Investigating the Effect of Acid Concentration on Marble Chips

Investigating the Effect of Acid Concentration on Marble Chips
Planning: The aim of this practical is to investigate the effect of varying acid concentrations on marble chips (CaCO3) 2HCL + CaCO3 à CaCl2 + CO2 + H20 Hydrochloric Acid + Calcium Carbonate à Calcium Chloride + Carbon Dioxide + Water Increasing the concentration of an acid increases the number of collisions between particles, therefore, the rate of reaction increases. The reaction will start faster in comparison to the speed at the end of the reaction. This is because when the two reactants are first mixed, their concentrations are both at their highest. As the experiment goes on, the concentration of the reactants will slow down, so the reaction will slow down until it finally comes to a stop. Constants: o The size of the marble chips ? all the marble chips used will be 6mm in length. This is because the surface area of the substance affects the rate at which the reaction takes place. A larger surface area means the reaction goes more quickly, so I need to keep the size of the chips equal so as to make it a fair test. o The mass of marble chips ? obviously, if the total mass of the marble chips is varied, the reaction will not be a fair test, so I must keep the mass the same.
By using the above methods, the amount will be kept the same o Volume of acid ? although the amount of hydrochloric acid needed to neutralise the marble chips differs according to the strength of the acid, the volume must be kept the same so as to make it a fair test. In order that the acid should not run out on the weakest concentration of acid, the volume will be 200cm3, which is enough to accommodate all of the concentrations. o The apparatus that I use will be kept constant throughout the experiment so that it is a fair test. o The surrounding conditions ? Air humidity, Temperature, Pressure. Although these may not have much of an effect, it is worth keeping these constant so that the experiment is perfectly fair. o Temperature of acid ? If you increase the heat of a reaction, it will speed up and so the test will not be fair. Prediction: This is an experiment designed to investigate the affect of varying concentrations of acid on marble chips (CaCO3). I predict that as the concentration of the acid is increased, the rate of the reaction will also increase. This is because of the number of collisions. As I said before, as you increase the concentration of the acid, the number of collisions will increase and so the rate of the reaction will increase. I predict that the resulting graph will look like this. [image] I decided to use 5g of marble chips, each sized 6mm. The ratio of hcl: CaCO3 is 2:1. This is because it is 2HCL + CaCO3 Therefore amount of CaCO3 = 5g ÷ 100g/mol = 0.05mol Therefore the amount of hcl needed to neutralise 5g of marble chip = 0.1mol I can therefore deduce that: Ø 100cm3 of hcl of concentration 1mol/dm3 will neutralise 5g of CaCO3 Ø 50cm3 of hcl of concentration 2mol/dm3 will neutralise 5g of CaCO3 Ø 33.3?cm3 of hcl of concentration 3mol/dm3 will neutralise 5g of CaCO3 Ø 25cm3 of hcl of concentration 4mol/dm3 will neutralise 5g of CaCO3 Ø 20cm3 of hcl of concentration 5mol/dm3 will neutralise 5g of CaCO3 Ø 16.6?cm3 of hcl of concentration 6mol/dm3 will neutralise 5g of CaCO3 I have therefore decided that I am going to use 6 different concentrations of hcl: 1, 2,3,4,5 and 6mol/dm3 I am going to use 200cm3 of each concentration. Method: Making up the concentrations - Volume required: 200cm3 To obtain: 1Mol/dm3 à mix 100cm3 of hcl concentration 2Mol/dm3 with 100cm3 of water 2Mol/dm3 à mix 200cm3 of hcl concentration 2Mol/dm3 with 0cm3 of water 3Mol/dm3 à mix 150cm3 of hcl concentration 4Mol/dm3 with 50cm3 of water 4Mol/dm3 à mix 200cm3 of hcl concentration 4Mol/dm3 with 0cm3 of water 5Mol/dm3 à mix 166.66?cm3 of hcl concentration 6Mol/dm3 with 33.33?cm3 of water 6Mol/dm3 à mix 200cm3 of hcl concentration 6Mol/dm3 with 0cm3 of water Experiment - 1. Make up the concentrations as above. 2. Assemble the apparatus listed below (fig 1). 3. Remove the bung and fill the conical flask with 200cm3 of the acid concentration 1mol/dm3. 4. Weigh out 5g of the marble chips, size 6mm. 5. Add the marble chips to the acid, start the stop watch and reseal the bung. 6. When the gas syringe reaches 50cm3 stop the stopwatch and remove the bung so that the gas can safely escape. 7. Record the time and repeat the experiment each time. 8. Do no.?s 2-6 with the other concentrations of hcl ? 2,3,4,5 and 6cm3. 9. Record all the results in a table. Preliminary work: For the preliminary work I looked at which size of marble chip should be used so that I obtained the best range of results. I tried out three sizes of marble chip: 2-3mm, 6mm and 9-10mm. The 2-3mm meant that the reaction happened too quickly, especially at the higher concentrations. This is because they have a larger surface area. With the larger size of marble chip, 9 ? 10mm the reaction went too slowly at the lower concentrations ad took at least fifteen minutes to generate 40cm3 of gas. I have therefore decided to use 6mm length marble chips. I also looked at what range of acid concentrations to use and I decided that the most suitable range was: 1,2,3,4,5 and 6Mol/dm3. By using these concentrations, I will be able to get a good range of readings and my results will be relevant. I also found out that there is a possibility of anomalies, for this reason I am going to take three readings for each concentration so that I can identify anomalies.. Apparatus: Þ Conical flask Þ Gas syringe Þ Rubber piping to link the above two Þ Safety goggles Þ Retort stand to hold up the gas syringe ? Paper towel to be wrapped around the parts that hold the syringe so as not to break it. Þ Volumetric flask Þ Pipette Þ Mass balance Þ Beakers of different size. Þ Stop Clock Þ Rubber Delivery Tube Þ Bunk [image] Safety note: Concentrated acid is very dangerous, when doing the experiment, I must be careful to ensure no acid is spilt as this could lead to corrosion work surface and skin. Also, I must be sure to wear safety goggles at all times when doing the experiment. Obtaining Evidence: Table of results: Concentration of acid mol/dm3 1 2 3 4 5 6 Time taken to reach 40cm3 s 1st reading 84.00 21.00 14.40 11.40 9.60 7.20 2nd reading 90.00 22.20 14.40 12.00 10.20 7.20 3rd reading 81.60 25.20 13.20 12.00 9.00 6.60 average 85.20 22.80 14.00 11.80 9.60 7.00 Using a precision maximising procedure: The following things should be done as well as the plan:
When doing the experiment I used a pipette to ensure that the
solutions were exactly the right volumes and concentrations.
I measured the time to the nearest millisecond. This meant that
the results would be more accurate.
I obeyed all of the constants so that the experiment was as fair a
test as I was able to produce.
Safety: When doing the experiment, I was as safe as possible. I made an effort not to spill any acid and I wore goggles throughout the experiment. Analysing Evidence: [image] The results show that as the concentration of the acid increases, the time taken to reach 40cm3 decreases. The graph shows that the inverse of the time taken and the concentration of the acid are in direct proportion. This means that as the concentration of the acid increases, the inverse of the time taken will increase proportionately. This shows, in theory, that the whole experiment is in equilibrium and that everything is balanced. However, this would only be the case if every point lay exactly on the straight line which the graph forms. This is not the case, however, the points do not all lie directly on the line, instead they are surrounding or very close to the line. This shows that the experiment is almost exactly in equilibrium but not quite. The equation of the line is y = 0.0243x ? 0.0085. This means that it is a straight line with gradient 0.0243 and it intercepts the y axis at -0.0085. The line does not go through the origin. This does not agree with theory, which suggests that the line should either go through the origin or it should cut the y-axis at a positive value. However my graph shows that the when the concentration of the hcl is anything less than 0.4Mol/dm3, the reaction will not occur. Theory suggests that an increase in acid concentration will cause an in increase in the rate of reaction. The reason that I plotted the inverse of the time on the y-axis was so that the gradient would be the rate of reaction. The gradient on my graph is 0.0243; this means that the rate of the reaction was 0.0243s-1/mol/dm3. The percentage difference in gradient between the actual line and the line of best fit is 0.0243/0.1 *100 = 24%. This is very large because I predicted that the gradient would 0.1, however, the actual 0.0243. I did not account or the fact that when I converted the time to the inverse of the time the gradient would change dramatically. I can therefore say that although I predicted that the graph would be a straight line, I did not correctly predict the gradient or the y intercept, as I suggested that the line would run through the origin. Evaluating: The experiment shows that the as the acid concentration increases, the time taken for 40cm3 of gas to be produced decreases. There were no real anomalies in this experiment as all of the points were very close to the line. This shows that the experiment was not necessarily very accurate, but it was a fair test and the experiment was conducted in almost exactly the same way each time. There was one fault in my method which was that from when I put the marble chips into the acid until when I put the bung in and started the time, some gas was lost. It also meant that I was not timing from the beginning of the experiment but from when the bung was put in. The reason that this did not affect my results was that I did it consistently; this meant that the results were correct in relation to each other but not fully accurate. A way to solve this would be to put the marble chips into a large weighing bottle and lower the weighing bottle into the conical flash. This would mean that the marble chips were in the flask but had not started the reaction. I would then put the bung in, and tip up the conical flask whilst starting the stop clock. This would tip up the weighing bottle and release the marble chips into the acid and the experiment would start. It would mean that no gas would be lost and the results would altogether be far more accurate. The experiment would also be able to remain a fair test at the same time. The repeat readings were, more or less, scattered in exactly the same time zone as the 1st reading. By this I mean that apart from the readings at concentration 1Mol/dm3, the repeats were all to within 5seconds of each other. I feel that, perhaps I could have used a two more repeat for each acid concentration as this would have made the results more reliable. At concentration 1Mol/dm3, the readings were scattered over a time range of 15seconds. This was expected as at the lower concentrations, it is harder to reproduce the experiment in exactly the same way so that the results are the same. As an extension to this experiment, I would go on to measure the change in mass over the course of the experiment. This means that I would put the conical flask on a mass balance and measure the mass after certain periods of time. There would not a bung in the top of conical flask, instead some cotton wool as this would allow the gas to escape but no air to get in. This would give another way to find out the rate of the reaction taking place between hcl and CaCO3. I can therefore conclude that the experiment was successful and I found out the aim of the experiment.

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