Determine what affects the rate of reaction when magnesium and hydrochloric acid are combined to form a solution.
Magnesium and Hydrochloric Acid Coursework
Aim
The aim of this investigation is to determine what affects the rate of reaction when magnesium and hydrochloric acid are combined to form a solution.
The variable that I am going to change shall be the concentration of acid (the acid used in this experiment is going to be hydrochloric acid). We are going to control the temperature, the amount of acid and the amount of magnesium. We are going to measure the amount of hydrogen given off from the experiment.
Prediction
I expect that the magnesium will displace the hydrogen in hydrochloric acid, to form magnesium chloride.
I expect this to happen because magnesium is higher in the reactivity series than hydrogen. If an element is higher in the reactivity series than an element from a compound, it will displace it. In this case, magnesium will be displacing hydrogen. This is called a displacement reaction. Magnesium is an element and hydrochloric acid is a compound, made up of hydrogen and chlorine. The hydrogen will be displaced, and the magnesium will connect with the chlorine to form the compound, magnesium chloride.
This is the formula for the reaction:
Mg + 2HCl MgCl2 + H2
magnesium hydrochloric acid magnesium chloride hydrogen
There are several factors that could possibly affect the rate of reaction between magnesium and hydrochloric acid. It depends on the surface area of the magnesium used, the mass of the magnesium, the temperature of the hydrochloric acid and the concentration of the hydrochloric acid.
Particles need to collide with enough velocity so that they react. As the temperature is increased the particles move faster; because they have more energy. This means that they are colliding more frequently and more of the collisions have enough velocity to cause a reaction. Since there are more collisions the chemical reaction takes place faster; therefore increasing the rate of reaction.
I will also be expecting the time taken for 100cm³ of hydrogen given off to decrease as the concentration of acid is increased. I also predict that all of the experiments within this investigation will gradually slow down after a rapid beginning to the reaction.
This is because of the collision theory. This theory states that reactions should slow down because towards the end of a reaction there are fewer particles to collide. At the beginning of a reaction, none of the particles have reacted. This means that all of the particles at the beginning of the reaction are available to react; so there is more of a chance that the particles will collide and therefore increase the speed of the rate of reaction. Towards the end of the reaction, most of the particles will have already collided, making collisions more infrequent; which resultantly slows down the rate of reaction.
Method
Before I begin the main experiment, I am going to perform a smaller preliminary experiment first. In my preliminary work I am going to measure the speed at which gas is produced at three different concentrations molar acid when reacted with 0.1g magnesium. The concentrations that I am going to use will be 2.0, 1.6 and 1.2 molar acid. I am doing this to check that the volume of acid that I have chosen (100cm³) will be sufficient for the experiment.
Concentration of acid (molar)
Time taken to release 100cm³ gas (seconds)
2.0
...
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Method
Before I begin the main experiment, I am going to perform a smaller preliminary experiment first. In my preliminary work I am going to measure the speed at which gas is produced at three different concentrations molar acid when reacted with 0.1g magnesium. The concentrations that I am going to use will be 2.0, 1.6 and 1.2 molar acid. I am doing this to check that the volume of acid that I have chosen (100cm³) will be sufficient for the experiment.
Concentration of acid (molar)
Time taken to release 100cm³ gas (seconds)
2.0
3.63
.6
32.64
.2
40.35
These preliminary results show that for the weaker concentrations of hydrochloric acid, 100cm³ of hydrogen is only just being given off. I will need to perform a mole calculation to work out a suitable volume of hydrochloric acid to use.
Here is a mole calculation that I will use to find an appropriate volume:
moles = mass / R.A.M.
= 0.1 / 24
= 0.0041666
volume = moles * 24 000
= 0.00416 * 24 000
= 100
After having completed a mole calculation, I have decided that 100cm³ of hydrochloric acid will be sufficient for my main experiment.
For my main experiment, I am going to take several steps to complete each individual experiment (i.e. each concentration). The order in which I will perform the experiments will be from the highest concentration of hydrochloric acid to the lowest concentration (2.0 molar to 1.2 molar):
> Firstly, I will get out all of the appropriate apparatus (listed below) and set it up as shown in the below diagram. In order to set up the measuring cylinder correctly, I will have to cup my hand over the top, put it in the water and then remove my hand.
> I will then cut off a strip of magnesium, weighing 0.1g. I must make sure that the weight of the magnesium remains constant throughout the investigation, so that here is no bias.
> I will also add 100cm³ of hydrochloric acid to the conical flask.
> Then, I shall drop the strip of magnesium into the conical flask (containing the hydrochloric acid).
> As soon as the strip of magnesium has been added to the hydrochloric acid, I will immediately attach the bung to the conical flask.
> I will then start the stop-clock.
> I shall then carefully watch the measuring cylinder, to make sure that I do not miss the point where 100cm³ of hydrogen is reached. This is particularly important during the experiments involving the higher concentrations of acid as the reactions are much faster and so it will be easier to miss the point at which 100cm³ is reached.
> When 100cm³ of hydrogen given off is reached, I will stop the stop-clock, and record the time it took for the hydrogen to be given off.
Here is a list of the apparatus that will be used for the experiment:
> Conical flask
> Measuring cylinder
> Longer measuring cylinder
> Delivery tube
> Plastic tub
> Water
> Magnesium
> Hydrochloric acid
> Stop-clock
> Safety goggles
Here is a diagram of the apparatus that I am going to use for the experiment:
As with all science experiments, safety precautions need to be used. It is very important that safety is considered as dealing with chemicals in a chemistry lab can cause harm.
Because of this, I am going to ensure that I am wearing safety goggles during the whole of the experiment, including when I am getting the apparatus out and putting the apparatus away. I shall also make sure that care is taken when handling glass (for example the measuring cylinder) in order to ensure that the glass is not broken. The chemicals shall be cleared away thoroughly and safely.
As well as safety, I must ensure that the test is made as fairly as possible. In order to do this, I shall be keeping the amount of magnesium constant (at 0.1g). In addition to this, I shall be performing all of the experiments at room temperature, so that there is no bias. It is also easiest to perform all of the experiments at room temperature because I will not have to change the temperature of the hydrochloric acid for every experiment. I will not be using any way of controlling the temperature, apart from relying on room temperature remaining constant.
Results
Concentration of acid (molar)
st experiment (time taken to release 100cm³ of acid (seconds))
2nd experiment (time taken to release 100cm³ of acid (seconds))
3rd experiment (time taken to release 100cm³ of acid (seconds))
Average of three experiments
2.0
7.06
8.75
9.10
8.30
.8
1.06
1.00
0.80
0.95
.6
3.94
6.45
4.54
4.98
.4
20.10
22.66
21.04
21.60
.2
25.94
30.04
29.94
28.64
The results on this graph are accurate to 2 decimal places.
I have decided that there are no anomalous results in my results table. I considered dismissing the first experiment of the 1.2 molar concentration of acid as an anomalous result (25.94 seconds) but decided against it, as the result did not overlap with any of the other concentrations.
Graph
Conclusion
As I expected, hydrogen was given off as a product of the reaction and the higher concentrations of hydrochloric acid took less time to give off 100cm³ of hydrogen than the lower concentrations did, so the general trend is the weaker the concentration of acid, the longer it takes for 100cm³ of hydrogen to be given off. This is what I predicted before I carried out the investigation. All of my results followed the same general trend, although some of the results were not as accurate as I would have hoped.
What happened was; when the magnesium was added to the hydrochloric acid, particles of magnesium collided with the molecules of hydrochloric acid. A molecule of hydrochloric acid contains one particle of hydrogen and one particle of chlorine. When one of these molecules combines with a particle of magnesium, the magnesium displaces the hydrogen, to form magnesium chloride and hydrogen. This type of reaction is called a displacement reaction. A displacement reaction will only take place if the element is more reactive than one of the compounds. The magnesium in this reaction was able to displace the hydrogen because it is more reactive than hydrogen.
When there is a higher concentration of hydrochloric acid, there are naturally going to be more molecules of hydrochloric acid. So, when the magnesium is added to the solution, there is a higher chance of collisions occurring (because there are more hydrochloric acid molecules to react with magnesium particles), therefore it is likely that more collisions shall occur and so the rate of reaction is likely to be higher than the rate of reaction for a weaker concentration. This is why my graph shows that the higher concentrations of acid take less time to give off 100cm³ of hydrogen than the lower concentrations of acid.
Evaluation
I thought that overall the investigation went well, although the results of my main investigation did not tie in with the results of my preliminary experiment. I decided that none of the results that I obtained from the experiment were anomalous, but some of them were inconsistent or did not directly follow the general trend.
If I were to carry out the experiment again, there would be several things that I could potentially change in order to make the experiment more accurate. To do this, I would have to make changes to the method. To start with, I could keep the temperature constant at a temperature other than room temperature. The fact that I chose to carry out the experiments at room temperature could have made the investigation inaccurate because room temperature varies and so there could have been bias. Considering the fact that the investigation was carried out over several days, there is a likely chance that this factor made the investigation inaccurate. This factor may explain why the preliminary results are so different to the results of the main experiment, and also why some of my results did not follow the same general trend as the other results.
My evidence may not be entirely reliable for other reasons as well. For example, after I had added the magnesium to the hydrochloric acid, there was a moment before I was able to attach the bung and consequently some of the hydrogen given off could have escaped. There will have also been a moment before I could have started the stop-clock, so all of the results are probably slightly inaccurate.
When I was weighing out the 0.1g of magnesium each time, I used scales that were probably inaccurate to some extent. This could have made the investigation inaccurate because I could have been using different amounts of magnesium for each experiment.
However, in general I believe that I have gathered enough information from this investigation to come to a firm conclusion; that magnesium if is added to higher concentration of acid, then there is likely to be less time taken for a certain amount of hydrogen to be given off than if magnesium was added to a lower concentration of acid.
If I really wanted to confirm the trend I could perhaps carry out more experiments for each concentration, in order to gain a more accurate average. However, I believe that I have gained sufficient evidence already with the results that I have. The correlation of the line of best fit on my graph proves this.
Jordan Hoose
