Rates of Reaction

Planning:

The aim of this experiment is to investigate the rates of reactions. When marble chips are mixed with hydrochloric acid, a reaction happens in which carbon dioxide is given off. During this investigation, I will have to find out whether the concentration of the acid varies the rate of the reaction and whether the rate of reaction is proportional to the increase of concentration.

           In a chemical reaction, the starting materials are called reactants, and the finishing materials are called the products. It takes time for a chemical reaction to happen. If the reactants take only a short time to change into products, that reaction is a fast reaction. The speed or rate of that reaction is high. If a reaction takes a long time to change the reactants into products, it is a slow reaction. The rate of that reaction is low. A rate is a measure of change over a certain period of time.

Some examples of chemical reactions are:

Ø      Magnesium + sulphuric acid à magnesium sulphate + hydrogen

Ø      Iron + oxygen à iron oxide

Ø      Copper oxide + sulphuric acid à copper sulphate + water

Naturally, all of these reactions occur at different speeds. The one with magnesium, for instance, is a very rapid reaction, because you can see the magnesium disappear quickly. While the oxidation of iron is extremely slow, since it takes days for patches of iron oxide to form. So there are two different ways of judging how fast a reaction takes place. Rate of reaction is the rate of disappearance of a reactant or the rate of appearance of a product.

However, what is it that happens during a chemical reaction? First of all, the particles of the reacting substances must collide with each other and, secondly, a fixed minimum amount of energy (called the activation energy) must be reached if the reaction is to take place. If a collision between particles has sufficient energy (i.e. if the particles collide fast enough and in the right direction), a reaction will take place. Not all collisions will result in a reaction. Therefore, a reaction can be speeded up if the number of successful collisions is increased. This theory is known as the collision theory for rates of reaction.

Concentration and Pressure:

Multiple factors can alter the rate of reaction. For example, the more concentrated the reactants are, the greater the rate of reaction will be. This is because increasing the concentration of the reactants increases the number of particles in a given volume and consequently, it increases the number of possible collisions as well. This increases the rate of reaction.

This also explains why the fastest rate of reaction is usually as soon as the reactants are mixed, i.e. they are both at their highest concentrations. As the reaction dies out, the concentrations of the substances decrease.

The effect of concentration can be shown by doing several experiments using equal masses of magnesium ribbon and hydrochloric acid of different concentrations.

Increasing pressure can speed up gas reactions. The reaction between nitrogen and hydrogen is used to make ammonia (this is known as the Haber Process). Ammonia is an important material in the fertilizer industry.

nitrogen + hydrogen à ammonia

N₂ (g) + 3H₂ (g) à 3NH₃ (g)

To speed up the reaction, we can increase the concentration of the reactants. In gases, that is done be pressurizing it. High pressure forces the gas particles closer together, making the gas more concentrated and increasing the number of suitable collisions.

Temperature and Light:

Furthermore, an increase in temperature also produces an increase in the rate of reaction. When a mixture of reactants is heated, the particles move faster. Since the particles are moving faster, they will travel a greater distance in a given time and so will be involved in more collisions. Consequently, the rate of reaction increases.

The rates of some reactions are increased by exposure to light. Actually, light has a similar effect in increasing the temperature. A mixture of hydrogen and chlorine does not react if kept in the dark, but in the presence of light an explosive reaction takes place.

Surface Area:

Additionally, the surface area of a particle has an effect on the rate of reaction as well. When one of the reactants is a solid, the reaction takes place on its surface. By breaking up the solid into smaller pieces, the surface area is increased, giving a greater area for collisions to take place and so causing an increase in the rate of reaction.

Catalysts:

Another way to hasten up reactions is to use a catalyst. One way of making oxygen in the laboratory is by the decomposition of hydrogen peroxide, H₂O₂:

hydrogen peroxide à water + oxygen

2H₂O₂ (aq) à 2H₂O (l) + O₂ (g)

Under normal conditions, this reaction is very slow. It can be speeded up by using more concentrated hydrogen peroxide, or by heating it. But a much easier way of speeding it up is to add manganese (IV) oxide, MnO₂. Manganese (IV) oxide is a fine, black powder and if you add it to hydrogen peroxide solution, the hydrogen peroxide starts to decompose rapidly. However, the manganese (IV) oxide is not used up. When the reaction finishes the black powder is still there.

Manganese (IV) oxide is an example of a catalyst. In the Haber Process, Iron is used. A catalyst is a substance which speeds up a chemical reaction without being changed or used up.

Now, in this experiment, I will be investigating the effects of concentration on the reaction of Calcium carbonate (marble chips) and hydrochloric acid:

calcium carbonate + hydrochloric acid à carbon dioxide + calcium chloride + water

CaCO₃ (s) + 2HCl (aq) à CO₂ (g) + CaCl₂ (aq) + H₂O (l)

            Only the concentration of the acid will be altered, while the mass of the marble chips will remain as constant as possible. Additionally, I intend to keep the amount of time the same for all each trial that I will carry out. This is so that I can keep my experiment as fair as possible. In addition to this, I will also test the water with which I diluted the acids to see whether it is actually neutral, since a minor difference of pH level could influence the outcome of the experiment. In order to record the change in the reaction, I can either look at the mass loss during the process or at the quantity of carbon dioxide produced.

Prediction:

            In my opinion, the results will show that as the concentration increases, the rate of reaction increases as well. I also believe that the concentration of the acid will be proportional to the rate of reaction. That is, if the concentration is doubled, then the rate of reaction should be double as well. I also think that after a certain period of time, we should see the curve’s slope become more and more gentle, until it reaches a perfect horizontal line. Actually, I believe that my results will show that the increase in the rate of reaction is proportional to the increase in concentration. However, this probably will not be visible in my graphs, since the amount of time that I am using is only three minutes, but I will do a further investigation using electronic methods, and therefore I should be able to see whether my theories are correct. Anyhow, I believe that my graphs will look something like the one shown to the right.

Naturally, I will carry out various trials, three for each concentration, so that I can have an average of my results. I will also carry out a trial with water in order to check its acidity, and its possible influence on the outcome of the experiment. This will make my results a lot more reliable. I will record my observations in a table like this:

From the results obtained, I will draw up various graphs, in order to compare the data in as many ways as possible in order to come up with a valid conclusion. On my graphs, I will also draw error bars on each point in order to see how reliable my averaged results are: the bigger the bar, the less reliable they must be.

Method:

         For this experiment you will need 25g of marble chips (calcium carbonate), 50ml of each concentration of hydrochloric acid, a conical flask, a balance, measuring cylinder, some cotton wool and a stop-watch. Having only one concentration of acid (2M HCl), I will have to dilute the HCl in order to obtain different concentrations. The dilution can be easily done following the steps below:

1.       Take two burettes. Fill one with 2M HCl and the other one with normal tap water.

2.       Take a conical flask in which the acid will be used for the experiment and pour a certain quantity of water followed by a certain amount of acid. These quantities should be calculated with some mathematical operations i.e., in order to make 1M HCl from 2M HCl, 25ml will be water, while the remaining 25ml will be acid. To make 1.5M HCl, 37.5ml will be acid, while only 12.5ml will be water and so forth.

 In this investigation, I will study the reactions with concentrations of 0.25M, 0.50M, 1.00M, 1.50M, 1.75M and 2.00M. I will also do a trial with water in order to see whether it is acidic and could affect the results. Once you have all of your concentrations follow these steps in order to carry out the actual experiment:   

1.       Measure out in a measuring cylinder 50ml of HCl of a certain concentration (1M for instance). Then pour the acid into the flask.

2.       Weigh out as close as possible 5g of calcium carbonate.

3.       Place the flask on the balance. Put some cotton wool at the opening. This prevents any liquid from splashing out during the reaction. Place the marble chips at the side of the flask, but still on the balance. Record the mass.

4.       Then, quickly take out the cotton wool, put the marble chips in the flask and put the wool back in place. Once you have done this start your stop-watch.

5.       Every 10 seconds record the mass loss (that is the mass recorded subtracted from the total mass).After about 3 to 4 minutes, stop the experiment.

6.       Rinse the flask out, and repeat steps 1 through 5 at least five times, each trial using a different concentration of hydrochloric acid.

7.       Once you have all of your results, graph them.

Results: 

Analyzing evidence:

During this investigation, I analyzed the reaction that took place between various concentrations of hydrochloric acid and calcium carbonate, which produced calcium chloride and water and gave off carbon dioxide in the form of a gas. The reaction can be explained in the following equation:

calcium carbonate + hydrochloric acid à carbon dioxide + calcium chloride + water

CaCO₃ (s) + 2HCl (aq) à CO₂ (g) + CaCl₂ (aq) + H₂O (l)

Whilst carrying out the experiment, I noticed that the calcium carbonate was literally being eroded by the acid. In fact, as the reaction continued on going, I realized that the marble chips were slowly diminishing, and once the reaction was terminated, the chips had vanished, completely consumed by the corrosive hydrochloric acid.

Furthermore, something that surprised me was the duration of the reaction. After having repeated the experiment also using electronic instruments (i.e. a pH meter), I discovered that the reaction continued for hours. My results look only at the first couple of minutes of the reaction, when it actually takes some hours to reach completion. However, if we compare this reaction to the corrosion of iron, it seems to be a very rapid reaction.

As I had stated in my hypothesis, the concentration of the acid is proportional to the rate of reaction, that is, if I double the concentration, the reaction should double its rate. This theory of mine was confirmed by the graphs. In fact, if we look at the final graph of the rate of reaction versus the molarity of the acid (values shown below) we can see that the rate of 1M was more or less the double of the rate of 0.5M. In fact, the reaction with 1M HCl had an average rate of 0.00217 grams per second, whilst the reaction with 0.5M HCl had an average of 0.00085 grams per second, which is approximately half the value of the former reaction. Another similar comparison can be made between the rate of reaction of 1M and 2M HCl, where the value of the latter is more or less the double of that of the former.

concentration   α   rate of reaction

This means, in other words, that the concentration of a reactant is proportional to the rate of the reaction. Consequently, I would say that my predictions were quite correct. In fact, I had predicted that the concentration would be directly proportional to the rate of reaction.

                        If we also look at the various rates of reaction, we can see that 2.0M HCl had the highest and fastest rate, while 0.25M had the lowest and slowest rate of reaction. This, consequently confirms that the higher the concentration of a reactant, the faster the reaction will be.

These results can be explained by the collision theory mentioned earlier. In fact, with a higher concentration, there are more particles in the same amount of space, thus ‘crowding’ that volume. Consequently, it is more likely that the particles will collide with each other, and therefore, also increases the possibility of a successful collision. With a greater number of successful collisions, the rate of reaction increases just as much. This also explains why they are related proportionally.

In addition to this, if we look again at the graphs, we can see that in most cases the error bars were rather small, usually with only a 0.01g margin of error. This means that overall, the results were rather accurate and consequently reliable. There are, however, a few anomalies, like in the results for the reaction with water. These are probably due to the fact that the changes in mass were so minute, that not even the scale could detect them well, and consequently it continued to change value. To contrast this problem, I could have repeated the trials many more times, than just three times. This would give me an even broader sample of results which would naturally increase the accuracy of my experiment.

The inaccuracies throughout my experiment might have been due to multiple factors. One reason could be that more acid was added than in the other trials, or maybe it was due to the weight change forced upon the table. In fact, one can simply change the mass reading of a balance by exercising  pressure on the surface upon which it is placed. However, it could have also been simply due to my carelessness in taking the readings at the correct moments.

Overall, though, I would say that this experiment was a success, despite the little inaccuracies which resulted in the end.

Evaluating evidence:  

Overall, as I have already said, I would declare this experiment as successful. Nevertheless, I would make a couple of changes to it. This is because whilst carrying out the investigation, I was affronted with various problems, which had the negative effect of rendering my results less accurate. Consequently a couple of improvements are needed in order not to encounter the accuracy problems that I had.

First of all, it was extremely hard to get the same amount of marble chips. As a matter of fact, I only managed to get exactly 5.00g of calcium carbonate one time. Most of the time it was either a bit less pr a bit more, 4.98g or 5.01g for instance, values which are quite close enough, but not enough to have perfectly accurate results. To contrast this complication, I would have the marble chips cut up in little cubes all of the same size by some sort of machine. Having them all the same size and shape, would most certainly make them weigh almost the same.

Another reason for my inaccuracies was probably the measuring out of the acid used for the experiment. In one trial maybe I added more of it than in another run, simple by mistake, even if I measured the quantities out with a measuring cylinder. Consequently, to oppose this problem I would suggest to use a burette, not only for the dilution, but also for measuring out the amount of acid needed for the trial. With a burette, one can control very precisely the amount of acid dropped into the beaker, thus making the volume of acid much more accurate.

In addition to this, I also had some problems in actually taking the readings. In fact, especially with the lower concentrations, the changes in the mass of the marble chips were so minute that the balance could not detect them very well. Naturally, this caused a couple of anomalies in my results as we can see from the results shown below:

            A solution for this faulty reading could be to use a balance with a more accurate scale.

Another method to resolve all of these accuracy problems would be to turn to electronic means. It is widely known that computers and machines are usually a lot more precise than human beings, so with their help, we would be certain to gather fair results.

One method of carrying out this experiment using electronic means would be using a data logger and a pH meter. Actually, I, myself, carried out this experiment. The calcium carbonate neutralizes the hydrochloric acid, therefore, as the reaction went on, the pH increased. At completion, the pH of the solution should be approximately 7, neutral, since the reaction between the hydrochloric acid and the calcium carbonate is, after all, a neutralization reaction. The decrease of acidity would be recorded by the pH meter and the computer automatically graphed the results without having to move a finger on my part. I left the reaction to go on over night, and that is how I found out that the reaction lasted for hours. The results are graphed in the following page. This discovery, however, also influences the reliability of my results since they only look at the first couple of minutes of the reaction, when, really, it lasts for hours. Maybe there is a different behaviour at the beginning of the reaction than at the end. Consequently, this could actually render my conclusion completely wrong, since my ideas are based only upon the results on the first part of the experiment. Therefore, in the future, maybe it would be better to record results until completion in order to have a very detailed and complete table of results,  on which a solid and backed conclusion can be made.

However, despite the errors which I made and their possible influence on my results and deductions, I believe that my evidence is good enough for a rather accurate and precise conclusion. This is because, on the whole, the inaccuracies were rather small, hardly noticeable, and as we see from the results, there were few anomalies. The only ground on which my results could be criticized is that, as I have said just above, I did not follow the reaction to completion. Nevertheless, I believe that this does not influence the reliability of my investigation too much and consequently my conclusions are acceptable.

An extension for this investigation could be the measuring of the volume of carbon dioxide released over a certain period of time. A gas syringe could be used in order to measure the volume produced and then graphs of gas produced versus time taken could be produced and analyzed, so that we can come to a further and improved conclusion.

A second way to extend the investigation would be to see the effects of concentration in the reaction of Sodium thiosulphate with hydrochloric acid, described in the following equation:

Sodium thiosulphate + Hydrochloric acid à Sulphur + Sodium chloride + Water

Na₂S₂O₃(aq) + 2HCl(aq)  à S(s) + 2NaCl(aq) + SO₂(g) + H₂O

During the reaction, the solution becomes cloudy, or milky. It becomes so cloudy that you cannot see through it. In order to have a further study on the effects of concentration on the rate of reaction, you could vary the concentration of the hydrochloric acid and keep the same the concentration of the sodium thiosulphate, or vice versa. You would place the beaker over an ‘X’ drawn on a piece of paper, and you would record the time taken in order for the ‘X’ to disappear.

With the same experiment one could investigate the effects of temperature. In fact, you could heat the hydrochloric acid and then mix it together with the sodium thiosulphate in the same way as mentioned above. Naturally, in order to make the experiment fair, the concentrations of the solutions would always have to be the same.

                        A third experiment, still involving hydrochloric acid and calcium carbonate, could be the effects of surface area on the rates of reaction. In fact, if we crunch up the marble chips to marble dust, the reaction will probably go faster rather than if we only put a big chunk of marble. This is due to the fact that since the surface area of the marble chips is increased, there is more calcium carbonate in direct contact with the hydrochloric acid, and as a result it increases the chances of these two particles having a successful collision between them.

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