st
Simon Johnson
1 April 2009
RATES OF REACTION EXPERIMENT
Introduction
The rate at which a reaction occurs is governed by a variety of factors. These include the concentration of the
reactant (or pressure when in gaseous form), the temperature at which the reaction occurs, the surface area of the
solid reactants and the presence, or lack of presence, of a catalyst. If any one of these conditions is altered, the
rate at which a reaction occurs will be altered. This experiment will alter the concentration in the reaction of
hydrochloric acid and magnesium, and determine the effect of this change on the rate of reaction.
The effect of concentration change on the rate of reaction is simple to understand. In order for a reaction to occur,
there must be a collision between two particles. The rate of reaction is the rate at which these collisions occur. The
concentration is an expression of the amount of particles of one substance present. If the concentration of one
reactant is increased (and the volume is kept constant) there will be more particles, and hence more collisions
between particles. The rate of reaction will then increase. This can be observed through the rate of evolution of
the products of the reaction. This experiment will attempt to demonstrate this empirically.
Aim
The aim of this experiment is to investigate the affect of a change in concentration on the rate of reaction. This
experiment will be a reaction between a constant amount of magnesium and a constant volume of hydrochloric
acid, which will occur in the following equation:
Mg
+
2HCl
MgCl2
+ H2
The independent variable will be the concentration of the hydrochloric acid, the concentration of which will vary
between 5 mol and 1 mol. This will be measured by the collection of the evolved hydrogen using a waterdisplacement technique. The speed at which hydrogen is evolved will be recorded, and the dependant variable will
be the time taken for hydrogen evolution, to be measured with a stopwatch. The rate of hydrogen evolution will
be compared between concentrations, to draw a conclusion about the rate of evolution.
Variables
The independent variable of this experiment will be the concentration of the hydrochloric acid. Initially, 5 mol
hydrochloric acid will be used. This will then be diluted with water to 4 mol, 3 mol, 2 mol and finally 1 mol. Each of
these concentrations will be reacted with magnesium, and a rate of hydrogen evolution determined.
The dependant variable will be the rate at which the hydrogen is evolved. The evolved hydrogen will be collected
in a water-filled measuring cylinder and the amount present at 5 second intervals recorded and continued until no
reaction is ongoing. From this, we will be able to determine the rate at which hydrogen is evolved.
Four key variables will be kept constant. Firstly, the volume of hydrochloric acid must remain constant. If the
volume of acid is increased, the amount of hydrogen and chlorine atoms would increase, hence the rate of
reaction would increase. To control this, the amount of hydrochloric acid that is to be used will be recorded
carefully and kept constant. Similarly, the amount of magnesium must be kept constant. The magnesium will be
accurately measured before use, and a constant amount used.
The surface area of the magnesium must be constant. If this was increased, the rate of reaction would also be
increased. To control this, a constant amount of magnesium will be used in ribbon form throughout, rather than
using magnesium in powder form.
The temperature that the reaction is occurring at must be kept constant. If this was to increase or decrease
significantly, the rate of reaction will be altered. It is not practical to completely regulate the temperature, but the
experiment will be undertaken in one place at one time at standard room conditions. This will limit, as far as
practically possible, the impact of temperature on the reaction.
Reliability of Data
It is very important that sufficient, reliable, data is acquired to allow a reasonable conclusion to be drawn. In order
to ensure that the data collected is reliable, I intend to repeat each concentration of acid 3 times, in order for an
average amount of hydrogen evolved to be calculated. This will reduce the impact of any human error in one trial. I
will also experiment with 5 different concentrations in order to accurately represent any trends in the data.
Equipment Required
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100ml 5.00 mol dm ±0.005mol dm Hydrochloric acid
30 cm Magnesium Ribbon and 150mm ruler, marked in 1mm intervals
1x 100ml ±0.1ml measuring cylinder
3x test tubes
Delivery tube and bung
1x 25ml ±0.1ml pipette
1x boiling tube
1x Stopwatch
1x 2 litre Ice-cream container
5x 100ml beakers
Diagram
Method
1.
2.
3.
4.
5.
6.
7.
8.
9.
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Create dilute acids. Pipette 15ml of 5 mol dm acid into one 100 ml beaker, and set aside as a 5 mol dm
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acid. Pipette 12 ml of 5 mol dm acid into another 100ml beaker, and pipette 3 ml of water into the
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beaker. Set aside as 4 mol dm acid. Pipette 9 ml of 5 mol dm acid into another 100ml beaker, and
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pipette 6 ml of water into the beaker. Set aside as 3 mol dm acid. Pipette 6 ml of 5 mol dm acid into
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another 100ml beaker, and pipette 9 ml of water into the beaker. Set aside as 2 mol dm acid. Pipette 3
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ml of 5 mol dm acid into another 100ml beaker, and pipette 12ml of water into the beaker. Set aside as
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1 mol dm acid.
Cut magnesium ribbon into 15 20mm strips ±0.5mm
Fill measuring cylinder with 100ml ±0.1ml water. Invert inside an ice-cream container. Fill container with
water.
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Take the 5 mol dm acid and pipette 5ml ±0.1ml of acid into each 3 test tubes. Insert delivery tube into
measuring cylinder under water and prepare bung above the boiling tube.
Pour one test-tube into the boiling tube. Place one piece of 2cm ±0.5cm magnesium into the tube, affix
bung to the tube and begin the stopwatch
After 5 seconds, record the amount of water displaced on the measuring cylinder. This will be in millilitres
±0.1ml. Repeat this at 10 seconds.
Continue recording at 5-second intervals until 3 consistent readings are gained.
Repeat steps 4 through 7 with the other two prepared test tubes to give 3 readings per concentration
Repeat steps 3 through 8 with the remaining four prepared concentrations of acid.
Data Tables
The amount of hydrogen evolved at specified units of time at a variety of concentrations.
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5 mol dm acid
Time (seconds) ±0.5
seconds
5
10
15
20
25
30
Hydrogen
Evolved (ml)
±1ml
Trial 1
5
9
13
15
15
15
Trial 2
7
9
12
14
14
14
Trial 3
7
9
15
15
15
15
15
9.00
13.3
14.67
14.67
14.67
Average Hydrogen
Evolved (ml) ±1ml (half
range method)
Observations: very fast reaction, no visible colour change. The test tube was noticeably warmer.
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4 mol dm acid
Time (seconds) ±0.5
seconds
5
10
15
20
25
30
Hydrogen
Evolved (ml)
±1ml
Trial 1
4
7
9
9
9
9
Trial 2
4
6
9
9
9
9
Trial 3
4
7
9
10
10
10
4
6.67
9
9.67
9.67
9.67
Average Hydrogen
Evolved (ml) ±1ml
Observations: reasonably fast reaction, no visible colour change. The test tube was notably cooler than previously.
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3 mol dm acid
Time (seconds) ±0.5
seconds
5
10
15
20
25
30
Hydrogen
Evolved (ml)
±1ml
Trial 1
4
8
9
15
15
15
Trial 2
5
9
11
15
15
15
Trial 3
6
8
10
15
15
15
5.00
8.34
10.0
15.0
15.0
15.0
Average Hydrogen
Evolved (ml) ±1ml
Observations: bubbles and fizzing noticeable during the reaction. No noticeable change in heat.
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2 mol dm acid
Time (seconds) ±0.5
seconds
5
10
15
20
25
30
35
40
Hydrogen
Evolved (ml)
±1ml
Trial 1
3
7
8
9
10
12
12
12
Trial 2
3
9
10
11
12
14
14
14
Trial 3
4
8
9
11
12
13
13
13
3.34
8.00
9.00
10.34
11.3
13
13
13
Average Hydrogen
Evolved (ml) ±1ml
Observations: reaction bubbled and fizzed, but noticeably slower than previously
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1 mol dm acid
Time (seconds)
±0.5 seconds
Hydrogen
Evolved
(ml) ±1ml
5
10
15
20
25
30
35
40
45
50
55
60
65
70
Trial 1
3
5
9
10
11
12
12
13
15
15
16
17
17
17
Trial 2
3
6
8
10
12
13
15
16
17
18
18
18
18
18
Trial 3
4
5
7
8
9
9
10
11
12
16
17
17
17
17
3.34
5.34
8.00
9.34
10.6
11.3
12.3
13.3
14.6
16.3
17
17.3
17.3
17.3
Average Hydrogen
Evolved (ml) ±1ml
Observations: some bubbling, but noticeably less than before.
Each line has an
associated random error
of ±1ml of hydrogen
evolved.
Conclusion
The aim of this experiment was to determine experimentally the affect on the rate of reaction of a change in
concentration of the hydrochloric acid. The data, as presented in the graph above, supports the hypothesis. The
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gradient of the line for the 5 mol dm acid is considerably steeper than the 3 mol dm acid which is steeper than
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the 1 mol dm acid line. This shows the difference in rate of reaction: as the gradient is steeper, the rate of
reaction is faster.
Why is this the case? It can be explained by reference to the scientific theory behind the experiment. The
concentration expresses the amount of hydrochloric acid particles in the solution For the reaction to occur, there
must be a collision between the hydrochloric acid particles and the magnesium particles. When the concentration
increases, there are more particles present in a constant volume. This means that there are more collisions
between particles and the rate of reaction will increase.
The error in this experiment varies between random and systemic errors. The random error in this experiment, as
shown above, is quite small: each point on the graph can be given XXXXX. However, as even a cursory glance at the
graph shows, there is considerable deviation from what would be expected. It would, therefore, be justifiable to
say that there was a significant amount of random error in this experiment.
Evaluation
One key error in this experiment is the procedure for measuring the amount of hydrogen evolved. This was
measured at 5 second intervals. However, this is a particularly crude measure: the reaction may have completed at
5.01 seconds, but not be noticed until 10 seconds. This would have the effect of slowing the rate of reaction by up
to 5 seconds each time.
The other key error came from the execution of the experiment. The assumption made in the method was that the
bung would be applied instantly and the stopwatch started simultaneously as the magnesium was added. This was
a key weakness to my method: it proved impossible to instantly perform three different tasks. This meant that
some hydrogen was lost to the air. This would have significantly affected the recording of the evolved hydrogen: if
considerable amounts hydrogen were evolved quickly, they would have been lost to the atmosphere. This is the
explanation for the differing final amounts of hydrogen: slightly different amounts of hydrogen were lost to the
atmosphere.
There is one key random error worth addressing. The hydrogen was measured on a measuring cylinder with the
usual random error of ±0.5ml. However, due to the haste that the results had to be recorded in, the random error
was ±1ml. This is reasonably significant on this scale, and is a key area for improvement.
One variable was unexpected. The magnesium was given partially covered in plastic. In order to be used, this had
to be sanded and the plastic removed. However, this was not done systematically and some considerable amount
of plastic remained. This will have decreased the amount of magnesium in contact with the acid and would explain
the slower rate of reaction noticed in some cases.
Overall, the data was reasonably reliable. Within the constraints of the random error noted above, each trial
produced roughly similar results to the other two. However, in two cases, the trend did not fit the pattern: the rate
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of reaction for the 4 mol dm acid was considerably slower than the others and in 1 mol dm acid, more hydrogen
was produced than previously. I feel that this can be explained by the point noted above, the impact of plastic
covered magnesium and the problem of loss of hydrogen in the experiment.
In order to reduce the random error, a more finely graduated measuring cylinder could be used. However, whilst
this would reduce the random error, it would fail to reduce both the systemic error in this experiment, and the
problems inherent in the method.
One simple solution would be for two people to undertake this experiment. That way, one person could affix the
bung to the boiling tube whilst the other person starts timing. This would significantly reduce the amount of
hydrogen lost to the atmosphere, and would increase the accuracy of the results. However, this does not take into
account the problem of lack of sensitivity of results- the 5 second interval for recording.
To combat this problem, a datalogger could be used with a flowmeter. This would accurately record every 0.5
seconds the amount of gas generated by the experiment and hence give a much more accurate set of data for
comparison of the rate of reaction. Such a method would involve a fundamental redesign of the experiment, and
would render the use of a water displacement method redundant. Instead, the gas would be fed into a flowmeter,
allowing the accurate amount to be calculated.
One final small modification is needed. The key variable of magnesium’s surface area must be kept constant. As
was noted above, the magnesium was delivered covered in plastic. This had the effect of reducing the surface
area. It would be a very simple modification to the method to ensure that all of the plastic is sandpapered off, prior
to the experiment.