Investigation Into Factors Affecting Rates of Reaction.

Christopher Bell 11C

Chemistry: Investigation Into Factors Affecting Rates of Reaction

Planning

This is an investigation to find out how a certain factor affects the rate of a chemical reaction. There are many different factors that do affect the rate of reaction. These include: Temperature at which the reaction takes place, catalyst used in the reaction, surface area/particle size of the reactant and concentration of the reactant. I have carefully eliminated 3 of these factors to leave me with the one I will use in my investigation. I have decided not to use temperature because although it is not impossible, it is very hard to achieve an exact temperature in the school lab. The catalyst factor will not be used because it is not quantitative and the results would be very difficult to analyse. Finally I will not vary the surface area/particle size of the marble chips I will use in this investigation because it would not be able to be very precise. Therefore I have decided to change the concentration of the solution because it is easy to do, and the results can be easily analysed.

I predict that as the concentration of the hydrochloric acid increases, so too will the rates of reaction. I think this because in order for two substances to react, they must collide with each other, and the collision must have enough energy for them to react. Therefore in a less concentrated acid there are less acid particles than a more concentrated acid meaning in the more concentrated acid there is a greater chance of a successful collision between them. The more successful collisions there are, the faster the reaction. The number of particles doubles as the concentration doubles meaning I can predict that the relationship between reaction time and concentration will be directly proportional, and the graph will look roughly as follows:

In order to keep this a fair test, and for the results I will obtain to be reliable, I must keep the remaining factors constant including a few more. The temperature of the reaction must be kept the same because of the collision theory that states that at low temperatures particles of reaction substances do not have much energy. If temperature of the reaction increases however the particles take in energy causing them to move faster meaning there is greater chance of a successful collision making the reaction more fast. When they have taken in more energy and are moving faster, they collide more often. The collisions also have more energy meaning more of them are successful. Therefore the temperature must be kept constant. The temperature can be measured using a thermometer. The reaction I will do is not exothermic and so I will therefore not need a water bath to keep the temperature constant. A thermometer could be used to measure the temperature. Instead I will perform the experiment all in the same day to make it the most accurate temperature constant without using a water bath. The surface area of the marble chips must be kept the same because the acid particles can only collide with the outer particles of the chip. However, when there are more pieces of marble, even though they have same mass, many more atoms are exposed for the acid to react with meaning a greater chance of successful collision and a faster reaction. I will keep this factor constant by using the same number of marble chips making up the same mass each time. A catalyst can alter the speed of reaction because a collision needs less energy in order to be successful. For this reason I have decided not to use a catalyst. The reaction is quick enough as it is anyway. The volume of acid used must be kept the same throughout the experiment. This is because if the volume is increased, there are a greater number of particles to react with. This could increase the rate of reaction, and make it an unfair test. Also the size and type of equipment must be kept the same. The only factor that I will change is the concentration of the acid. Also, the experiment will be repeated and the average result taken to make the results more reliable.

The apparatus needed for this experiment is: top-pan balance, boiling tube, delivery tube with bung, gas syringe, stop clock, clamp and a measuring cylinder. This apparatus will be set up as follows:

For this investigation, I will use the reaction between calcium carbonate (marble chips) and hydrochloric acid to form calcium chloride, water and carbon dioxide.

2CO3 + 2HCl → CaCl2 + H2O + CO2

In order to make sure that this experiment will work, I carried out a preliminary experiment. ...

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The apparatus needed for this experiment is: top-pan balance, boiling tube, delivery tube with bung, gas syringe, stop clock, clamp and a measuring cylinder. This apparatus will be set up as follows:

For this investigation, I will use the reaction between calcium carbonate (marble chips) and hydrochloric acid to form calcium chloride, water and carbon dioxide.

2CO3 + 2HCl → CaCl2 + H2O + CO2

In order to make sure that this experiment will work, I carried out a preliminary experiment. I did this by doing the experiment described as above but only using the two extremes of the concentrations, 0.5M and 2.5M of hydrochloric acid. The results were as follows:

0.5M HCl 2.5M HCl

As can be seen from this table there are no results for the volume of CO2 produced for 2.5M HCl past 30 seconds. This is because it reacted too fast and the full volume of the gas syringe had been used up before the next reading. For this reason I have had to change the way I will measure the rates of reaction. I have decided to measure the length of time it will take for 50cm3 of CO2 to be produced. I feel that the volumes of acid used and the mass of the marble chips used is fine for the investigation.

As I have already stated, the mass and number of marble chips will be kept the same. I propose to use 5 marble chips amounting to a mass of 3grams with a tolerance of ± 10%. The concentrations of hydrochloric acid I will use will be of 0.5M intervals between 0 and 2.5M. I will use 25cm3. I intend to measure the rate of reaction by measuring the volume of carbon dioxide produced in 3 minutes. The volume of gas will be measured using the gas syringe. I will perform the experiment as follows:

Weigh out 5 marble chips coming to the mass of 3grams using the top-pan balance.
Set up the apparatus as above, ensuring the gas syringe is completely empty and that the bung fits securely into the top of the boiling tube.
Remove the bung and place the 5 marble chips into the boiling tube.
Measure out 25cm3 of hydrochloric acid using the measuring cylinder. Pour the acid into the boiling tube whilst simultaneously starting the stop clock and putting the bung into position.
Note down the volume of gas in cm3 produced from each concentration of acid after 3 minutes by reading it off the gas syringe.
Repeat the whole experiment again.

Analysis of Results

It can be seen that the results show that as the concentration increases, the rate of reaction also increases. This is because in a more diluted acid, there are not so many acid particles. However in a more concentrated acid, there are more acid particles in it. This means there is a more successful collision occurring. In my experiment, when the concentration of the acid is at 0.5M, there is a smaller acid particle to calcium carbonate ratio. When the concentration of the acid is 2.5M, the ratio is much greater, meaning there is a greater chance of a successful collision between an acid particle and a calcium carbonate particle. This therefore creates the pattern being as the concentration increases, the rate of reaction also increases. Although my results are not very accurate, as can be seen from the first construction line, my graph does show a directly proportional relationship between the concentration and the rate of reaction, answering the aim of the investigation. I have answered my prediction, in being able to prove that as the concentration increases, the rate of reaction also increases, although my line of best fit does not show a directly proportional line, coming from the origin. This can be seen where at 0.5M the rate is 0.005s-1, then at 1.0M the rate is 0.0167s-1 and finally at 2.0M, the rate is 0.02525s-1. In no way, does this show direct proportion. Therefore part of my prediction has not been proven. This is probably something to do with a human error that will be discussed in my evaluation.

Evaluation

I do not feel my results have been very reliable. Despite the fact that there are no anomalies, and the results are all fairly close to the line of best fit, they are not directly proportional, and come from the origin as predicted, and as they should do. There are many reasons for this. One could be that the bung has not been inserted properly at the beginning of an experiment. At the beginning of each one, I found there to be too much to do. For example, I had to pour in the acid, insert the bung and start the stopwatch almost simultaneously. It comes therefore as no surprise to see the results so unreliable. Although the help of a fellow student could have been sought, they were all to busy to help. For this reason, if I were to do this investigation again, I feel a group of three would be better in order to do the three things previously described. The temperature in the classroom could also have changed during the course of a lesson. This could be because before my lesson the room was vacant making it cold. However when my class, the heat from our bodies could have warmed it up. This would show the repeated experiment to have faster rates of the reaction compared to the first experiment due to the increased heat. This problem could be overcome by using a thermostatically controlled water bath. This keeps the temperature to a very close constant. If I were to repeat this investigation, I would look into using one of those. Unfortunately they were unavailable for the present investigation. The concentration of the acid could also have played a part in my unreliable results as the 1.0 and 2.0M acids I made myself. In the process of making them, I could have made a mistake. Readily made concentrations of acid, like the ones used for the 0.5, 1.5 and 2.5M could be used to make it more accurate. The experiment could also be improved by using cubed marble pieces. This saves having to weigh out marble chips and would make the surface area much more constant. However they are very expensive, and were not available to me. These are all ways in which I might be able to show a directly proportional relationship in my results, instead of the unreliable results I currently have. This investigation could be expanded in a few ways. An extended range of concentrations could be used with a smaller interval between them. For example 0.25, 0.5, 0.75, 1.0, 1.25, 1.50, 1.75M etc. up to 5.0M could be used. Although this would be very painstaking, it would give a wider range of results, and enable us to make a much more accurate conclusion. Also, different metals could be used like magnesium. Instead of measuring the length of time to produce a certain volume of gas, the mass of magnesium lost could be recorded. A greater mass of magnesium like 7.5g could be used to make it easier.

Overall, I feel the experiment has been quite successful, despite not being able to prove a directly proportional relationship. However many different ways of doing the investigation have been found, meaning a more accurate investigation could be planned.