Calcium Carbonate and Hydrochloric Acid

Surface area affecting the reaction rate

Introduction

In this experiment calsium carbonate (CaCO3) will be put into a flask and then mixed with Hydrochloric Acid (HCl).

The reaction is:
CaCO3(s) + 2HCl(l) --> CaCl2(aq) + H2O(l) + CO2(g)

The purpose of the experiment is to determine whether or not increasing surface area of marble chips will increase the reaction rate.

Reaction rate is a way of measuring how fast the reaction takes place. So the higher the reaction rate the faster the reaction takes place and the lower is the reaction time is.

Research Question

How does reaction rate change with increasing surface area of calcium carbonate in hydrochloric acid ?

Hypothesis

With increasing surface area of marble chips the reaction rate will also increase.

Justification

For the same mass of the calcium carbonate, powder has a larger surface area than marble chips. As the

collision theory states for a chemical reaction to occur particles need to collide at a certain speed and

angle. With increasing surface area, more surface area is available for the collisions to occur on. So when the surface area increases more collisions happen and therefore the reaction rate increases.

Materials

Hydrochloric acid HCl(l),
Calcium carbonate (marble chips) CaCO3(g)
Calcium carbonate (powder) CaCO3(g)
250 ml3 flask
50 ml3 measuring cylinder
Electronic scale
Stop clock
Cotton

Variables

Reaction time is the dependent variable, as it depends on the surface area of the marble chips.

The surface area of calcium carbonate is the independent variable, as it does not depend in anything. This can also be concluded from the fact that it is the surface area that is changed in this experiment.

The concentration and the amount of the acid used are the controlled variables, as they have to stay constant throughout the experiment

Method of data collection

It is important to carry out this experiment in a safe way, due to the fact that hydrochloric acid is being used. Hydrochloric acid is highly dangerous as it can damage skin and eyes. To carry out the investigation in a safe way goggles and an apron were on all the time. The area where the experiment took place was cleaned before the experiment. The desk was whipped and any obstacles that could affect the experiment from being carried out without any disruptions.

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Preparations for the reaction

Measured 50 ml3 (± 1,0 ml3) HCl in the 50 ml3 measuring cylinder
Weighed 1,0 g (± 0,0001g) of marble chips on the electronic scale (which accounted 4 decimals)
The flask with cotton on top was placed on the scale and the scale was adjusted to 0,0000 g
The calcium carbonate (marble chips) was put into the flask
The stop clock and cotton were placed next to the digital scale so they are ready for use

Now the reaction was ready to take place.

HCl was poured into the ...

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Preparations for the reaction

Measured 50 ml3 (± 1,0 ml3) HCl in the 50 ml3 measuring cylinder
Weighed 1,0 g (± 0,0001g) of marble chips on the electronic scale (which accounted 4 decimals)
The flask with cotton on top was placed on the scale and the scale was adjusted to 0,0000 g
The calcium carbonate (marble chips) was put into the flask
The stop clock and cotton were placed next to the digital scale so they are ready for use

Now the reaction was ready to take place.

HCl was poured into the flask with the marble chips
As soon as all the HCl was in the flask the cotton was placed in the tube hole, on top of the flask, so that only gas (CO2) could escape, and the stop clock was started
The weight of the solution was recorded with 30 (± 1,0 sec) second intervals until the solution has stabalised did not change weight 3 recordings in a row.
The same experiment was then repeated with calcium carbonate powder instead of calcium carbonate marble chips.

As calcium carbonate powder has a bigger surface area than calcium carbonate marble chips, comparing the two experiments will answer the research question. It is important to use the same amount of calcium carbonate powder in the second experiment as there was calcium carbonate marble chips in the first one, otherwise the surface area of the powder might not necessarily be higher than marble chips.

Data processing and presentation

The results of both experiments are as follows:

Fig. 1 Powder Fig. 2 Marble Chips

These results show the change in weight every 30 seconds.

The fact that hydrochloric acid and chips weighted 0,0979g more that hydrochloric acid and powder, could have been caused by inaccuracies in measurements of hydrochloric acid and marble chips and powder. As it is not known which of the two weights at time 0 is the most correct and which of the measurements caused that difference it is not possible to say how it will affect the results, because:

If there is too much acid in the marble chips experiment it would increase the reaction rate (collision theory)
If there is too much marble chips in the marble chips experiment it would increase the loss of weight over time as more product would be produced
If there is too little acid in the powder experiment it would decrease reaction rate (collision theory)
If there is too little powder in the powder experiment it would decrease the loss of mass over time as less product would be produced

In this experiment the rate of reaction is determined by the formation of the product (CO2(g)). When CO2(g) is formed inside the flask, the gas leaves the flask. As it is only CO2(g) that leaves the flask, a graph with the loss of weight over time will show how quick the product (CO2(g)) was formed. But it is important to note that it is not the final loss over time, of the reactions, that is to be compared, as it should be the same for both reactions. The reason for that is, that the same mass of calcium carbonate produces the same mass of the product (CO2(g)), whether it is chips or powder. So to find which of the two (marble chips or powder) have the fastest reaction rate one must either compare the loss over time at a given time (but before one of the reaction is over, as when the reaction is over no more product is produced and the comparison of the two will show irrelevant data) or compare the slopes of the two reactions.

The loss over time for the two reactions is illustrated by fig. 3 (powder) and fig.4 (chips) below.

Fig. 3 Powder

Fig. 4 Marble chips

Analysis

Fig. 3 shows the loss of weight over time using calcium carbonate powder. One can see that all recordings, besides one (0,1779g), are on the trend line, which means that the recordings are not random and that the experiment with the powder most likely went good without noticeable errors. The recording that is not on the trend line could have been caused by a too late reading, meaning that the recording took place a second or two later than it should have.

The same analysis (as the ones for fig.3) can be applied to fig.4 and the two anomalies on that graph. The 0,0149g reading (on the graph) could have been recorded a second or two too late and the 0,0828g reading a second or two too early.

Another explanation for the anomalies, where the loss of weight over time was smaller than expected (lower than the trend line) could be that some of the gas that is produced and is escaping, falls down again causing lower loss of weight at that moment. But as its only the 0,0828g reading that fits into that category it is unlikely to be the reason.

Looking at the two graphs one must notice that the final loss of weight over time for the two reactions is not the same, which it is supposed to be (see explanation in “data processing and presentation” above). This suggests that the marble chip reaction was not over when the recording of data stopped. The reason data recording stopped was that 3 recording in a row showed the same data. The possible reason for the 3 recording to show the same data could be that the gas that was trying to escape fell down again and the gas that escaped in that time period could not be detected by the scale. This should though not interfere with the methods used to answer the research question (see methods and explanations below).

As mentioned before, to answer the research question, “How does reaction rate change with increasing surface area of calcium carbonate in hydrochloric acid ?”, one must either compare the loss of weight over time at a given time (but before one of the reaction is over, as when the reaction is over no more product is produced and the comparison of the two will show irrelevant data) or compare the slopes of the two reactions.

Both above mentioned methods are to be used.

The first method is to compare the loss of weight over time at a given time: the chosen time is 90 seconds. As one can see on the graph the loss of weight over time after 90 is 0,1281g in powder experiment and only 0,0266g in chips experiment. The powder experiment has lost approximately 5 times more weight than the chips experiment after 90 seconds, meaning that more product was formed in the powder experiment, hence the reaction rate of the powder experiment is higher that chips experiment.

The second method is to compare the slopes. Comparing the two graphs one can clearly see that the slope is much steeper in fig.3 (powder experiment), suggesting that the reaction rate is much quicker in the powder experiment. This is because the steeper the slope is, the more weight is lost over time, hence more product is formed and the quicker the reaction rate is.

Conclusion

Even though there were some inaccuracies in measurements of reactants the overall experiment went according to the plan and the results are in agreement with the hypothesis and the collision theory mentioned above (in justification). Note that although the experiment supports the collision theory it does not prove it, as experiments can only support theories, not prove them.

Inaccuracies and Possible improvements

Surface area is the only independent variable, so all other measurements and actions have to stay constant throughout the 5 experiments. These measurements and actions are:

The exact amount of hydrochloric acid and calcium carbonate used (both chips and powder)
How quick the stop clock was started, after the reaction started.
How precise the intervals were in between the reading (as explained in the analysis this could be the reason for some of the recording being a little bit off)
Gas rising and falling again as it couldn’t escape through the cotton

As all these could not possibly have stayed constant throughout the experiment these are the inaccuracies that could have caused the results not to be as exact as possible.

As explained before the experiment was conducted without any noticeable errors, so the only improvement, that could take place next time this experiment is conducted, is the one that concentrates on more precision of the above mentioned inaccuracies.

Another improvement that could make sure that there is no confusion, whether the reaction has stopped or not, which caused the stop of the recordings of the marble chips experiment, is to either increase the amount of the same recordings before stopping from 3 to 5 or say that the weights has to stay the same for a few minutes.

Neuss, Geoff. Chemistry for the IB Diploma, 2001