'Investigating factors that affect the rate of chemical reactions.'

‘Investigating factors that affect the rate of chemical reactions.’

Matthew J Hampton

Silverdale School

Class 11L

Chemistry Coursework

Scientific Knowledge.

The rate of a reaction is a measure of how fast the reaction between the reactants occurs and so how fast the products are produced. Some reactions are very slow, such as the rusting (oxidisation) of iron to ferrous oxide (rust) in the presence of water. Some reactions are very fast, such as the reaction between sodium metal and water or when dynamite explodes. Both these reactions would not be good for us to study as they occur too slowly or too quickly for us to measure accurately. A reaction that occurs at a reasonable rate should be studied, such as the reaction between dilute hydrochloric acid and calcium carbonate, in the form of marble chips or powder.

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

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

Another advantage of this reaction is that carbon dioxide is a product. Carbon dioxide is a gas and thus the rate of reaction can be measured by collecting, and measuring, the amount of gas given off as the reaction proceeds. This is one of three ways of measuring a reaction rate. If a gas is given off, the gas can be collected and measured. If one of the reaction products is cloudy then the rate at which the reaction becomes cloudy can be measured. The reaction between calcium carbonate and hydrochloric acid does not product a precipitate, but the reaction between sodium thiosulphate and hydrochloric acid produces a yellow precipitate of sulphur and so the reaction rate can be measured by the rate of cloudiness of the reaction products.

If neither of these methods can be used, the change in mass of the reaction can be measured. It would be possible to measure the rate of reaction in the reaction between calcium carbonate and hydrochloric acid using this method, as the weight of the reaction mixture would decrease as the carbon dioxide is given off, but collecting the gas is easier and more reliable.

Calcium is an alkaline earth metal from group II of the periodic table. It is quite reactive, but not as reactive as the group I alkali metals. It forms ionic compounds such as calcium carbonate. Calcium reacts with acids (all of which contain hydrogen) to give off hydrogen gas. However in the reaction of calcium carbonate with hydrochloric acid water and carbon dioxide are formed.

All chemical reactions depend on the interaction of the chemicals to enable bonds between them to be broken and others to form, so the products are different to the reactants. Thus the number of interactions or collisions between the reactants is very important and the rate of reaction is controlled and explained by Collision Theory. This states that the rate of reaction simply depends on how often and how hard the reacting particles collide with each other. The basic idea is that particles have to collide with other in order to react, and they have to collide hard enough as well.

There are four methods of increasing the rate of a reaction:

Temperature: a rise in temperature increases the number of collisions because when the temperature is increased the particles all move quicker. If they move quicker, they will have more collisions. Temperature increase also results in the particles having more energy, so the collisions are also harder and more likely to lead to a reaction.

Increasing concentration (or in a gas pressure) increases the number of collisions in a given time. If a solution is made more concentrated it means there are more particles of reactant between the water molecules, which makes collisions between important molecules more likely.

Size of the particles (surface area) increases the number of collisions. If one of the reactants is solid then breaking it up into smaller pieces will increase its surface area. This means the particles around it in solution will have more area to work on so there will be more collisions in a given time.

Catalysts increase the number of useful collisions. A catalyst works by giving the reacting particles a surface to stick to where they can bump into each other. This increases the number of useful collisions.

The reaction between calcium carbonate and hydrochloric acid in solution is a reaction that occurs at a moderate speed. No catalyst is necessary for this reaction to proceed, and it occurs at room temperature.

In the preliminary experiment I will alter the concentration of the hydrochloric acid and the surface area of the calcium carbonate (marble chips) to ensure that the experiment works and gives reasonable, measurable results.

In the final experiment I will only use one size of calcium carbonate particles, using the same mass each time, but with more concentrations of hydrochloric acid, but using concentrations between those used in the preliminary experiment. So in the final experiment only one variable will be changed.

Conducting a fair test.

Any experiment needs to be carefully planned and carried out so the results are accurate and reproducible. This means that if the experiment was carried out again or done by someone else, they would get the same result, if they used the same concentrations ...

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Conducting a fair test.

All the weights and concentrations should be as accurate as possible, and the measurements of the gas given off should be accurate and recorded at exactly the right times, using a stopwatch. Sometimes even with care, things might go wrong. For this reason it helps to perform the experiment several times and measure the results, then average them. This increases the reliability and decreases the chance of drawing a false conclusion of there is a wrong or outlying result.

Apparatus.

The experiment involves reacting a liquid (aqueous dilute hydrochloric acid) with a solid (marble chips = calcium carbonate). This reaction should take place in a conical flask, as the hydrochloric acid is dangerous if spilled. The product given off is carbon dioxide gas, which needs to be measured, so a stopper with a tube leading to an inverted water filled collecting cylinder is necessary to collect the gas and allow its volume to be measured.

In addition a stopwatch and a balance is needed to make the measurement of time and weight accurate and so to control the variables.

Conical flask with side arm

Stopper

Glass tube

Beaker of water

Inverted measuring cylinder full of water

Measuring cylinder to measure quantity of hydrochloric acid

Balance to weigh mass of calcium chips or powder

Stopwatch to record time

Pencil and paper to record results.

Marble powder and small and medium chips

Dilute hydrochloric acid (0.2M, 0.4M, 0.6M, 0.8M, 1M)

Safety.

To perform this experiment safely it is necessary to behave in a sensible manner when handling dangerous chemicals. The floor and bench should be clear and uncluttered and all hazards removed from the area. Hydrochloric acid is corrosive and should be handled with care. Dilute solutions are safer than concentrated ones, so special care should be taken with the more concentrated solution of hydrochloric acid. Also eyes should be protected and hands kept away from dangerous chemicals. Any spillage should be reported and dealt with immediately and hand or eyes washed in water if they come into contact with the acid. Finally after the experiment is finished, the acid must be disposed off safely and carefully. I will not use concentrations of hydrochloric acid greater than 1.0M as these are too corrosive and also too dangerous if spilt. There are some important safety symbols these are illustrated below.

Recording the results.

The amount of gas given off is collected in the inverted water filled measuring cylinder. The amount of gas could be recorded in one of two ways: either the amount of carbon dioxide collected after a certain time, or the recording could be made continuously, say every ten or twenty seconds. The latter will give more data and the first method can be used after all the data has been collected and compared.

The time should be recorded very accurately and the start of the experiment recorded by starting the watch at the same time as adding one of the reactants and sealing the reaction flask. The interval between recordings should not be too short to allow the results to be recorded. The rate of reaction should not be too slow to produce measurable changes in gas volume between measurements, nor too quick to enable accurate measurements to be made.

Predicting the results.

The two variables we are going to change in the preliminary experiment are the surface area of the calcium carbonate, using medium sized chips (small surface area to weight) small chips and marble powder (large surface area to weight), and the concentration of the hydrochloric acid, using the strongest concentration I can use, 1.0M and a weaker concentration of 0.2M. If the quantity of calcium carbonate, the concentration of the hydrochloric acid and the temperature and pressure are kept the same and the experiment is accurately performed I would predict the reaction rate will increase as does the surface area, so the amount of carbon dioxide gas given off will be fastest with the marble powder and slowest with the big chips and in between with the small chips.

The other variable is the concentration of the hydrochloric acid. I predict that the more concentrated acid will give a faster rate of reaction that the weaker acid solution. This is because the stronger solution will contain more acid ions and so more collisions will occur in a given time and the reaction will be faster.

Both the predictions I have made about the preliminary experiment are predicted by collision theory.

Preliminary experiments.

Method

I set up the apparatus as shown below. For the preliminary experiments we used 20 mls of two concentrations of hydrochloric acid solution, 0.2 and 1.0 molar because these are the extreme measurements. If the 0.2 molar reaction is not too slow and the 1.0 molar is not too quick, then any concentration in between should give a satisfactory measurable result.

The calcium carbonate was available in three forms, medium marble chips, small marble chips and marble powder. We decided to use 1.2 grams in each experiment. From this experiment I should be able to find out which marble state to use and what acid concentration will be best to use, in my main experiment.

The apparatus is as shown above.

We recorded the volume of gas given off every 10 seconds for a total of 200 seconds. I used 1.2g of marble chips each time.

RESULTS.

Before I could interpret these results I drew them as a graph.

Conclusion from preliminary experiment.

It is clear that the powder causes a faster reaction rate than the small or medium chips, which are the slowest. It is also clear that 1.0 molar HCl gives a faster reaction rate than 0.2 molar, indeed the 1.0 molar with the marble powder reaction is so quick the measuring cylinder is full after 50 seconds and then the gas overflows and cannot be measured any more. Therefore this experiment should not form part of the final experiment.

What would be ideal for the final experiment is to have a reaction rate that produces a steady flow of bubbles that can be measured accurately, and the final volume should be less than that of the measuring cylinder.

I can achieve this result by using the small marble chips a 1.2g each time, that is the calcium carbonate with a moderately large surface area, larger than the medium chips but not so big as the powder. If I use these chips the experiment will work in a predictable and measurable way with 20 mls of 1.0 M down to 0.2M HCl and so I can use a range of concentration in between, such as 1.0M, 0.8M, 0.6M, 0.4M and 0.2M, using the same experimental conditions as in the preliminary experiment.

The Main Experiment.

Method

I will use the same apparatus as in the preliminary experiment and the same apparatus. I will use the same weight (1.2g) of small marble chips and the same amount (20 mls) of dilute HCl.

I will use five different concentration of HCL, namely

1.0M hydrochloric acid x 3

0.8M hydrochloric acid x 3

0.6M hydrochloric acid x 3

0.4M hydrochloric acid x 3

0.2M hydrochloric acid x 3

I will use 1.2g of marble chips in my main experiment because in my preliminary experiment this amount gave me good and reliable results and I think that it will do the same in my main experiment.

I recorded the amount of CO2 given off every 10 seconds for a total of 200 seconds. I performed each experiment three times and averaged the results to get more precise and reproducible results from which to plot the reaction rate graphs.

Diagram

Prediction for main experiment

From my preliminary results and my background knowledge of collision theory, I predict that in this experiment the reaction rate, and so the rate at which the CO2 gas collects in the cylinder, will be quickest in the 1.0M HCl experiment and slowest in the 0.2M HCl experiment, and the other concentrations will range in between in an orderly fashion. This is because the concentration of the dilute aqueous hydrochloric acid is a measure of the number of particles of HCl in the liquid. Thus the 1.0M HCl has more molecules in it than the lower concentrations and so there will be more useful collisions between the HCL and the calcium carbonate, resulting in a faster rate of reaction and so the production of more reaction products and the formation of more CO2 gas. As the concentration lowers, there will be less particles and so less collisions and so a slower rate of reaction and less CO2 gas produced.

I also predict that because I have chosen the small chips and the right concentrations of HCl that all the experiments will be possible in with the equipment being used. I know this because of the results obtained in the preliminary experiments.

Finally I predict that the results of each of the three repeats of each concentration will show results very close to each other. I know this because the amount of particles of HCl and calcium carbonate will be the same each time and no other variable is being changed and so the number of collisions and the reaction rate and so the production of CO2 gas will be the same each time.

Results (obtaining)

The anomalous result in the 0.8m column is not included in the average because I realised immediately from looking at the results of that individual experiment that I think I used1.0M acid concentration and not 0.8M HCl because the reaction proceeded at the same rate as the 1.0M experiments and faster than all the other 0.8M reactions. I decided this was an anomalous result and I have not considered those results further in my analysis.

To ensure the experiment was as accurate as possible I made sure that every time I measured out the marble chips the balance was set to zero before use, I took all my volume readings from eye level so that I could get the most accurate view of the cylinder, and every reading I took was to the nearest ml (appropriate for cylinder used).

Another way of presenting the results would be as reaction rates by working out and comparing the amount of gas produced per minute by each experiment. This could be done over just the first 60 seconds, or by measuring over 180 seconds and dividing the result by 3. If the rate is constant throughout, then the line will be straight on the graph and the result of 1 and over 3 minutes will be the same.

CO2 production in ml/ minute

It can be seen that for most of the experiments the results are very similar, showing that the rate of gas production is the same throughout the experiment and the amount of gas produced was higher with each increasing concentration of HCl. The only exception to this is for the first minute of the 0.6M HCl experiment, where less gas seems to have been given off than expected and also at a slower rate in the first minute than in the rest of the experiment.

I believe that the results of this experiment are in keeping with my predictions and also what I know about collision theory, in that the more particle there are to make useful collisions, the faster the rate of reaction. In this experiment, when all the other variables are kept the same, I have shown that the higher the concentration of acid used, the faster the reaction rate.

Conclusions.

My results show that the higher the concentration of acid the greater rate of reaction, therefore the greater the volume of gas produced in a given time in each experiment. I can clearly see this from the graph of my results that the 1.0M HCl curve is much steeper then the 0.2M curve. Also I noticed that the 0.2, 0.4 and the 0.6M curves are much closer together then the curves for the 0.8M and the 1M results. I can clearly see that the anomalous result for the experiment including the 0.8M of acid was most likely a mix up and the 1M acid was used instead of 0.8M HCl, but it could also be that people poured excess acid back into the bottle therefore making it more dilute, but this is less likely because there is a very strong resemblance between the anomalous curve and the 1M curve.

All the lines are straight lines from the beginning until the end of the experiment. This indicates that the rate of reaction is constant, and that neither the marble chips nor the acid are used up. The results are in keeping with my predictions that the reaction rate will be quicker as the concentration of the acid increases. This is what I would predict form collision theory.

Evaluation.

This experiment was performed to the best of my ability. I believe that the results I have obtained are accurate, fair and reproducible, because of my prediction. I think somebody else who read this project could repeat my experiment and would get the same results. The rate of reaction in this experiment was measured by recording the volume of gas given off over time. I think the apparatus used was appropriate. However in the preliminary experiment one of the reaction rates was too quick and too much gas was produced when 1.0M HCl was used with the marble powder, with the result that the cylinder overflowed and the experiment had to finish earlier than the other experiments. This could have been avoided by using a larger cylinder, or less marble powder or less volume of HCl. This would have resulted in the other experiments being too slow, so for the main experiment we used only the small marble chips, those with the middle surface area.

The main weakness in the method we used was in collecting and measuring the gas produced. Firstly it was necessary to have the top of the reaction flask open when the acid was poured onto the marble chips. Thus in the first few seconds any gas produced was able to escape from the flask before the stopped was put on. Although the time to put the stopped with the side arm on was roughly the same in each experiment, the faster the reaction, the more gas would have escaped in the first few moments, while in the slowest reactions almost no gas would have been produced or escaped.

The other area of error was in reading the amount of gas produced. As the gas was produced continuously but reading were only taken every 10 seconds, it was difficult to determine exactly how much gas was in the cylinder at that precise time point, as some bubbles were still coming up the cylinder. This error was greater the faster the reaction went. However I do not think this had much effect on my results as we measured the experiment for 200 seconds and so there were a lot of results for each concentration used and the three times we did the experiment produced results that were very similar every time.

The only anomalous result we obtained was one experiment using 0.8M HCl when the reaction rate was faster than expected. I have already explained that I think this was because I accidently used 1.0M HCl and if this was so then I would expect a faster rate, as I saw in that run. Because I decided that was an anomalous result, I did another experiment at that concentration, which gave similar results to the other 0.8M experiments and I used that one in my averages.

I think this was a good experiment as it gave results that were the same as my predictions and agree with what I known about reaction rates from my knowledge of collision theory, that if you increase the number of particles reacting ( the concentration of the HCl) then there will be more useful collisions and the reaction will go faster resulting in the production of more CO2 gas, in a given time.

It would be possible to extend the readings I obtained using higher or lower concentrations of acid, but this would mean using different equipment, as higher concentrations of acid would be dangerous as HCl is corrosive and also a bigger cylinder would be necessary, while using lower concentrations of acid would need a more accurate way of measuring the smaller volumes of gas given off, perhaps by using a smaller, but long and thin measuring cylinder. Also other variables could be investigated that alter collision theory, such as changing surface area of the marble, or changing the temperature at which the reaction is carried out or using a catalyst to aid the reaction. A gas syringe could be used to give me more accurate results to the volume of gas given off.

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