The equation for this reaction is…
Mg + 2HCl → Mg2 + H2 ………Hydrogen is given off.
The whole experiment must be as fair as possible. As this is a reaction between two chemicals, I will wash out the apparatus (measuring cylinder and conical flask). This is to make sure that there is no acid or magnesium still left inside. If there were, it could alter the experiment, as there may be more Magnesium or Acid than needed. I will also repeat the entire experiment twice to make sure it is accurate. I will then draw the graph using the averages.
I predict that the higher the concentration of acid, the faster the reaction. I.e. it will take a shorter time for a certain amount of hydrogen to be produced. The prediction is supported by particle theory, which shows that as there are more acid particles when the concentration is higher, then there will be more chance of them colliding with the Magnesium particles. However, if the concentration is lower, so there are less particles, the likelihood of them colliding with the Magnesium particles is slimmer. This means that it will take longer for the particles to find and hit each other, as there are less of them. When the Magnesium and Acid particles collide, they react. Therefore, with more concentration of acid, the reaction occurs faster as the particles collide sooner.
There will need to be some safety precautions during the experiment. As with all reactions, it is best to wear goggles. We are varying acid concentration, so this may mean that high concentration could be dangerous. Even though we are using sensible acid levels, I will use goggles just in case.
Apparatus
- Shallow, large plastic bowl
- Magnesium strips (5, each of the same length)
- Hydrochloric Acid (same concentration)
- Water (to dilute the acid)
- Rubber bung
- Conical flask (must be the same sort each time – must have attachment to tubing)
- 2 Measuring cylinders (with markings in cm cubed)
- Stopwatch
- Bee hive shelf
- Micrometer (to measure width of Magnesium)
- Ruler (to measure length)
- Electronic scales
In the experiment, the acid is given as the same concentration. The water is used to dilute it and therefore change the concentration. This is done by percentage. To start with I will just use 100% acid and magnesium. I will then use 80% acid and 20% water. The total volume must always add up to 50cm cubed. Therefore 80% and 20% can be translated as 40 cm cubed of acid and 10cm cubed of water. The percentage of acid should decrease by 10 cm cubed with each reaction, so the water percentages will increase at the same rate.
Method
- Set up apparatus as shown in diagram below making sure that the water in the upside down cylinder is full up and that there are not any air bubbles.
- Measure the width, length and weight of the Magnesium strip you are using by using a micrometer, ruler, and a pair of scales. Then record the result.
- Add one strip of Magnesium into the conical flask, immediately, put on the bung and start the stopwatch. (This should be done with more than one person.)
- Stop the stopwatch once the water level reaches 50cm cubed on the measuring cylinder.
- Record the time taken.
- Wash out the conical flask and measuring cylinder.
- Repeat stages one to six but replace the concentration of acid with the next one down. This should be a lower concentration than before. Continue until all five concentrations have been measured.
- Repeat the experiment (stages 1 to 7) twice to make sure the results are accurate.
OBTAINING EVIDENCE
I recorded the length, weight and width of the Magnesium before each reaction, in case it was different. For every time it was measured, I found that the length was 5cm, the weight was 0.09 grams and the width measured 0.23mm. Therefore, the time taken for the water to be pushed out of the cylinder until 50cm³ was not altered by any of these things.
This is a table of all three experiments and a table of repeats. Each time, the water level had to reach the same place. I have written ‘time taken’ instead of rate of reaction because the whole reaction is not being recorded. It is only timed until the water level reaches a certain point.
EXPERIMENT 1
(s) = Seconds
In the last result, there is no acid. There is no reaction between water and Magnesium, so a time is not recorded.
EXPERIMENT 2
(s) = Seconds
These results varied from my last experiment. Each reaction is generally a lot faster.
EXPERIMENT 3
(s) = Seconds
Here is the averages table for my results. I have worked out the averages using a calculator and they are done to two decimal places.
AVERAGES OF ALL THE EXPERIMETS
(s) = Seconds
I did not observe any other changes during the reaction besides the varying ferocity of the bubbles in the conical flask. I found that as the concentration of acid increased, so did the amount of bubbles. This shows that the reaction was occurring faster.
I have decided to draw a line graph to show the results, as the gradient of the lines will help me show whether my prediction was correct. This makes it possible to do a line of best fit, which I would be unable to do on a bar chart.
ANALYSING EVIDENCE
My results were very hard to place into a graph, as there was such a wide range of results. However, from this graph, you can see that there is a positive trend in the way the results progress. As the concentration of acid increases, the time taken decreases. This agrees with my prediction. When there is a higher concentration, there are more acid particles, therefore there is a higher chance of one of them colliding and reacting with a Magnesium particle. This is why the reaction occurs so fast to begin with. Then, as there are less acid particles and more water, there is less chance of these collisions, so the time taken is longer. This can be seen in the graph.
The graph shows that the increase in time is not so substantial at the beginning, when the concentration only just begins to decrease. I was not really expecting this. However, this is very important in showing whether the right range of acid was used. When using 40cm cubed of acid (80%), the reaction was still extremely fast. Therefore, there may have been too many acid particles when there was 100% acid to make a difference. There was a high chance of the particles colliding with 40cm cubed. This means that using 50cm cubed would not make much difference to the likelihood and the acid would outnumber the Magnesium.
Towards the end, there is a big incline in the time taken, as the concentration is extremely low. This can be observed when the line of best fit gets dramatically steeper. The amount of acid is so low, the chance of collisions means that the time taken increases. I explained this in my prediction so the graph supports it.
We can see here that it is the Hydrochloric Acid that has the activation energy, as they are the particles, which are moving and colliding with the Magnesium. It was better to use a curved line of best fit here, as there was such a step gradient at the end, a straight line would be inaccurate. However, I think that if the graph and the experiment were to continue by using more Hydrochloric Acid, the graph would begin to become a lot less steep. This is because the extra acid particles are not needed due to the Magnesium particles already reacting with other ones. The remaining acid particles would therefore have nothing to react with. We can see that this may happen by using the graph. The higher concentration’s time progress higher a lot slower due to this.
EVALUATION
I do not think that there were any unusual results in any of my experiments. The results seemed to support the prediction, and the repeats followed the same trend as the first experiment. However, there was a substantial variation between the actual figures from each experiment. For example, with 60% acid concentration, I recorded 88.78, 50.06 and 36.47 seconds. However, compared to the other results from each of their individual reactions, they fitted in well. I think that this may be due to the original concentration of the acid that we were given. This may have varied, so that when we diluted the acid, it was not the same every time. This probably altered our results.
However, the general standard of our observations was good. We started the stopwatch as soon as possible by working with more than one person, and made sure the test was fair by measuring other factors (weight, length and width.) However, the results could be even more accurate by having more decimal places on the scales and improved observations of the water and acid measurements. Also, air bubbles in the measuring cylinder may mean that there was not the correct amount of water meaning that we would stop the stopwatch at the incorrect time. These things could have affected the results I got.
I feel that the range of readings that I used was okay. Due to what I found on the graph, I realised that perhaps, using 40cm cubed and 50cm cubed was slightly inappropriate. This is because the graph showed that the difference in their times was not all that much, meaning that by 50cm cubed, the acid particles outnumbered the Magnesium ones, so the remaining particles were ineffective. It would have been better to not do 50cm cubed at all. However, it did help me to realise this. I also noticed that when I chose to time the reaction until the water level reached 50cm cubed, the reactions took slightly too long towards the end. This meant that we could not do another repeat and were slightly rushed.
I think my method was suitable and was appropriate in relating to what we were supposed to find out. To make the method more detailed and accurate, it would be better to keep the conical flask the same throughout and perhaps even measure the temperature of the room as well. This is because variation in temperature can change the speed of a reaction as the particles have more energy. To get more evidence, I could widen the concentrations to see if my prediction about higher concentrations was correct and perhaps see how fast the reaction is when having a concentration of below 20%. I could also investigate the same thing by doing a gas syringe experiment, which might be more precise. Another way to monitor rates of reaction is by seeing how the factors of temperature and surface area can alter it.
