The molecules must also be positioned properly in relation to each other, this is called the steric factor. A dibasic acid has twice the concentration of H+ ions than a monobasic acid this increasing the chance of a collision in the right orientation and with the activation energy required.
To find a suitable set of apparatus to use, I tested a few different arrangements of apparatus, using 2cm of magnesium ribbon and 20cm3 of hydrochloric acid. The first method I tried, was a conical flask with a delivery tube connected to a measuring cylinder submerged in water. I will put the acid in the conical flask, and then drop in the piece of magnesium and collect the gas in the measuring cylinder.
This method was not very successful, as it was extremely difficult to set up, and if too much gas was produced it was necessary to change measuring cylinders, which caused the loss of some of the gas and therefore inaccuracies. The next method I used, was the same as the method before, except using a gas syringe to collect the gas instead of a measuring cylinder.
This method was much better than the first one, as it was far easier to set up, and none of the gas was lost. There was one problem that was consistent through both of these methods, which was that it could be quite difficult to drop the metal into the acid and then put the bung back into the conical flask, and start the timer all without losing any of the gas. Overall, the second method was definitely best, so that is the one I will be using. While doing this, I also found the amount of gas give off before the reaction finished with the quantities used. I found that approximately 10cm3
To sufficiently test whether the concentration of the acid affects the order of reaction, I will need to test more than one acid and preferably different types of acid (mono, di or tribasic) to ensure that the conclusion I reach is not just for the acid tested. To find the correct acid to use, I tested three different acids, at two different concentrations with magnesium using the method specified earlier. The acids I will be testing are hydrochloric acid and sulphuric acid, and the concentrations I will be using are 0.2 mol/dm3 and 2mol/dm3. This gave me the following results;
I will use the acids that I tested, hydrochloric and sulphuric. Although, at 2mol/dm3, sulphuric acid was almost too fast to take accurate measurements for. Unfortunately, I will not be able to use a tribasic acid for my experiment as it is too dangerous for laboratory use.
After doing my laboratory trials, the reagents I will use, will be 20cm3 of acid (hydrochloric and sulphuric) with 2cm of magnesium ribbon. The concentrations I will use, will be 0.2mol/dm3 to 2mol/dm3 with intervals of 0.2mol/dm3. The apparatus I will use, will be
- Conical flask
-
100cm3 gas syringe
- Tubing
- Stopwatch
- 2 Clamp Stands
To make the correct concentrations of acids, I will need to mix them with water according to the table shown below.
As I am only changing the concentration, I will want to keep all other variables constant. One of the variables that can change during the reaction is temperature as the reaction is exothermic. This could result in inaccuracies in the results because the heat produced will give the particles more energy which will result in the experiment being faster. To prevent this, I will use a water bath set at room temperature. When I was doing my laboratory trials, I found that the magnesium floated on top of the acid meaning that only half of the surface area of the magnesium was exposed to the acid. To make it sink, I will need to fold the magnesium. I will need to make sure that I fold the magnesium in exactly the same way for each reaction so that every time there is exactly the same surface area in contact with the acid. If it was not the same, the reaction may be faster or slower making the results inaccurate. This will make the reaction slower, as there will be less surface area for collisions to take place, but as long as it is the same for every reaction, this will not affect the overall result. The acid for each reaction must come from the same stock solution, otherwise the concentrations may be slightly different making the results inaccurate. I will make sure that I use exactly the same pieces of apparatus for each reaction so that the conditions will be the same each time.
To measure out the magnesium to a suitable degree of accuracy, I will use a ruler and a scalpel to cut it. To measure out the solutions, I will use measuring cylinders.
At the concentrations I will be using it at, sulphuric acid is corrosive, and hydrochloric acid is an irritant. I will need to avoid contact with the chemicals and wear eye protection at all times. When diluting the acids, I must also make sure that I add the acid to the cold water slowly.
Implementing
Tables of Results
The table below is for 20cm3 of hydrochloric acid with 2cm of magnesium ribbon at room temperature.
The table below is for 20cm3 of sulphuric acid with 2cm of magnesium ribbon at room temperature.
There are no results for the concentration of 0.2mol/dm3 as this took to long to produce a suitable result, or did not produce a result at all.
To find the rate of reaction, I needed to work out 1/time, this is done with the average results and is shown for the two different acids in the table below.
To work out the order of reaction, I need to plot the natural log of concentration against the natural log of rate of reaction. The data for these values is in the table below.
The gradient of this graph gives me the order of the reaction, to find the gradient, I use the equation
Gradient = y2 – y1
x2 – x1
where (x1,y1) and (x2,y2) are points on the line of best fit.
This gives me;
For HCl: -1.85 - -5.30 = 2.16
0.69 - -0.91
For H2SO4: -1.66 - -4.84 = 1.99
0.69 - -0.91
Observations
When the magnesium ribbon was added to the acid, there was effervescence and bubbling as it was clear that there was a gas given off. The conical flask containing the acid heated up.
Conclusion
For both acids, the rate of reaction increases as the concentration increases, this was expected, because as the concentration rises there are more hydroxonium ions in solution, and therefore more hydrogen ions available to react with the magnesium. The rate determining for this reaction, is therefore most likely the reaction between hydroxonium ions forming hydrogen gas, as an increase in hydroxonium ions speeds the reaction up.
The way I think the reaction takes place is shown below for HCl;
1st Step
HCl dissociates in water: 2HCl → 2H+ + 2Cl-
2nd Step
Hydrogen ions bond with water to form hydroxonium ions: 2H+ + 2H2O → 2H3O+
3rd Step
Magnesium reacts with chloride ion to give MgCl2 leaving two spare electrons from the magnesium: Mg + 2Cl- → MgCl2 + 2e-
4th Step
Two hydrogen ions dissociate from two hydroxonium ions and react with the spare electrons to form hydrogen gas:
2H3O+ + 2e- → H2 + 2H2O
My results gave the order of reaction for hydrochloric acid to be 2.16, and the order of reaction for sulphuric acid to be 1.47. This result was not what I expected, I expected the order of reaction to be greater for sulphuric acid, because it produces twice as many hydrogen ions when it dissociates. The results are also not consistent with the reaction mechanism I stated above.
In the graph for log concentration against log rate of reaction, I noticed that there was a kink in the graph for sulphuric acid at about 1.0mol/dm3. Either side of this kink, the line of best fit would be at very different angles, if they were treated as two different parts. If I work out the orders for them individually, I get an order of 1.77 between 1.0mol/dm3 and 2.0mol/dm3, and an order of 2.14 between 0.4mol/dm3 and 1.0mol/dm3. This would lead me to believe that sulphuric dissociates differently at different concentrations. It dissociates more at lower concentrations (up to about 1.0mol/dm3) than it does at higher concentrations (above about 1.0mol/dm3) I cannot say whether this is the case for all dibasic acids, so further experimentation would be required to come to a more definite conclusion for the investigation.
Evaluation
Overall, I feel that my experiment was performed to an acceptable degree of accuracy, but there were definitely a few areas that could be improved to make the results and therefore conclusion reached more reliable.
The main problem I found was the process of dropping the magnesium into the conical flask, putting the bung in, and starting the stopwatch all without any hydrogen gas escaping. There are a number of ways in which I could overcome this, I could have someone else assisting me, by doing one of the bits for me. I could also have started the stopwatch and then dropped in the magnesium when the timer reached 1 or 10 seconds, and then taken that number off the final time. There is still also the human error involved with starting the stopwatch that cannot be removed.
Even though I was using a scalpel to cut the required length of magnesium, it was still difficult to get the length exactly, especially using a ruler, as the markings on the ruler are 1mm apart, so it is possible to cut the magnesium to slightly the wrong length, and when the length of magnesium that is being used is so small, this could make a significant difference. A ruler with a smaller scale would help improve accuracy on this, or possibly measure out the amount of magnesium using scales rather than cutting. If scales are used, powdered magnesium would probably be easier to use.
There was the same problem on the gas syringe, as the markings were 0.1cm3 apart, this meant that the gas evolved in the time recorded could be out by 0.1cm3. To solve this, a gas syringe that is accurate to smaller volumes.
When measuring out volumes of solutions used, I used a measuring cylinder, but it would have been far more accurate to use a pipette, as the measuring cylinders were only accurate to 0.2cm3.
I was using a water bath at room temperature to help prevent the effect of the exothermic reaction heating up the solution providing more energy making the results inaccurate, but I do not believe this had much effect. Perhaps a larger water bath would help overcome this, and maybe this could be enclosed my another insulating material.
To further develop my experiment, I could repeat the experiment using more concentrations, this would make the line of best fit more accurate, and therefore working out the order of reaction more accurate. I could also repeat the experiment for more mono and dibasic acids to make sure that the conclusion I came to was not specific to the acids I used. It would also be interesting to see if there was a difference between strong and weak mono or dibasic acids. I could also extend the investigation to include tribasic acids.
