How Concentration of Acid affects rate of reaction.

Chemistry Coursework – How Concentration of Acid affects rate of reaction

Plan

I shall investigate whether the concentration of an acid affects the reaction rate between an acid and a carbonate.

I shall vary the concentration of acid, but keep the other potential variables constant. These include:

Time – if an experiment has longer to take place than another, unless the reaction has finished, differing the time over which the experiment will take place will distort the results. This is because it has longer to react. So there is longer for the reactants to come into contact with each other. I shall use a stopwatch to keep the time constant between experiments.

Heat – if the heat of the solution and/or chips is different between experiments, this will affect the result. Increased heat means that molecules are moving faster. This will lead to more molecules of acid coming into contact with the carbonate in the same timeframe. Crucially, it will also mean that more molecules collide with enough force to power a reaction. Both of these reasons mean that heat should be kept constant between experiments.

Ideally, the conical flask should be placed in a water bath to regulate the temperature. However, due to time constraints, this was not possible, so I could not use water baths and had to rely on a constant room temperature.

Surface area of chips – if one chip has a higher surface area but the same mass, than another, and they are both placed in an acid solution, the chip with the larger surface area will come into contact with more acid molecules, so it will react faster as there are more possible areas for the reactions to take place.

Mass of chips – if two chips had the same surface area at the start of the experiment, but one had a higher mass than the other, it would take longer to dissolve and so during the experiment it would have a higher surface area, meaning more area to conduct reactions. This would mean that it would be an unfair experiment.

Amount of acid – during the experiment, if the reduction in the amount of acid in the conical flask means that all parts of all chips are not in contact with acid, it will also distort the results. The chips will invariably be slightly different sizes, so in one experiment, more % of the chips could be in the solution than in others, leading to more surface area being in contact with the solution. This leads to an unfair experiment.

Material of Carbonate – changing the material the chip is made of will change its properties and so it will probably change the reaction time too.

Changing the acid – N.B. not the concentration of the acid, as this is the independent variable. Changing the acid will mean a change in the amount of H+ ions dissociated and so will change the strength of the acid.

Shaking / Stirring – this will mean the carbonates will come into contact with more molecules of acid (although at one time they will only be in contact with the same amount), increasing the chance of a carbonate molecule colliding with an acid molecule, with enough combined force to react. Although it is possible to stir/shake with the same vigour and for the amount of time, for each experiment, it is highly unlikely, so the easiest way to uniform the amount of shaking/stirring is simply to do none at all.

Equipment

Hydrochloric acid – I tried using sulphuric acid in the preliminary experiment, but the reaction was over too quickly to accurately record the results for the quickest acting(most concentrated) acid. Remember to wear safety spectacles when using hydrochloric acid, as it is an irritant if it meets the eyes.

Distilled water – to dilute hydrochloric acid to different concentrations.

2g of 2-4mm calcium carbonate chips – after selecting the acid, I chose this size of chip because it was the smallest possible size after powdered calcium carbonate (which reacted too fast in the preliminary ...

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Equipment

Hydrochloric acid – I tried using sulphuric acid in the preliminary experiment, but the reaction was over too quickly to accurately record the results for the quickest acting(most concentrated) acid. Remember to wear safety spectacles when using hydrochloric acid, as it is an irritant if it meets the eyes.

Distilled water – to dilute hydrochloric acid to different concentrations.

2g of 2-4mm calcium carbonate chips – after selecting the acid, I chose this size of chip because it was the smallest possible size after powdered calcium carbonate (which reacted too fast in the preliminary experiment to accurately record the results). This means that it is the easiest size with which to accurately reach 2.0g. As the chips get larger, it becomes increasingly difficult to make the total mass of the chips to equal 2g (If the total mass of the chips in the plastic tray was 1.7g, and the large chips had a mass of 0.6g, the nearest value 20 2.0g would be 0.3g away from it. If the chips weighed .4g (the smaller ones) it would mean another could be added to the tray and the mass would only be 0.1g away.) I also chose the 2-4mm size as the bigger chips reacted too slowly (as they had a lower surface area : mass ratio) to record 6 values of concentration.

Plastic tray – to place chips in so they can then be placed on the digital scales and carried back to the experiment.

Scales – to find the total mass of the chips in the plastic tray. The scales are sensitive to the nearest 0.1g, which is acceptable as it has an error bound of only 0.05/2.0 x 100 = 2.5%.

20ml-measuring-cylinder – to measure acid and distilled water in order to produce accurate concentrations ready for the experiment. As I was measuring 20ml of solution, it would be the most sensitive measuring cylinder that I could use.

Conical flask – to hold the solution and the calcium carbonate. It has a great advantage over other instruments in which the reaction takes places for this experiment, as the conical flask has a rubber tube exiting from it and a bung. This means the amount of gases created in reactions inside the conical flask can be measured if the correct instrument(s) are placed at the end of the tube.

10cm3, 20cm3, 50cm3 and 100cm3 measuring cylinders-to collect gas produced. (These will be filled with water at the start of the experiment.) Different measuring cylinders were chosen as this means I can more accurately collect the amount of gas produced, as some measuring cylinders are more sensitive than others are, but some can hold more than others can. This could have changed the results slightly, (if the marking were not consistent with the volume) however, I felt this was justified because it gives increased sensitivity.

Pneumatic trough (filled with water) – this keeps the water in the measuring cylinder until it has been displaced by the gas passing out of the tube from he conical flask. When this happens, it gives the displaced water a place to go.

Stopwatch – to inform when to take readings (30s). I chose 30s from the preliminary experiment because the lower concentrations of acid did not create significant amounts of gas before this. It also allows more values of concentration to be made (if the experiment took longer, thee would be less time to retake results or to take more values of concentrations of the solutions).

Working Method Used

Set up ‘permanent’ apparatus (does not need to be changed between experiments)

Fill pneumatic trough with water
Clamp boss clamp to retort stand

Set up ‘moveable’ apparatus (needs to be changed between experiments)

Fill required measuring cylinder with water and clamp it to boss clamp, so open end of measuring cylinder is in water in pneumatic trough. Take care not to spill any water from the measuring cylinder. (From preliminary experiment I found that I should use 10ml measuring cylinder for 0.0M and 0.4M solutions, for 0.8M solution, the 20ml measuring tube is used, for 1.2M and 1.6M solutions, the 50ml measuring cylinder is used and for the 2.0M solution, the 100ml is necessary. I chose these cylinders for the concentrations because they were the lowest measuring cylinders available for the amount of gas produced (including a little room for safety)).
Thread tube from conical flask through pneumatic trough and into measuring cylinders.

Place plastic tray on scales and add 2.0g of 2-4mm calcium carbonate chips onto it (it is a good idea to ‘zero’ the scales onto the plastic tray)

Measure solution into 20ml measuring cylinder. For a concentration of 0.0M, 20ml of distilled water is needed. For a concentration of 0.4M, 16ml of distilled water and 4ml of 2.0M hydrochloric acid is needed. For a concentration of 0.8M, 12ml of distilled water and 8ml of 2.0M hydrochloric acid is needed. For a solution of 1.2M, 8ml of distilled water and 12ml of 2.0M hydrochloric acid is needed. For a concentration of 1.6M, 4ml of distilled water and 16ml of 2.0M hydrochloric acid is needed. For a concentration of 2.0M, 20ml of 2.0M hydrochloric acid is needed. Pour the solution into the conical flask.

Drop chips into conical flask, place bung on top and start stopwatch.

Monitor stopwatch and when it reaches 30s, look to see how much gas was displaced from the measuring cylinder and record this.

Clean apparatus and repeat steps 2-7 for different concentrations of acid.

Other Notes

I am measuring the amount of carbon dioxide produced. This is because it is the gas formed in this reaction: calcium carbonate + hydrochloric acid → calcium chloride + water +carbon dioxide.

The strength of the acid is governed by the % hydrogen ions which dissociate. Hydrochloric acid is a very strong acid because the hydrogen ion dissociate from HCl to form H3O+ ions when combined with water. There are no molecules of HCl found in the solution (they are now Cl- and H3O+). The remaining water molecules remain as H20 This means it is considered a very strong acid. A weaker acid, like ethanoic acid, only slightly dissociates. Some hydrogen ions dissociate from ethanoic acid, CH3OOH, to form H3O+ ions, (and leaving CH3OO- ions). However, there are still many CH3OOH and H20 molecules left. This means it is considered a weaker acid.

Observation

I used 10ml, 20ml 50ml, and 100ml measuring cylinders to take readings. This helped to ensure greater accuracy as the amount of gas produced could be read to a smaller degree. However, this could also lessen the reliability as the set-up changes between experiments. I decided to repeat the first reading of the 1.6M solution as this was anomalous, a considerable change from the other results for 1.6M.

I think enough readings were taken to ensure reliability; the 3 results for each concentration were normally relatively close together and this also means that the procedure was also quite accurate. I have not covered the whole range, because the acid solution continues to get stronger as the concentration (measured in molars: moles/litres-1) rises. However, stronger acids could not be used in the experiment, only the 2.0M was available. This could be due to safety reasons, so it would not be possible to use more concentrated solutions. If the reason was merely due to supply, more similar experiments could be conducted with stronger concentrations this would expand the range for the results.

Analysing Evidence and Drawing Conclusions

The line of best fit on the first graph (how concentration affects initial rate of reaction) suggests a y = x2 graph. As the concentration increases, the initial reaction rate of reaction increases. The rate at which it increases is also continually rising (for instance there is only a 0.08ml/s increase between the concentrations of 0.0M and 0.4M whereas between 1.6M and 2.0M, there is an increase of 0.48 ml/s) . This is why I drew the second graph.

The second graph produces a directly proportional straight line. This shows that concentration2 is directly proportional to the initial rate of the reaction.

The graph shows positive correlation if the concentration of acid is higher, more molecules of acid will come into contact with the marble chips. This leads to more collisions between acid molecules and carbonate molecules, so if the concentration of acid is higher, the reaction will happen faster.

All the points are on the line, or very close to it at the beginning, however, as the line progresses, the results become increasingly further away, although the results towards the end of the line are still reasonably close to it. This suggests the results are accurate.

I did not predict that the results would rise in a curve, or that concentration2 would be directly proportional to the initial rate of reaction. However, I did predict that there would be positive correlation. This is because water does not react with carbonates, so it will dilute the acid. Therefore if there is less water, the acid will be stronger. This leads to a greater reaction taking place between the acid and the carbonate, as there will be more acid molecules coming into contact with the carbonate.

Evaluating Evidence

The second graph shows that the results are near to the line; this suggests that the results are accurate.

I used different sized measuring cylinders to collect the gas. This increases the sensitivity of the measuring cylinders, as an appropriate choice can be made for each concentration of acid. This means it has a higher accuracy. Therefore the results using the weaker solutions could be measured to a high degree of sensitivity, but the higher solutions could not be measured to such a degree (the 100cm3 measuring cylinder was less sensitive than the 10cm3 measuring cylinder).

There was one anomalous reading, however, this was realised when it was taken, so it was retaken and the anomalous result was not included when calculating the averages of the initial rates of reaction. Therefore it did not influence the graphs in any way. The last average seems to be quite far from the best fit line compared to the rest. This could be because it was difficult to read the 100cm3 measuring cylinder accurately. It could also be due to the 100cm3 measuring cylinder having slightly different markings for the volume of water than the other measuring cylinders (for instance all the measurements could be 2cm3 high.)

The results are reasonably reliable as 3 readings were taken for every concentration and the means of these were plotted on the graphs. There was one anomalous result, however, I retook this and found it to be wrongly measured.

I believe a large improvement could be made to the experiment by using a 100cm3 burette to collect the data from. This would provide all the data to a high level of sensitivity and it would also stay constant between experiments. This eliminates both problems highlighted by the result for a concentration of 2.0M.

The end of the burette might not fit onto the end of the tube from the conical flask, so an adaptor could be used to make sure none of the gas escapes.

The experiment could also be repeated with water baths for the conical flasks with a thermostat in the conical flask. This should ensure that the temperature in the conical flask is regulated as hot or cold water could be added to maintain a constant temperature between all experiments. This would ideally have been in my experiment, however, due to time constraints I could not include it.

I think I took enough measurements to the correct degree of accuracy and over an appropriate range, because I managed to find out that concentration2 is directly proportional to the initial rate of reaction. The results I took followed the line of best fit, so that suggests that they are accurate too. The experimental uncertainties are within acceptable limits as they follow the line of best fit closely. The only result I found which was not within acceptable limits, I retook and as it was anomalous, I disregarded it when drawing an average.

I would change the measuring cylinder to a burette, for increased sensitivity and accuracy. I would also prefer to use powdered calcium carbonate, as there are no problems of surface area: mass ratio because the powder will have the same surface area: mass ratio (because the powdered chips are relatively the same size). However, in order to use the powder, an automatic stirrer would be needed to stop the powder clinging together. The acid should also be changed to one that disassociates less, in case the powdered calcium carbonate reacts a lot more than the small chips of calcium carbonate. It could also be useful to test some other concentrations of acid, especially ones which are a lot stronger, to see if the conclusion is valid for these values. Higher values have another advantage because any small error in the line when it is shown with small values of concentration will not be so pronounced, it could be due to experimental error. However, when the concentration increases, any small error in the line of best fit on the graph will be more pronounced.