For the reaction to take place the particles of the reactants must collide together and there must be a specific amount of energy in the particles called Activation Energy (Ea) to cause the reaction. If the same amount of hydrochloric acid and magnesium ribbons are used then the same volume of hydrogen is given off. It just takes different amounts of time due to the concentration of the acid.
Variables:
The three main variables that can change the rate of reaction are:
1. The concentration of hydrochloric acid
2. The temperature
3. The size magnesium ribbon.
Concentration of hydrochloric acid - With a more concentrated acid, the number of acid particles is greater, so the number of collisions is greater and the rate of the reaction is faster. In a less concentrated acid, the number of collisions is low, so the rate of the reaction is slower.
Low Concentration High Concentration
The particles are spread The particles are crowded
out and will collide less close together and will
often resulting in fewer collide more often resulting
successful collisions. in more successful collisions.
Safety:
I need to take precautions when carrying out this experiment, in order to conduct this experiment safely I must:
* Wear safety goggles as the acid is corrosive and could damage my eyes
* Take great care when using glass wear as it could shatter and cut someone due to the sharpness of glass when it breaks.
* Take great care when dealing with the acid
* Use a paper towel just in case some acid spills, then it would go onto the towel
* Make sure that the whole working are is risk assessed, with no obstructions
Fair test:
For my results to be of any use, one must ensure that the investigation is carried out impartially, therefore it is important to control the variables so that only one variable changes.
Therefore I must:
* Use the same volume of hydrochloric acid throughout (4cm")
* Use the same lengths of magnesium ribbon for each test
* Always start the timer as soon as the acid is poured onto the magnesium
Apparatus:
Hydrochloric Acid (0.5m to 3.0m)
The reactant
Burette
To get accurate dilutions
250ml Conical Flask
To do the experiment in
Magnesium Ribbon (3cm)
The reactant
Sodium Biocarbonate
The reactant / only used for pilot test
Gas Syringe
To measure the amount of Hydrogen released
Bung
To make sure no Hydrogen escapes
Thermometer
To measure the temperature
Timer
To time the experiment
Goggles
To protect my eyes
Paper Towels
To absorb any spilt acid
Method:
1. Put 25ml of 3.0molar hydrochloric acid into a 250ml conical flask.
2. Connect the syringe and the thermometer onto the bung
3. Add a strip of magnesium into the conical flask which has 25ml of hydrochloric acid already inside.
4. Place the bung on top of the conical flask as soon as the magnesium has been dropped and start the timer.
5. Record every ten seconds how much the syringe has moved until the magnesium strip has gone and record how long it took for the magnesium to disappear.
6. Clean Equipment
7. Repeat experiment changing only the concentration of acid using this set of dilutions.
Preliminary Work
With the purpose of embarking on this investigation with some background knowledge, I set up an experiment to decide what was the best concentration level to use for my actual experiment and the ideal volume of hydrochloric acid to use. I used the following concentrations in my pilot experiment: 1M, 2M, 3M and 4M with a fixed volume of 4cm" of hydrochloric acid.
Results of preliminary experiment:
Concentration of acid (M)Time taken for Magnesium to react (s)
1155
262
336
413
Fom my results I can clearly see that anything between the concentrations of 1M-3M would be fine, the reaction for 4M was very quick, and using it in my real experiment would not be worthwhile. I will use 0.5M, because the time taken for 1M was decent and it would probably provide me with a decent set of results from which I can draw up conclusions. In conclusion the concentrations that I will use for my experiment are: 0.5M, 1M, 1.5M, 2M, 2.5M and 3M. The volume of hydrochloric acid that will be used will stay as 4cm".
Results:
Concentration of acid (M)Time taken for Magnesium ribbon of 1cm to react (s)
1st reading2nd readingAverage reading
0.517898631326
1112116114
1.5117100108.5
2415749
2.5543142.5
3442333.5
I recorded the times in seconds; this was so that it would be easier for me to calculate the rate of reaction. The rate of reaction will be calculated using the following formula:
1/t- where t=time
I will use the formula to find the average rate of reaction for each Molar of acid, below are my results:
Concentration of acid (M)Rate of reaction equationRate of reactionRate of reaction multiplied by 1000Answer to 2 decimal places
0.51/13260.00075410.75410.75
11/1140.00877198.77198.77
1.51/108.50.00921659.21659.22
21/490.020408120.408120.41
2.51/42.50.023529423.529423.53
31/33.50.029850729.850729.85
Analysis:
With my results from the experiments, I can conclude that my prediction was correct since increasing the concentration of hydrochloric acid decreases the time for the magnesium ribbon to react. I found out that the 3.0M concentration of hydrochloric acid reacted fastest with magnesium ribbon than 0.5M, 1.0M, 1.5M, 2.0M and 2.5M. The higher the concentration the quicker the reaction. This happened because there are more hydrochloric atoms around to collide with the magnesium ribbon so there is a high chance of a collision.
By looking at the graph which displays my average results, I can conclude that increasing the concentration of hydrochloric acid decreases the time for the magnesium ribbon to react. I can conclude this by looking at the line on the graph which connects my results together. I can also see by looking at the graph that there is a big difference in the time taken for the magnesium ribbon to react between 0.5 molars and 1.0 molars of hydrochloric acid, however, this is not an anomalous result because this difference is shown in the results from both experiments I carried out.
Evaluation:
Overall, I feel that this experiment was quite successful; the results obtained were noteworthy as they enabled me to explain a valid conclusion drawn from my results table. Repeating the experiment twice and then calculating an average ensured that the results were reliable.
For this investigation, I took results for the concentrations of 0.5, 1, 1.5, 2, 2.5 and 3 moles which again helped ensure that all observations and conclusions made were meaningful as the results were of a suitable range. Results for a few more different concentrations such as taking some for 1.25M hydrochloric acid and 1.75M hydrochloric acid would have improved the experiment. It would also be interesting to see how the rate increases compared to 1M and 1.5M hydrochloric acid.
For the results to be of any use, I also had to ensure that the investigation was carried out impartially. It was important to control the variables so that only one variable changed. I made sure that I:
used the same amount of hydrochloric acid (4cm")
used the same size boiling tubes/ conical flask
used the same length magnesium strips (1cm)
However, one variable I did not make sure was constant was temperature. This could have altered the results and could be one of the reasons as to why I obtained an anomalous result for 1M hydrochloric acid. I have highlighted that particular point on graph 1 in blue. Another possible reason is that when the reaction takes place, bubbles of H2 are given off, which might stay on the magnesium, therefore reducing the surface area of the magnesium so the acid cannot react properly so this affects the results. The final reason for my anomalous point is that there may have been some slight human error when stopping the stopwatch.
In order to improve the accuracy of my results I could:
use a computer to time the reaction.
repeat the experiment three times and then calculate an average time between the three tests to have a larger range of results.
the magnesium used is very reactive and as it is stored in a bottle which is not fully covered, oxygen can reach the element. This is bad due to it being known as an element that can react with oxygen. This reaction is known as oxidation. The magnesium has been oxidised and this affects my results slightly. However, the results obtained were accurate.
If I were to do a similar experiment I would like to test the affect of concentration on the rate of reaction on other Metals. As there are also some other reactant metals, some even more reactive than Magnesium and also because they too are Elements. I would expect the results graph to have a similar shape or trend. For example I could test the metal Sodium, Which I could probably predict from now a more reactive metal than Magnesium. Sodium follows the same rules as magnesium except for the fact that it is not in strips.
