Apparatus
For the experiment we will use the following equipment/chemicals:
A test tube, a test tube rack, a delivery tube, a measuring cylinder, a timer, a tray, a thermometer, a gas syringe, a micrometer(to measure the thickness of the magnesium), a pair of scissors, a pair of safety goggles, water, magnesium and hydrochloric acid.
Safe Testing
To ensure that the test is safe we will:
a) Carry out each of the basic safety procedures required before carrying out any experiment (tucking ties in, tying back long hair, wearing a pair of safety goggles etc.)
b) Hold the magnesium carefully and not let into go into the water/hydrochloric acid until we are carrying out the reaction.
c) Handle the hydrochloric acid very carefully, by using a pipette to transfer the hydrochloric acid from the bottle into the test tube.
Fair Testing
To ensure that the test is fair we will:
a) Not let any air into the measuring cylinder when turning it upside down into the water.
b) Keep the temperature at 22 degrees celsius throughout the experiment.
c) Keep the volume of hydrochloric acid at 1.5cm3.
d) Keep the concentration of the hydrochloric acid at 2%.
e) Keep the time for each reaction at 1 minute.
Collecting Our Results
I will be taking down results for five different surface areas(stated below). I will then be repeating the experiment two more times, in order to confirm that my results are accurate and precise. There are three different methods which I could use to collect my results:
a)We could use a timer to see how long it takes for magnesium to disappear.(We will not be using this method as it is inaccurate, and also too basic and would be used at a lower level.)
b)We could collect the hydrogen using a syringe and measure how much gas there is.(Although this would be the most accurate method, we will not be using this method as there may be a shortage of syringes if everyone was to use one.)
c)We could fill a beaker with water, attach a delivery tube, turn the measuring cylinder upside down and see how much gas is collected.(We will be using this method, as it is more accurate than the first method and we have the equipment required to use this method.)
Brief Method Plan(done before experiment)
Before I start the experiment I will make sure I have all the equipment required and will
prepare a results table to record my results into.
1) Get a pair of safety goggles, tie back long hair and tuck in ties(see safety section).
2) Fill a tray up with water and place a test tube into a test tube rack. Collect all the equipment you will need.
3) Place into the test tube 1.5cm3 of 2% concentrated hydrochloric acid.
4) Fill the measuring cylinder with water and turn it upside down into the water, making sure no air is let in. Get a delivery tube and attach one end to the measuring cylinder and the other end to the test tube.
5) Place the magnesium into the water, cover the top of the test tube with the end of the delivery tube and start the timer.
6) After the reaction has been going for 30 seconds, measure the amount of hydrogen collected so far and record it into your results table.
7) Measure the amount of hydrogen collected again at the end of the minute and record it once again into your table.
8) Repeat steps 2-7, keeping everything exactly the same, changing only the surface area of the magnesium by cutting one extra piece out of it.
9) Repeat steps 2-8 two more times, in order to make sure the results are accurate and precise.
Obtaining The Evidence
Calculating The Surface Area Of The Magnesium
I required a micrometer to calculate the thickness of the magnesium. In order to calculate the surface area of the magnesium I first had to calculate the area of faces A, B & C on the diagram shown below, and then add all three areas up and multiply them by two as there are six sides on the surface area. My calculations are shown below my diagram.
Area of face A=10 x 0.3=3cm2
Area of face B=10 x 0.02=0.2cm2
Area of face C=0.3 x 0.02=0.006cm2
face A + face B + face C=3 + 0.2 + 0.006=3.206cm2
Total surface area=3.206 x 2=6.412cm2
Each time we then cut off another piece of magnesium, two extra areas will be created, with each one having the same area as the area of face C. Therefore, every time we cut another piece of magnesium, we will have to add onto the surface area the area of face C multiplied by two, which gives 0.012cm2(0.006 x 2).
Method Used(done after experiment)
Firstly, before I started the experiment, I carried out the basic safety procedures listed above within the safety section, in order to decrease the chances of getting hurt. Next, I began to prepare for the experiment. Firstly, I collected a tray and filled it halfway up with water. Then I got some magnesium and cut off a 10cm strip. Then, I collected all the other equipment I needed for the experiment: a test tube, a test tube rack, a delivery tube, a measuring cylinder, a timer, a gas syringe and a pair of scissors. I was then ready to start the experiment.
I started by placing the test tube into the test tube rack, before filling the measuring cylinder up with water. I then covered up the top of the measuring cylinder with my hand, turned it upside down and placed it into the water. I did not let any air get into the measuring cylinder as this would have tampered with my results, making them inaccurate and I would have had to
repeat this step. I then measured out exactly 1.5cm3 of 2% concentrated hydrochloric acid and poured it into the test tube. Then I put one end of the delivery tube into the measuring cylinder and I was then ready to start the reaction.
I dropped the magnesium into the test tube, quickly covered the top of the test tube with the other end of the delivery tube and started the timer. After the reaction had been going for 30 seconds, I measured the amount of hydrogen collected so far and recorded it in my results table, though I did not stop the reaction or the timer. After the reaction had been going for 1 minute I once again measured and noted, the amount of hydrogen collected.
I then repeated this process for each of the different surface areas of the magnesium. Each time I kept all the other factors the same, changing only the number of pieces/surface area of the magnesium, though keeping the mass the same each time.
Finally I repeated the whole process two more times to make sure my results were accurate and precise.
Results Table(Analysis given below within ‘Analysis of Results’ section)
A Graph To Show How The Surface Area Affected The Rate Of A Reaction
Volume Of Hydrogen Produced(cm3)
Surface Area Of Magnesium(cm2)
A Graph To Show How The Rate Of Reaction Had Been Affected Half Way Through The Reaction
Volume Of Hydrogen Produced(cm3)
Surface Area Of Magnesium(cm2)
An analysis of the graphs is given within the conclusion section.
Analysing Evidence And Drawing Conclusions
Summary And Explanation Of The Reaction
The equation for the reaction between magnesium and hydrochloric acid is:
magnesium(s) + hydrochloric acid(aq)--------->magnesium chloride(aq) + hydrogen(g)
Mg + 2HCl ---------> MgCl2 + H2
A displacement reaction occurs when the magnesium reacts with the hydrochloric acid. The reason for this is because magnesium is higher in the reactivity series than hydrogen, so it displaces the hydrogen in the hydrochloric acid, forming magnesium chloride and hydrogen as the products of the reaction. Another example of this would be reacting aluminium with hydrochloric acid, where the aluminium would displace the hydrogen, forming aluminium chloride. Anything below hydrogen in the reactivity series(copper, silver, gold & platinum) would not react with hydrochloric acid as they are too unreactive, and anything above magnesium in the reactivity series(calcium, sodium & potassium)would be too dangerous to react with a strong acid.
Analysis Of Results
My results table shown above, shows that I carried out a fair test by keeping the temperature, time, and the volume and concentration of hydrochloric acid the same throughout the experiment. The table also shows that my prediction was correct, as the table proves that as you increase the surface area of the magnesium, the rate of reaction/amount of hydrogen produced increases.
Looking at the graphs, you can see that a pattern has emerged in my results. The pattern was that as the surface area of the magnesium increased, the amount of hydrogen produced also increased. The pattern in my results also agrees with my prediction. I received no anomalous results whilst carrying out the experiment. I have also included the results of my retest in the graphs.
Comparing the first graph to the second, you can see that the reaction began quite slowly before eventually increasing, as the majority of the collisions between particles occurred within the last 30 seconds of the reaction. The reason for this is because the reaction between magnesium and hydrochloric acid is an exothermic reaction. This means that as the reaction progresses the particles will heat up, and the hotter the particles are, the more energy they have and so the higher the rate of reaction will be, due to a larger number of collisions, because of an increase of energy within the particles.
Conclusion
Having analysed the results table and graphs, I can now conclude that as you increase the surface area of the magnesium, the rate of reaction also increases i.e. a larger volume of hydrogen bubbles are produced. The reason for this is because increasing the surface area gives a larger area for the acid particles to collide on, which will lead to more successful collisions, and also, the magnesium molecules and hydrochloric acid molecules can only collide with each other on the surface of the magnesium.
So, smaller particles of magnesium, lead to a larger surface area, which leads to more collisions, which leads to more successful collisions(collisions with sufficient velocity), which eventually leads to a higher rate of reaction.
My conclusion is also very similar to my hypothesis, as my hypothesis was about what I predicted my conclusion would be and as they are similar, this again proves my prediction was correct.
Evaluating The Evidence
Evaluating Our Choice Of Method
I think the method that we used to obtain our results was quite successful, although it could have been better. The most important thing to consider whilst selecting an appropriate method is that the method is accurate enough to give a valid conclusion, as this method did. Our results were more accurate than they would have been had we used the method of timing how long it took for the magnesium to disappear, as this would have been the most inaccurate method of the three main methods. There was a method which was more simple and also, more importantly, more accurate, than the method we chose. This method would have been to collect the gas using a syringe and then measure the amount of hydrogen produced. The reason we were unable to use this method was because there would have been a shortage of syringes, had everyone required one.
Although the results were not always exactly the same in each of the re-tests, I think, overall, the method was sufficient, as it provided me with a valid conclusion, as well as a reliable set of results, and it also gave no anomalous results.
Investigation Extensions
The following investigations could be carried out in order to provide additional evidence to the conclusion:
-The experiment could be repeated, though this time using a wider range of surface areas, as the range of surface areas we used wasn’t very large.
-The experiment could be repeated with a different metal, other than magnesium, to ensure that the conclusion does not just apply to magnesium.
-The experiment could also be repeated changing the type of acid used, to ensure that the conclusion does not just apply to hydrochloric acid.
-The experiment could be repeated keeping all the factors, including the surface area, the same. My prediction would be that the rate of reaction would be the same. If it was, then this would again prove that the conclusion is correct.
