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Things like heat, stirring and maybe sun light could act as catalysts, however again catalyst will be a difficult variable to experiment on because it is again hard to vary just like for example temperature, or light intensity or amount of stirring.
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Concentration can be kept constant consistently throughout because the same concentration of acid will be used which will be in the same industrially produced bottles for the experiment. The concentration is easily changed too, so it can be varied by diluting with different amounts of water. We are very good and have used different concentrations of liquids in our life before for example with orange squash concentrate. We have learnt and perfected the amounts of water and concentrate we need to make the best drink. Therefore this will be a very self explanatory, and good variable to alter.
We can investigate the effect of concentration by looking at the reaction of magnesium ribbon and different concentrations of dilute hydrochloric acid. This is the formula for the reaction of Magnesium and Hydrochloric acid:
Magnesium + Hydrochloric acid → Magnesium Chloride + Hydrogen
Mg (s) + 2 HCl (aq.) → MgCl2 (aq.) + H2 (g)
We can use different concentrations of hydrochloric acid in each test, keeping all other factors the same. We can measure concentration in moles per cubic decimetre (or moles per litre), abbreviated to mol dm−3. A mole isn’t the mass of a substance, but in fact is the certain number of particles of a substance. So for example, one mole of hydrogen chloride (HCl) dissolved in one cubic decimetre of water will contain the same number of chloride ions as one mole of sodium chloride (NaCl) dissolved in one cubic decimetre of water. Because the volume of water is the same in each case, both of the solutions will have the same concentrations
If we were to investigate on the rate of reaction of an experiment due to concentration, we could use the following experiment (example). We would measure amounts of gas collected at time intervals say for example every 10 seconds, and would probably find that the reaction which is faster i.e. gas collected at a higher rate (higher gradient on a graph) is likely to be the one with the higher concentration.
This is a potential experiment that could be done to investigate the effect of concentration on the rate of reaction.
This is the most sensible method of measuring the rate of reaction, provided that we alter some of the methods and use more accurate equipment rather than the apparatus showed in this.
In this particular method of measuring rate of reaction due to concentration, I would expect that the more concentrated the solution is the faster the rate of reaction is likely to be. The 2M concentration of acid will use the magnesium strips up quicker than the 1M, and the gas (hydrogen) would be collected at a quicker rate in the 2M concentration as compared to the 1M. The amount of gas collected should be the same however the rate at which it is collected will be higher in the higher concentration.
This experiment if altered to be performed under more strict equipment will be the most accurate method of obtaining results to clarify the rate of reaction due to changes in concentration.
Just like before we can explain that rate of reaction increases when we increase concentration using the collision particles theory. In a more concentrated solution there are more particles in the same volume of solution. This increases the chances of collisions between reactant particles. This means that we will get more collisions at any given time, resulting in a faster rate of reaction.
It will be advisable to address the individual issues (variables) and then group them together to come up with a conclusion if the results are good enough. Each variable will have to be kept constant and consistently apart from the one variable which we are investigating on, concentration.
Possible ways of measuring rate of reaction
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Glass syringe- measuring the amount of gas collected in the initial stages of the reaction. This is the most accurate way and the least time consuming too!
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Measuring mass loss - this isn’t a very practical way of doing it, because the amounts we are using are very little, and also the weighing scales won’t be as accurate as we might expect them to be.
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Upturned cylinder in trough of water. This could pose problems whilst trying to measure accurate amounts of gas collected.
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Timing how long it takes for a reaction to stop. However this is the least useful out of the four. The reason for this being that it doesn’t give you initial rate of reaction.
The rates of reaction due to changes in Concentration (Final Plan)
Aim: To devise an experiment to measure the effect of changing one variable on the rate of reaction between magnesium ribbon and hydrochloric acid.
Formula: Mg (s) + 2 HCl (aq.) → MgCl2 (aq.) + H2 (g)
Preliminary experiments and results
The preliminary experiments were done in order to investigate and experiment on the values of Mg and HCl that should be used in order to make it an effective experiment. It provided an opportunity to play around and perfect the actual experiment that was to come because it acted as a practice run. The preliminary experiments gave me a better understanding of the equipment and the reaction to be investigated. Any mishaps in these would allow us to modify the actual experiment so that when a student performs this experiment he obtains optimum results, giving him/her a better understanding into the rates of reaction. By doing the preliminary experiments it was necessary to make the fine tuning and adjustments that were needed to make it a good experiment.
Chosen way of measuring the rate of reaction:
I have decided to use a gas syringe connected to a conical flask with HCl inside it and will measure the rates of reaction with magnesium ribbon. The gas syringe will be held up with a Stand, clamp and boss. The conical flask will contain the different concentrations of HCl, and will be put in a water bath, to try and control temperature. A bung will be put on the conical flask which will have a tube connecting it to the gas syringe. Two burettes also hung on a stand will be used to measure the amounts of HCl and water to dilute the acid in order to obtain the different concentrations. The acid and water will be collected in a measuring cylinder to double check that I have put the right amounts in using the burette. This will then be transferred into the conical flask, ready for the experiment to be carried out, and timed.
Ranges of variable and number of different values:
For this experiment 2M acid will be used, which is diluted into different concentrations. I initially plan to use 5 different ratios of a mixture of acid and water, each will have 2 re-runs in order to make check for mistakes and reliability, which will enable us to check for mistakes and amend them. It will also enable us to find any anomalous results which can be checked and altered by making improvements to the experiment.
We are going to have a range of concentrations and not have them close together, as this will indicate more clearly whether concentration has an effect on the rate of reaction. If we did a small range of values then, a little mistake could make the experiment go wrong, obtaining the same results when not intended to, for example the same rate of reaction of a slightly less concentration, due to a mistake in mixing the two by a very little amount.
How will the other variables be controlled if they were to be used?
-Surface area: Magnesium can be used in the same length from the same roll, so the width and length are kept constant. Although we are not measuring the effect of surface area on a reaction, we will still have to keep this variable the same as with each and every experiment, because we are investigating the effects of concentration on the initial rate of a reaction.
-Temperature: This can be controlled in a advanced water bath however a water bath is not really going to be considered because it takes too much time. Therefore we could just possibly do it in class laboratory in the open air, measuring the temperature at the start and end of the experiment too see whether we were able to control the temperature to an acceptable degree of accuracy. Otherwise we could put the conical flask in a water bath measuring the temperature at the beginning and end of the experiment. It is considered that this reaction will be an exothermic reaction; therefore it will be suitable to perform all experiments in a trough of water.
-Concentration: This is what we are going to be varying, since it is the easiest to control but also at the same time keep consistent because it comes from the same industrially produced bottle which will have the right concentration throughout accurately. We will do this by simply using water to dilute the acid. The formula we will be using will be; Concentration = 2 x volume acid / (volume acid + volume water). This however is too time consuming figuring out the volumes of acid and water. So, by simply using 100cm3 as a total of acid an water and putting the acid and water into ratios of a 100 will be much simpler and quicker, e.g. 70cm3:30cm3 (acid: water).
-Catalyst: This must be kept constant!!! We will have to keep the sequence of events the same all the way throughout, i.e., whether acid is put in first or magnesium. We will also have to avoid stirring the two, and try and keep heat and light constant too.
What is expected to happen when concentration is changed?
I would expect that the more concentrated the solution is the faster the rate of reaction is likely to be. For example a 2M concentration of acid will use the magnesium strips up quicker than a 1M concentration, and the gas (hydrogen) would be collected at a quicker rate in the 2M concentration as compared to the 1M. The amount of gas collected at the end of the experiment should be the same however the rate at which it is collected will be higher in the higher concentration.
We can explain that rate of reaction increases when we increase concentration using the collision particles theory. Reactions can only occur if collisions between reactants occur. In a more concentrated solution there are more particles in the same volume of solution. This increases the chances of collisions between reactant particles. This means that we will get more collisions at any given time, resulting in a faster rate of reaction, because of the collisions more will occur with energy greater than the Ea and therefore the rate of reaction will increase.
APPARATUS
Method
- Put all of the above on the safety mat apart from the stop watch and thermometer.
- Suspend 2 burettes using a stand and a burette clamp, leaving enough space below the burettes and the safety mat for a measuring cylinder.
- Suspend the gas syringe on the stand using the boss and clamp to secure it firmly but not too tight.
- Connect the rubber delivery tube with a bung from the gas syringe to the conical flask.
- Fill the plastic container ¾ full of water.
- Place the conical flask which is connected to the gas syringe into the trough of water.
- Get enough acid (2M) and tap water from the same bottle to last you a couple of experiments before re-filling again.
- Collect in the measuring cylinder one by one (water and acid) from the burette by subtracting the amount needed from the initial volume …double checking using the scale on the measuring cylinder so that you have the correct amount of both.
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Following the above step collect the concentration of 1M (acid: water ratio → 50:50) by putting the acid and water in to the conical flask…ratios being volume in cm3 which add up to a 100cm3.
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When you have 1M acid (50cm3 acid and 50 cm3 water) in a conical flask put it into the trough of water and measure the temperature for 20 seconds and note it down)
- Put the gas syringe back to zero.
- Collect Mg strips and cut in to lengths of 3cm.
- Have the stop clock handy, start it as soon as you have placed the Mg into the HCl and put the bung on the delivery tube on.
- Note down the volume of gas collected in the gas syringe every 10 seconds for 60 seconds.
- Repeat the same concentration (1M) twice more, i.e. have two more runs of the above experiment using the same values of Mg and acid.
- After the minute is over measure the temperature of the acid again for 20 seconds and note down your results.
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Do steps 8-16 again for the concentrations 1M, 1.1M, 1.2M, 1.3M, 1.4M (ratios 55:45, 60:40, 65:35, 70:30).
I MUST KEEP THE CHRONOLOGICAL ORDER THE SAME!!! (ie. The sequence of events of the actual experiment must be done in exactly the same order as the method shows, i.e. for example the order which the acid or Mg is used.
A graph to show my predictions which I made just before the instructions and apparatus:
What is expected to happen when concentration is changed? (A COPY OF THE PREDICTION FROM BEFORE)
I would expect that the more concentrated the solution is the faster the rate of reaction is likely to be. For example a 2M concentration of acid will use the magnesium strips up quicker than a 1M concentration, and the gas (hydrogen) would be collected at a quicker rate in the 2M concentration as compared to the 1M. The amount of gas collected at the end of the experiment should be the same however the rate at which it is collected will be higher in the higher concentration.
We can explain that rate of reaction increases when we increase concentration using the collision particles theory. Reactions can only occur if collisions between reactants occur. In a more concentrated solution there are more particles in the same volume of solution. This increases the chances of collisions between reactant particles. This means that we will get more collisions at any given time, resulting in a faster rate of reaction, because of the collisions more will occur with energy greater than the Ea and therefore the rate of reaction will increase.
Overhead is a picture of what the experiment should look like!
Preliminary experiment results
Summary of Preliminary Results
These preliminary results have provided a very good opportunity to amend any mistakes, and by the looks of the results there are definitely things that could be changed and bettered to make it a better experiment. Although, the above results of the preliminary experiment are pretty much ok, and show us the general trend of rates of reaction due to changes in concentration, there are some mishaps which do need to be sorted out to make it a better experiment.
Problems and Changes
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One of the main problems was doing four things at once, in other words starting the stop clock, putting the Mg in, sealing the conical flask, and then finally measuring the results. This was very hard to do, and sometimes resulted in hurrying the initial method up. The magnesium sometimes got stuck in between the bung and flask; because of how quickly I wanted to set the experiment rolling in order so that the magnesium dropped in at the same time as the stop clock started. This could mean unreliable results, because I could have started clock and placed Mg in the HCl all at different times, so the reaction would ultimately be measured all at different times in each run in each concentration.
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Firstly a good thing to do would be to allow you to have some time to place the Mg in to the acid properly without it getting stuck. Therefore I will start the stop clock, and drop the Mg at 5 seconds instead of at 0; however the 5 second mark will actually be recorded as 0. This will allow me to have one less job in my hands especially at the start of each run. As soon as the stop clock is about go on 5 seconds, I will now just have to drop the Mg in and quickly put the bung on. This will mean that I won’t have to start stop clock, put Mg in, and close the bung all together at the same time. Another suggestion could have been to ask someone to start the stop clock for you; however that was disallowed, although it would again as above be one less job, and would allow the timing and reaction to start at the same time.
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Another main problem was that the Mg was being used up far too quickly than the allotted 60 seconds I had given for the reaction. The Mg strips were either too short or I had put too much acid in. It was a case of both, although mainly the length of the Mg strips. In the more concentrated acid the Mg was being used up in as less as 15 seconds, meaning that the amount of gas would stay the same till the 60 seconds allotted time. This would provide a poor graph, because we would just see a rising graph, then it would be flat for the next 45 seconds. Also, the concentrations used are probably too high, and more diluted acid needs to be used, because as above the experiment wasn’t lasting the allotted time.
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To amend these little niggles and to make the experiment better and more reliable, I had to change the lengths of Mg and the concentrations of the acid used. Firstly I doubled the lengths of Mg strips to 6cm however did not fold the magnesium. This was because if I folded it then, it would be inaccurate, because if the Mg wasn’t folded in exactly the same way then it could alter the surface area, and that isn’t what we want because we want to keep every variable the same apart from the concentration. The concentration since it was providing results which weren’t of much use had to be reduced. There fore for my final results I decided to change the concentrations to 0.8M, 0.9M, 1.0M, 1.1M, and 1.2M.
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Another little problem that occurred was that whenever the bung was inserted into the conical flask, it would cause the volume inside the gas syringe to change slightly, so that it wasn’t on 0 to start of with. Another little problem I encountered was that I forgot to sometimes put the syringe back to 0 before doing the second and third runs. This although wasn’t a problem which I could sort out.
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The solution to the problem of the gas syringe was that the volume shown after the bung had been inserted, I would record the value on the syringe and use that as the starting point (0) and add the same volume of gas to each result noted down on the intervals (5 seconds). The problem of forgetting to put the syringe back to zero had no solution but to remember!
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Another problem experienced was that the temperature was hard to keep exactly the same, because of the classroom environment and not done under any controlled temperature environments. Although there were slight changes, the general temperature of the acid before and after was roughly 20°C.
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There weren’t any major things I could undergo in order to keep the temperature variable as constant as possible. Although, presuming that the temperature of the tap water was fairly constant, once it had been given time to flow, I decided to try and see whether changing the tap water in the trough of water, after each concentration, would keep the temperature more constant therefore keeping the results constant hopefully.
The temperature could have acted as a catalyst. A catalyst speeds up the rate of a reaction without being used up itself in the reaction. It can be reversed chemically unchanged at the end of a reaction. It works by providing an alternative reaction pathway of lower activation energy. At a given temperature , more of the collisions will have sufficient energy (Ecat) to be successful when a catalyst is present. There are more successful collisions per second, and therefore the rate is faster.
