I can investigate the effects of these factors on rates of chemical reactions; however, using several variables will make the situation hard to handle. Therefore, I have decided to investigate the affect of one of the four variables on the rate of reaction.
Aim:
I intend to investigate how different concentrations of hydrochloric acid will affect the rate of chemical reaction of magnesium ribbons once dropped into the solution, producing magnesium chloride and hydrogen gas.
Fair Testing:
In order to keep this experiment as adequate as possible, I must control all variables involved in this experiment. The only variable that is being altered is the concentration of the acid. I must keep in mind that the following factors are the same throughout the experiment:
- The temperature of the hydrochloric acid.
- The volume of the dilute hydrochloric acid used.
- The length of magnesium ribbon is kept the same.
- That I scrap off the magnesium ribbons as much as possible with sand paper, before experimenting with them.
- Using the same apparatus, in order to maintain consistency with results.
Prediction:
I predict that as I increase the concentration of the acid I am using, the rate of reaction will also increase.
-What makes me say this is that, if I increase the concentration of the acid there will be more acid particles present in the same, fixed volume. If there is more acid particles present in the same fixed volume, then there will be more collisions per second, which will also mean that there are more successful collisions per second, resulting with a faster rate of reaction. This is proven to me through the collision theory, which states that, the more collisions between particles in a given time the faster the rate of reaction.
However, I also predict that as I double the concentration of the acid I am using, the rate of reaction will uniformly double as well.
-I consider this the case because if you double the concentration of the acid, then I’d suspect the number of acid particles within a fixed volume of the solution doubles. If I rely my opinion on this, then I would expect that the number of successful collisions will also double, resulting with a chemical reaction double as fast.
Apparatus/Chemicals Used:
I will need to take into account the apparatus I need to use for this investigation. Since I am dealing with different concentrations I will need to be very adequate with my measurements, and therefore, I will need to use very precise apparatus. The following apparatus will be of good use in order to carry out this investigation:
Chemicals:
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4M Hydrochloric Acid- prepared by lab technicians.
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Distilled water- this is for diluting the hydrochloric acid.
Apparatus:
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Safety Glasses- this experiment deals with an acid, which can cause damage if any, gets into the eyes.
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Measuring cylinder (10ml, 100ml)- 10ml for measuring very small volumes of acid/water. 100ml for measuring bigger volumes of acid/water.
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Conical Flask- for putting all the reactants in and observe experiment.
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Thermometer- needed for preliminary work to find the most suitable temperature to keep the acid at.
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Ruler- measure magnesium strips.
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Scissors- cut up the magnesium into strips
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Sandpaper- scrap off coating on the magnesium strips.
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Stop-Clock- to time how long the reaction takes for the magnesium to disappear once it is dropped in the dilute hydrochloric acid
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Glass Rod- helps stir the dilute hydrochloric acid before the reaction begins.
Method:
- Set up practical as shown in the diagram above.
- Cut up magnesium into strips of the same length with the ruler.
- Sandpaper each one so that the coating on the magnesium strips is completely removed.
- Measure the desired amount of water needed for balancing the acid concentration with a different 10ml/100ml cylinder.
(These ranges will be predicted through preliminary work).
- Measure the desired amount of acid needed for balancing the acid concentration with a different 10ml/100ml cylinder.
(These ranges will be predicted through preliminary work).
- Stir solution with the glass rod.
- Make sure that the stop clock is reset to zero.
- Once magnesium strip is dropped into hydrochloric acid solution, start stop-clock.
- Observe magnesium strip critically, once the strip disappears immediately stop stop-clock.
- Record concentration of solution, and the result.
- Repeat steps 1-10 for every concentration of the solution you have pointed out.
- AFTER ONE SET OF RESULTS HAS BEEN COLLECTED, ANOTHER SET
OF RESULTS FOR THE SAME CONCENTRATIONS MUST BE TAKEN. THIS WILL GIVE ME ACCURATE AND CONSISTANT RESULTS, WHICH I WILL BE ABLE TO RELY ON IN ORDER TO PUT TOGETHER A FINAL CONCLUSION TO THIS INVESTIGATION.
Safety:
Safety is an area where every person taking part in practical work should be cautious of, not only for their safety but the safety of others around them. In this particular experiment, hydrochloric acid will be used and all sorts of glassware, therefore I have to keep in mind that:
- I use safety glasses; concentrated hydrochloric acid can irritate eyes.
- Be careful with the use of hydrochloric acid, if any gets on skin, must be washed away immediately. Irritant.
- I must be cautious of all the glassware I am using, breakable and can cut through skin.
- I dispose all chemicals safely once I am done with using them.
Preliminary Work:
Concentration:
I have found that the most reasonable range to take in my results is between the following concentrations (inclusive):
100 cm³ of hydrochloric acid + 0 cm³ of water
10 cm³ of hydrochloric acid + 90 cm³ of water
Temperature:
This variable must be decided on. Once we find the right temperature, we must maintain it throughout the experiment in order to conduct a fair test.
We will test this variable with a standard concentration of 50cm³ of hydrochloric acid and 50cm³ of water.
Room temperature (25ºC) is a reasonable temperature to work with. It will be very difficult to maintain temperatures that are above or even below room temperature. The practical work will become very complicated, and with the time allocated for practical work in class, we will surely run out of time.
Size of Magnesium Ribbon:
A reasonable size fore the magnesium ribbon is 5cm long. All preliminary work results were conducted using a 5cm long magnesium ribbon.
Results:
These results show the results of two tests of how long a 5cm magnesium ribbon took to disappear in each concentration. An average time was calculated by adding up both test results and dividing by two.
Analyzing Results:
(PLEASE TURN OVER TO VIEW THE GRAPH I HAVE PLOTTED USING THE RESULTS FOUND IN THE TABLE ABOVE.)
Conclusion:
The results I have obtained and the graphs I have plotted have showed me that as I increase the concentration of the acid, the rate of reaction increases. i.e. that as I increase the amount of acid found in a complete volume of 100cm³ of solution, the quicker it takes for the magnesium ribbon to disappear.
My range of results proved to be supportive to the first bit of my prediction, where I predicted that as I increase the concentration of acid, the rate of reaction also increases. The results clearly show that as I increase the amount of acid in a fixed volume of 100cm³ of solution, the reaction time decreases.
PROOF from the AVERAGE graph:
- With 50cm³ of hydrochloric acid combined with 50cm³ of water, the average reaction time was 37 seconds.
- However, with 55cm³ of hydrochloric acid, combined with 45cm³ of water, the average reaction time was 28 seconds.
Now looking more into the results the second part of my prediction seems somewhat incorrect. The second part to my prediction says that, if you double the acid concentration, the rate of reaction will uniformly double. From the results I obtained, I am not quite confident that the rate of reaction would ‘UNIFORMLY double’ if the concentration of acid were increased.
PROOF:
- With 10cm³ of hydrochloric acid combined with 90cm³ of water, the average reaction time
was 833 seconds.
If I DOUBLE the concentration of acid that would mean the volume of hydrochloric acid is now 20cm³ and the amount of water 80cm³, the average reaction time for this concentration was 109 seconds.
This is nowhere near what I have predicted.
- With 30cm³ of hydrochloric acid combined with 70cm³ of water, the average reaction time
was 69 seconds.
If I DOUBLE the concentration of acid that would mean the volume of hydrochloric acid is now 60cm³ and the amount of water 40cm³, the average reaction time for this concentration was 25 seconds.
This shows that as I doubled the concentration of the acid, the rate of reaction tripled. So, this shows there is no real pattern I could rely on to get the exact reaction time.
- Another thing I would like to point out which has convinced me that my second prediction
was wrong is that as I plotted the results for both tests and the average, I got a curve. Whereas if my prediction were correct I would expect a straight line through the origin.
In conclusion, the reality behind this, in general, follows the Collision Theory, which states that in order for particles to react they must collide and have plenty of energy to react.
Through this experiment, I kept increasing the concentration of the acid by 5cm³ and completing the rest with water to reach a total volume of 100cm³, and seeing how long it took for the magnesium ribbon to disappear. Looking now at the results, it shows that the increase in acid concentration has lead to an increase in reaction rate. The reason being for this is that when I increase the acid concentration, the amount of acid particles increases, and therefore, the number of collisions would generally increase, this would also lead to an increase in the number of successful collisions. This results with a faster reaction.
The reason why the second bit of my prediction was wrong is that, if you double the acid concentration, the number of particles is not exactly doubled. Even if they were doubled, this would not mean that the number of successful collisions would also double. It would only give a better chance for acid particles to collide and react with the particles on the surface of the magnesium ribbon. So nothing is really exact in this case.
Evaluation:
Looking back at this investigation and how accurate my results were, I would not really consider that my results were the most accurate. The reason is because:
- The equipments we were provided with by the school were quite worn out from overuse, such as the stop clock.
- The air conditioning system was put on and off several times during the lessons we had to carry out the practical work. This could have caused a bit of an interference with the results since temperature is a variable that can affect rates of reaction. This problem was something I could not get in control of.
- When measuring the volume of acid and the volume of water, I could have been a bit inadequate due to the lack of time. Things may have been rushed towards the end of the experiment.
There were some results that I was surprised with, and it was for the same concentration in both tests I carried out. The time for the reaction with 10cm³ of hydrochloric acid and 90cm³ of water seems completely out of range compared to the time for the reaction with 15cm³ of hydrochloric acid and 85cm³ of water.
My results could have been more reliable if I had repeated the test a third time. However, the fact that I was running out of time did not help. Despite all this, I think the method of gaining the results was quite a good approach. The range of results were quite spread out, and gave me a large span to compare my prediction with.
There were no exact patterns throughout the set of results I obtained. However, there was an obvious relationship, which is that as I increased the acid concentration the reaction time decreased.
If I were set more time to investigate other aspects of the original question I would try to look for other relationships, or patterns. For instance, the part of my prediction, which says that, if you double the concentration of the acid, the rate of reaction would uniformly double. If I were given more time, I would investigate that bit of the predication specifically, and try to be more accurate to see if that prediction is true in any way. Or maybe if there is other patterns such as, if I double the concentration, the rate of reaction triples or quadruples.
