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Rates of Reaction

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Whether the strength of Hydrochloric acid will affect the speed of the
rate of reaction when reacting with Magnesium Ribbon.

I want to find out whether the strength of Hydrochloric acid will
affect the speed of the rate of reaction when reacting with Magnesium
Ribbon.

Variables

There are two important independent variables in my investigation the
first of which is the concentration of the Hydrochloric Acid used
during the investigation. The concentration of the acid will partly
determine how fast the reaction takes place, if we put some of the
magnesium ribbon into the HCL at 2molar then it might have simply
dissolved within a few seconds and then I would not have had time to
record down any decent enough results to plot a graph with. The other
important independent variable that will affect the speed of the rate
of reaction is the length of magnesium ribbon. We need to put enough
Mg Ribbon into the HCL so that it doesn’t run out straight away but
also so that the experiment stops by itself once the gas syringe has
filled up.

The reasons for choosing these two independent variables are that it
would be too hard to try and find a catalyst for this experiment; a
catalyst is a substance that increases the rate of a chemical reaction
without being consumed in the reaction. Finding a catalyst for this
particular experiment could take months never mind the amount of time
we have been given for our experiment.

The independent variable that I will change is going to be the
hydrochloric acid, we will use 50ml of HCL at the following molarity
concentrations; 2molar, 1.6molar, 1.2molar, 0.8 molar and 0.4 molar. I
have decided to experiment with these different strengths of molar
because I think it will give a wide set of results to put into a
graph.

The dependant variable I shall be measuring will be the time it takes
for the magnesium to completely react with the hydrochloric acid, I
shall record the times for each concentration of hydrochloric acid
three times so that it makes it a fair test.

To ensure the investigation is fair we will use the same length of
magnesium ribbon for all the different strengths of hydrochloric acid
and the length will be the one that we picked out from the preliminary
experiments. Another way in which we will keep the investigation fair
will be to carry out an experiment at each concentration of
hydrochloric acid three times so that we can then get hopefully around
the same outcome from each experiment, and if there are any anomalies
we can try to explain why they happened.

The final way in which we will try to keep our experiment fair will be
to use the same gas syringe each time therefore we will not have one
which glides smoothly for one experiment and one that sticks and jumps
for a different experiment. Although we cannot always guarantee that
the experiments will be fair because we will not always get the same
bottle of hydrochloric acid because there will not be enough to carry
out all the experiments with just the one bottle so the different
bottles could be at ever so slightly higher or lower concentrations of
hydrochloric acid which will affect the outcome never the less.

Prediction

I think that my results will show that the hydrochloric acid with a
molarity of 2 will finish reacting the quickest because it is the
strongest hydrochloric acid and I predict that the slowest experiment
to finish reacting will be the 0.4 molar strength of hydrochloric
acid. I think that the results will go slower and slower the nearer
they are to the 0.4 strength of hydrochloric acid. The reason that the
experiment with hydrochloric acid at 2 molar will dissolve the
magnesium ribbon faster is because it is of a stronger concentration
and so the molecules in the hydrochloric acid move faster and so react
with the magnesium ribbon quicker.

Preliminary Experimental Work

I have carried out two preliminary experiments, the first reason that
I carried out these were to see which of the two would be easiest and
most accurate to carry out in the actual experiment. Then once I had
picked one of the two to take forward I then went on to try to get
some good measurements and molar strengths to use in the actual
experiment.

Burette Method

Equipment

Conical Flask, Bung, Connecting Pipe, Burette, Large Measuring Jug.

For this method we first set up the equipment needed to carry out the
burette experiment as shown below;

The burette method consists of the Magnesium ribbon being put into a
conical flask with 50ml of hydrochloric acid quickly put the bung in
once the magnesium ribbon has been put in, then the gas will go up and
around the pipes and go into the burette which will be filled with
water as air bubbles. This method was hard to take down results
because every time a bubble of the gas released went through the
burette it would simply stop and start, for example just as I was abut
to take a reading a bubble shifted the reading down 7ml and so this
gives the results a certain degree of inaccuracy. We took down
measurements in 5 second intervals. The burette has a volume of 50ml³.

Gas Syringe Method

Equipment

Conical Flask, Bung, Connecting Pipe, Gas Syringe, Hydrochloric Acid,
Magnesium Ribbon, Clamp, Stand.

For this method we set up the equipment as follows;

The first to do is to gather together all equipment, then once that is
done get the conical flask and put the bung into the top of it. Then
connect the pipe from the bung to the gas syringe, making sure that it
is air tight. Then once everything is set up get the stop clock ready,
measure out the 50ml of hydrochloric acid and pour it into the conical
flask and again the same as the burette method simultaneously put the
magnesium into the conical flask, put the bung into the top of the
flask and start the timer. We took down measurements in 5 second
intervals. The gas syringe has a volume of 100ml³.

GRAPH

This is another disadvantage of the gas burette method because it will
only measure up to 50ml³ we will not be able to get the full reading
for the experiment. Another disadvantage is that the burette method
changes readings in stages going from 9 to 17 instantly as soon as an
air bubble reaches the surface; this can make the experiment
inaccurate because if you about to take a reading and an air bubble
comes to the top and the reading drops then it will not be
scientifically accurate.

Gas Syringe Method

GRAPH

This is another example of an advantage of the gas syringe method as
it can hold up to 100ml³ of gas. Another advantage of the gas syringe
method is the accuracy that this method allows us to have because the
gas is constantly passing into the gas syringe the reading is steadily
and constantly going up so when the time comes you can take an
accurate reading.

How did the preliminary Experiments help with my decisions for the
actual experiment?

The preliminary experiments helped me to make a lot of decisions and
the most important decision of all which was with which method I would
carry out the actual experiment? I have decided to do the experiment
using the gas syringe method; I made this decision using several
things from the preliminary experiments the first was the fact that
the gas syringe method was quiet a lot more accurate than the burette
method. This means that it would be a good idea to use this method
because it is more accurate when taking down the results and so should
therefore give us a more accurate rate of reaction. Other things that
pushed me towards making the decision to use the gas syringe method
was that it was a lot easier to take down the results because the
readings on the side of the gas syringe can be set to any angle
whereas the readings on the side of the burette will always have to be
upside down and so this makes it a lot harder to try and read out the
readings when the time comes.

Equipment

Conical Flask, Bung, Connecting Pipe, Gas Syringe, Hydrochloric Acid,
Magnesium Ribbon, Clamp, Stand, Measuring tube and Stop clock.

The accuracy of the conical flask will go up in sets of 25ml, but we
will first use the measuring tube to measure out the quantities of
both the acid and the water we will use to dilute the acid. The
measuring tube is accurate down to 1ml. The accuracy of the gas
syringe is also down to 1ml but this is down to 1ml³.

Risk Assessment

The acid that we are using in this experiment is of 2 molar strength,
which is strong enough to at least cause irritation to the skin if
spilled on it, the hydrochloric acid would also cause irritation to
the eyes and mouth if ever spilled into those. Another risk is that
magnesium is flammable especially if in powder form, but we are using
it in a ribbon form and we are not using any flames in any part of the
experiment. The gas which is given off from the reaction between
Hydrochloric acid and Magnesium ribbon is Hydrogen which is a
flammable gas.

We will reduce the risk of these hazards by wearing protective eyewear
for example laboratory goggles, we will also measure out the
hydrochloric acid pouring away from the body, over the sink. We will
not be using flames in our experiment so the risk is reduced from the
hydrogen and magnesium but still other experiments could be using
flames so we will still have to be careful and make sure to stay away
from any other experiments using flames.

Method

Above is a labeled diagram of my equipment which has been set up.

The first thing that we shall do is to prepare our work area where we
will be doing the experiment, clear anything that could cause an
obstruction or any risk. The next thing to do is to simply gather
together, and not set up, all equipment that we shall need. When
getting the hydrochloric acid we shall take one container full and if
we need anymore we merely ask for some more, and we shall only get the
magnesium ribbon when we need it, this is not only for safety reasons
but also we shall be able to measure out the desired amount there and
then. Next we will set up the equipment that we have just gathered
together, as the diagram above shows.

Once this is done we will take the measuring tube and measure out our
first strength of hydrochloric acid, once measured out accurately we
shall then put it into the conical flask. Then we shall measure out
the 10cm of magnesium ribbon and gently fold it into a smaller shape.
Then this is where a moment of inaccuracy could enter into the
experiment we now have to put the magnesium ribbon into the conical
flask, whilst at the same time put the bunk into the top and start the
stop watch, but this is impossible so we just have to do it as fast as
we can whilst still being safe in what we are doing.

Once we have done this we then record results every 5 seconds until
the experiment runs out of either hydrochloric acid or magnesium
ribbon and so cannot carry on. I will take measurements ranging from
1ml³ to 100ml³, as this is the largest amount of gas that the gas
syringe will hold. I will check that my results are reliable by
firstly checking that everything went as predicted in which the
weakest solution took the longest time in order to give off all the
gas and that the strongest solution would finish reacting the
quickest.

Another way which we will check that the results are reliable will be
to check the results against the preliminary experiment. I shall make
sure that my results are accurate by keeping some aspects of the
experiment the same. The things that are going to be kept the same are
the amount of hydrochloric acid and water, there will always be 50ml
no matter what the concentration, The length of the magnesium ribbon
will not change either; there will always be 10cm of that and the
temperature at which the experiment will take place will always be
near enough the same because we shall carry it out at room temperature
and although we cannot always control this we will always do the
experiment in the same room as to try to lessen this possible change.
We will change however the concentration of the hydrochloric acid.

GRAPH GRAPH GRAPH GRAPH

One of the key calculations that I used was the calculation of how to
work out the rates of reactions which was, you have to take the amount
of gas given off after the first 5 seconds and divide it by 5.

For Example: At 2 Molarity strength of hydrochloric acid set of
results; 1.

The volume of gas was 64cm², divide this by 5 which was the amount of
time that elapsed for this much gas to be given off and you get
12.8cm²/second. Now to carry on working this out for the rest of this
set you have to find the difference between 64cm² which was the amount
given off after 5 seconds and the amount of gas released after 10
seconds which was 88cm², the difference is 24cm² and so we again
divide that by 5 and we then get 4.4cm²/second which is the correct
rate of reaction for this time period.

Conclusion

The most obvious trend that I can see is that the higher the strength
of the hydrochloric acid then the larger the rates of reaction will.
This is proven in the first and last sets of results, which are for
hydrochloric acid strengths of 2molar and 0.4molar. The highest rate
of reaction for any set of result in the 2molar is 14cm²/second
whereas the highest rate of reaction for hydrochloric acid strength of
0.4 is 1cm²/second which is a difference in rates of reaction of
13cm²/second. Another pattern that I saw was that no matter the
strength of the hydrochloric acid the volume of gas will always
increase and never decrease, it may stay exactly the same for a few
seconds but it should still increase. This will always happen because
this particular experiment is an irreversible reaction. This means
that once the magnesium has reacted with the hydrochloric acid and the
gas is given off then there is no way to get the magnesium back. My
results are agree totally with my original predictions, which was that
the 2molar experiments would react a quicker than the hydrochloric
acid that was 0.4molar. My prediction also said that as the
hydrochloric acid became nearer and nearer to 0.4molar then the
reaction would take place slower and slower. The only abnormality that
there was in the whole experiment was with hydrochloric acid strength
of 1.2molar, results set 3 the first rate of reaction result was
7cm²/second, which was a 4 cm²/second increase on the other 2 sets of
results.

Evaluation

I think my results were very accurate as the results for the gas given
off was accurate down to the nearest cm² thanks to the gas syringe and
the rates of reaction results were accurate down to the nearest tenth
of a cm²/second and that was down to how I worked out those particular
results. I had one odd results which didn’t match my prediction and
that was set of results 3 for with a molar strength of 1.2molar. This
result was 7cm²/second over the other 2 sets of results. One
explanation for this odd result could possibly be that we put the bung
into the top of the conical flask too late and didn’t start the stop
clock at the same time. My results I think are very accurate because
the volume of gas measured from the gas syringe and taken down were
accurate to the nearest cm² and the rates of reaction was accurate
down to the nearest tenth of a cm²/second. I think that my results are
good enough and accurate enough to give a firm conclusion that the
stronger the molarity of the hydrochloric acid is then the faster the
magnesium ribbon is going to dissolve. I think this because my results
show it, the starting strength which was 2molar took less than half a
minute whereas the finishing strength which was 0.4molar took 5 and a
half minutes, this was the longest results set out of all strengths of
hydrochloric acid. There are a few ways in which I could make my
reaction more accurate and reliable and the most obvious would be that
I would have to find a way to hang the magnesium above the
hydrochloric acid so that as soon s the bung goes in the acid drops
and the stop clock starts, this would therefore eliminate any possible
abnormalities. I think a way in which I could make this experiment
last longer and to give extra evidence would be to do the same
experiment only using smaller gaps in strengths of the hydrochloric
acid, in other words do the experiment again only use hydrochloric
acid of 2molar, 1.8molar, 1.6molar, 1.4molar…and so on until you got
down to 0.2molar. This would give you a much more wider set of results
especially if you did all of those strengths 3 times as we have done
in this experiment. That is another way of making the experiment more
accurate, by repeating it more than 3 times to so that we had enough
sets of results for each strength to rule out any abnormalities.

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