lower concentration small surface area
Higher Concentration
low temperature
Higher
surface area
higher surface area
Higher temperature
From this the predicted graph of the results should be a straight-line graph as shown in Figure 1
Preliminary Work
A similar experiment was performed to determine the effect on the rate of reaction of sodium hydroxide solution with marble chips. For this reaction the rate of reaction was indeed found to be proportional to the concentration of sodium hydroxide solution. The experiment about to be performed should show a similar relationship between the rate of reaction and concentration of the hydrochloric acid.
Method
The apparatus used for the experiment is shown in figure 2. This comprised of:
The experiment was set out as follows:
The conical flask was filled with a given concentration of hydrochloric acid solution to a volume of 50 cm3. The graduated measuring tube was filled with water, then inverted into the trough as shown in figure 2. So that the liberated hydrogen entered the measuring tube. The water in the measuring tube settled at a level, which was recorded. A 10 cm length of magnesium strip of cross-section was placed into the solution in the flask and the flask was stopped up. Immediately due to the reaction of magnesium with hydrochloric acid (HCl) there was liberation of hydrogen.
The timer was then started immediately, and the time taken for a given volume (50cm3) of hydrogen to be displaced into the graduated measuring tube was recorded.
This experiment was repeated with concentrations ranging from 0.4 moles to 2.0 mole of hydrochloric acid, (dilutions made from the stock solution as shown below):
Each time before starting the experiment the graduated measuring tube was inverted as quickly as possible so that the initial level of water was the same. This was to ensure that the pressure variation on the hydrogen was constant.
Considering that the stock solution of hydrochloric acid was 2M to start with, dilutions were made as follows:
cm3 of acid: cm3 of water
2.0M 50 : 0
1.8M 45 :5
1.6M 40 :10
1.2M 30 :20
1.0M 25 :25
0.8M 20 :30
0.6M 15 :35
0.4M 10 :40
The results are as follows:
From the graph on the next page, we see that there is a common trend, (with one exception, the last result). From the graph we see that as concentration of Hydrochloric Acid (HCl) increases, the time hydrogen takes to displace 50 cm3 of water in the measuring tube decreases. This means that if a higher concentration of hydrochloric acid was reacting with magnesium more hydrogen would be produced in a shorter time. From graph we see that as concentration increases so does the rate. The very last result on both graphs however indicates an error has occurred during the practical. Errors can be due to many factors making it an anomaly. The factors that affect the time taken to displace 50cm3 of water in my experiment are:
- Human Error
- Temperature
- Length of magnesium
- Concentration of acid
- Amount of magnesium in contact with HCl
Human error
As with any experiment, human error will always be a likely factor that will affect the time taken to displace 50cm3 of water. The time taken to look at the rate of water displacement, will always be a liability to the accuracy of the results i.e. the time lag in starting of the clock soon after dropping the magnesium in the acid and once 50cm3 of water has been displaced, the time taken to react, and press the stop button on the stop clock, will always affect the accuracy of the results.
Thus when using higher concentration the time taken to liberate the given volume of H2 being quite short (in my experiment less than 10 seconds) for concentration above 1.6 M and above, these results are more susceptible to human error than lower concentrations where you get longer times.
In my experiment I took care in the preparation in the various concentrations of HCl. I also took care when measuring the magnesium ribbon and cutting it to the required size, these too are subject to human error
Temperature
The effect of temperature will dramatically affect the time taken to displace 50cm3 of water. This is because the increase in temperature causes molecules to move faster resulting in the molecules undergoing more collisions, therefore more collisions have greater activation energy and so the reaction is faster.
There were no obvious changes in the temperature in the lab during the time I was conducting the experiment therefore I think it could be accepted that the temperature remained fairly constant during the experiment.
Length of magmesium
By having a longer length of magnesium, there will be more Hydrochloric Acid (HCl) reacting with the Magnesium ribbon (Mg), resulting in a higher displacement rate. This is because by increasing the length of Magnesium ribbon (Mg) you also increase the surface area. By increasing the surface area, you increase the amount of area molecules have to react with the acid. The larger the surface area, the higher the rate of reaction and so the faster the the liberation of Hydrogen and displacement rate of water in the measuring tube.
The lengths of the Magnesium ribbons were kept constant by measuring accurately, however the surface of the individual Magnesium ribbons was not the same. This could affect the surface area of the magnesium exposed to the acid.
During the experiment I noticed that the magnesium ribbon (Mg) was floating on the Hydrochloric Acid (HCl) while it reacted. And so, in theory, only half a piece of Magnesium ribbon (Mg) was reacting at one time. However this was the same for all concentrations and therefore could be taken as a constant.
In my experiment the time taken to liberate the given volume of hydrogen are very short in the higher concentration of acid, one way of increasing this time would be to use shorter lengths of the magnesium ribbon but this on the other hand would increase the time taken to liberate the amount of hydrogen to much longer in the very low concentrations of acid.
I have already dealt with the effects of concentration of acid on the liberation of hydrogen and also regarding surface area influencing the liberation of hydrogen.
conclusion
The results obtained from the experiment fit the general pattern of my prediction (exempting the last result) that with increase in concentration of the acid there is increased reactivity. The graphs produced reflect this.
However the graph was not linear, as I would have expected in accordance to my prediction. Though there is an increase in the rate of reaction with increase in concentration the graph appears to be steeper above one Mole of concentration of HCl than that of below 1.0 M. There appears to be an increase in the rate of reaction above 1.0 M of HCl. (I would think that optimum reaction between Mg with HCl is above 1.0M concentration of HCl. This would mean that the reaction is slower at concentrations of HCl below 1.0 Mole)
.
Evaluation
The accuracy and precision of the experiment was of a medium standard thus the results were of the same quality. The method was very efficiently performed to a good standard. Apart from the last result, the results obtained show that as concentration increases, the time taken to displace 50cm3 of water decreases. Obviously surrounding temperature around the Magnesium ribbon (Mg) could not be kept completely constant i.e. it was kept in room temperature (20o-25o).
I would improve the experiment by extending the concentration ranges to see if there is a limit to the rate of reaction and concentration. I.e. does the rate of reaction increase to infinity with concentration? I would also experiment by varying the length of magnesium. I would scratch the surface of the magnesium ribbon (Mg) so that any oxide formed will be taken away and only Magnesium ribbon (Mg) and Hydrochloric Acid (HCl) are reacting.
I could find out more by investigating the effect of temperature on the reaction between Magnesium ribbon (Mg) and hydrochloric acid (HCl) and measure the reaction rate at different temperatures.