and the more faster the reaction. A reaction that has more power (energy) produces gas faster increasing the rate of reaction, and more gas is produced in the same length of time.
Therefore a less concentrated solution gives off less gas after that certain length of time. At the start of the reaction, the line is steeper meaning that the reaction is faster as there is plenty of acid particles that can collide with the marble chips and more gas can be made. After time, there are fewer marble chip particles and the acid becomes less concentrated, as during the start of the reaction they get used up so towards the end, the reaction slows down. This is shown on the graph as the slope of the reaction curve decreases and eventually becomes flat. When the line continues to stay flat, this means all the reactants have been used up and no more gas can be produced.
The line graph to show the average rate of reaction steadily increases the more concentrated the solution becomes. More gas particles are being made faster so by dividing the volume of gas made with the time taken (3 minutes) means the rate will be quicker. Also if I compare the volume of gas collected to the concentration eg 20/30 (3 mins) gas collected average= 189. 5 and if you double the concentration eg 40/10 (3mins) gas collected average= 495 the volume of gas collected doesn’t exactly double. If I compare the concentration with the rate of reaction, from my evidence I can conclude that doubling the concentration does not double the rate of reaction exactly. eg 20/30 (3 mins) rate= 63. 2
eg 40/10 (3 mins) rate= 165
The conclusions that I have made from my results are:-
- The more concentrated the solution, the faster the rate of reaction.
- A reaction is always faster at the start and that is when most of the gas is made.
- At the end of the reaction, all the reactants have been used up so no more gas is produced.
- There is always some acid and marble chip particles left because not all of the collisions are successful and they don’t all have the activation energy.
- The energy is the same all the way through the reaction as the temperature was kept the same, it was only the concentration (the amount of acid particles) that was changed.
My conclusions compare to my earlier predictions as I predicted that if I increase my variable (concentration) then the rate of reaction would also increase, due to there being more acid particles in the same volumes, so more successful collisions and the reaction speed would increase. My conclusions overall clarified my prediction made earlier in the experiment.
Although there is a slight error in my results, in that the 40/10 concentration, at the beginning of the reaction, was more faster and produced more gas than the 50/0 and it wasn’t towards the end that the 50/0 average volume, overlapped the 40/10 as it had more acid particles that collided with the marble chips so therefore the reaction could continue for longer. As the 5 mins was running out and the reaction was still producing more of gas as it has more particles and more successful collisions which produced more gas in the same length of time; This error may well of been caused by temperature differences in the lab but despite this error I have enough results to confirm that increasing the concentration increases the rate of reaction.
EVALUATION
I thought that overall it was a successful experiment as I got a lot of results quickly and they were quite constant. I got some differences, but not many and I got a good line of best fit for my average rate of reaction. There were a few odd results but the main one was overlap of the average volume of gas between 50-0mls and 40-10mls solutions. The less concentrated solution started off producing more gas but towards the end of the reaction is began to decrease and the more concentrated solution continued to produce gas as it had more acid particles in the same volume so therefore it took longer for them to be used up.
Another odd result was on the average rate of reaction graph for the most concentrated solution. The error looked to be more greater, as the graph uses a smaller scale. This anomalous result could have been caused by human error such as not starting the stop clock straightaway or not sealing the conical flask with the rubber stopper fast enough so some of the gas may have escaped.
On the other hand, the error could have been caused by:-
- Gas dissolving in the water.
- The surface area not being constant throughout the experiment.
- Some of the gas could have been lost during the exchange of the glass measuring cylinders.
I could have made my experiment more accurate and reliable by:-
- Using powder instead of marble chips would prevent the variation in surface area as the powder has a larger and constant surface area making the rate of reaction much quicker. Powder can also be measured more accurately.
- Using a gas syringe to collect the gas given off from the reaction would remove the solubility problem, however, there is a slight error in that some gas will be lost during the exchange of the syringe. In our experiment we collected gas over water causing some of the gas to be dissolved as it rises to the surface which produces a lower result.
- Doing the experiment in an electric water bath would remove the temperature problem. With us doing our experiment over a period of two days, the temperature in the Lab may have changed, affecting our results by causing a slight catalyst, speeding up the rate of reaction.
Alternative measuring equipment could also compliment to the accuracy of the experiment. I could have used a burette to measure the exact concentration of the dilute acid instead of a measuring cylinder. It allows you to control the amount you add, drop by drop rather than pouring large amounts at one time that could be above or below your proposed measurement.
To measure the exact amount of gas made I also could have used a gas syringe, again this would be far more accurate than using glass measuring cylinders filled with water. As gas is being made from the reaction, it rises up the flask and pushes its way into the gas syringe forcing the plunger to move out and then the volume of gas can be measured accurately.
There is yet another way in which I could have approached this experiment and back up my conclusion.
Instead of collecting the gas given off in the reaction, I could have recorded the mass loss. This would enable me to record how much mass was being lost as the reaction occurred. The more gas given off, then the more concentrated the solution. If we measured the mass loss to 4 decimal places, this would be a very accurate way to follow the rate of reaction. (Also by using powdered marble chips, this would produce a good mass loss and give us a wider range of results that were more consistent).
Mass loss experiment
Place flask onto a balance and add the marble chips , straight away, once you’ve poured in the concentrated acid start the stop clock and seal the top of the flask with a plug of cotton wool to prevent too much gas from escaping.
As the reaction is occurring, gas is given off, so the mass will decrease as more gas given given off. A more concentrated gives off more gas so will also have a grater decrease in mass loss.
The evidence from my results are reliable and conclusive enough to support my conclusion that increasing the concentration increases the rate but I have not gathered enough accurate results to justify that doubling the concentration will double the rate of reaction; I would have to do more repeats of the experiment to accurately confirm this prediction
