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Investigating the Reaction Between Bromine and Cyclohexane

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Investigating the Reaction Between Bromine and Cyclohexane



By adding bromine to a mixture of Cyclohexane and water, and placing
the mixture under a bright light and shaking from time to time,
Hydrogen Bromide is formed. This reaction is shown below:

[IMAGE]C6H12 + Br2 C6H11Br + HBr

When the bromine has all reacted and the red colour has gone, the
hydrogen bromide can be titrated with sodium hydroxide. This reaction
is shown below:

[IMAGE]HBr + NaOH NaBr + H2O

From this titration I can then work out the HBr formed per mole of
Bromine.


Results of my experiment
------------------------

Class results.

Group Number

Mass of Br2 (g)

Titre (cm3)

1

0.99

32.50

2

0.94

29.80

3

1.03

32.60

4

1.04

34.20

5

1.17

35.70

6

0.94

31.25

7

0.95

30.50

8

1.04

32.50

9

1.10

32.20

Average

1.02 to 2dp

32.36 to 2dp


Working out the mean averages
-----------------------------

To work out the mean averages I used the following formula:

Sum of column / number of groups

My results- with raw data

Original mass of stoppered flask (g)

Mass after Bromine was added

(g)

Mass of Br2

(g)

Initial burette volume

(cm3)

Final Burette volume

(cm3)

Amount of NaOH added

(cm3)

136.78

137.81

1.03

100

67.4

32.6


Using the results
-----------------


Bromine

I used a mass of 1.03grams of Bromine.

I arrived at this amount by weighing the volumetric flask without the
bromine added then I weigh the flask with the bromine added. The
different between which was 1.03 hence the mass of bromine must be
1.03to 2dp.


Moles
-----

Mass of bromine used/ relative molecular mass of Br2=
1.03/(2x79.90)=0.00645to 3sf


Sodium Hydroxide
----------------

I used 32.60cm3 of NaOH during the titration.

I arrived at this answer by recording an initial and final burette
readings and then working out the difference, which was 32.60cm3.


Moles
-----

Number of moles = concentration x volume (in dm3)= 0.200 x 0.03260=
0.00652 mol


The reaction

The reaction between the sodium hydroxide and hydrogen bromide is
shown below:

[IMAGE] HBr + NaOH NaBr + H2O

Moles of Hydrogen Bromide produced in the reaction between Bromine and
Cyclohexane

Number of moles of Bromine used x2

6.44x10-3 x2 = 12.88x10-3 mol


What amount in moles of hydrogen would be produced from 1 mol of
bromine?

There would be 2x as many moles of Hydrogen Bromide produced from the
reaction of one mole of Br2, as you would need 1 mole of Br2 to react
to make 2 moles of HBr.

A Balanced equation for the reaction between bromine and Cyclohexane
is shown below:

[IMAGE]C6H11(aq) + Br2(aq) C6H11Br(aq) + HBr(aq)

This reaction is generally known as a substitution reaction as the
bromine has taken the place of the Hydrogen.

Why was water added to the reaction?

Water is not a reactant in this reaction, however as the HBr produced
is in a gaseous form, water is added in order to dissolve the HBr in
it so that it doesn't escape.

Evaluating my results and procedures.

The balance, the Burette and the measuring cylinders I used had fairly
precise measurements, as it is important for these results to be
accurate. This is shown below when looking at there percentage errors:

Percentage error = error x 100/reading

%Error for balance= 0.01x100/1.03=0.971%to 3sf

%Error for burette= 0.1x100/32.6=0.307%to 3sf

%Error for measuring cylinder (Cyclohexane)= 1x100/35= 2.86%

%Error for measuring cylinder (distilled water)=0.5x100/15=3.33%

As you can see the percentage error for each piece of equipment is
very low and therefore I feel that the techniques used are very
suitable.

How I worked out the error in readings for my equipment

The balance reads to 2dp hence the error each time I use it is 0.005g
as the balance rounds to 2dp. Therefore since I used the balance twice
the error is doubled and hence becomes 0.01g

The Burette reads to within 0.05cm3 as I record the volume of NaOH
twice this means that each time my readings could be out by 0.1cm3

The Cyclohexane measuring hasdivisions every 2 ml. Therefore I could
read accurately up to 1 ml or 1 cm3.

The distilled water-measuring cylinder has divisions every 1ml
therefore I can read accurately to within 0.5 cm3

From the percent errors above I can see that the major source of error
in regards to equipment was the two measuring cylinder as they weren't
as accurate as I would have hoped. Meaning that in the extreme cases
my measurement of Cyclohexane could have been 35.5 cm3 or 34.5cm3,
which would have altered my titre. As an increase of 0.5cm3 of
Cyclohexane will result in a slightly higher titre, as more sodium
Hydroxide is needed to titrate the extra 0.5cm3. By contrast a
decrease of Cyclohexane will mean that the titre is decreased as there
is less Cyclohexane to dissolve.

The main error in this experiment however is not the quality of
equipment but being able to accurately judge when the titre has
occurred properly, as the color change is never permanent and
therefore we record the volume at which the color change occurs for a
long duration. Sometimes it is hard to judge when this color change
stays for a long enough duration and therefore the titration is
sometimes incomplete and groups results vary as they except different
color changes (i.e. some groups color changes are more permanent than
others.) This is shown on the graph below:

The graph shows a very weak positive correlation of points (i.e.
points are spread out from the line of best fit), which suggests that
my experimental work isn't extremely accurate. I feel this is due to
the variation in depth of color changes for each group (as explained
above).

Other errors in the procedures could also have affected different
group's titres, thus making the group average slightly unreliable.
Firstly if the burette jet was not full every time they began their
titration then the amount of NaOH added would be inaccurate and
therefore titre may appear more than it actually is (i.e. causing an
increase in the titration). Secondly if the top of the stopper was
slightly off then the hydrogen bromide gas, that has not dissolved,
may escape from the volumetric flask. Thirdly there is a slight
variation in the amount of indicator in each drop hence each group
used slightly different amounts of indicator and therefore there were
slightly different variations in colour, hence different titres. An
increase in indicator would cause a premature colour change hence
titre would be lowered. Whereas a decrease in amount of indicator used
may mean that titre increases as colour change is not as recognisable.
Fourthly if the volumetric flasks have been put under different light
sources or for different amounts of time then some of the mixtures may
not be decolourised and therefore titration's may be decreased as more
NaOH is needed to be able to notice a colour change in the solutions.
Finally if some of the sodium hydroxide sticks to the top of the
volumetric flask whilst the NaOH is being added then the titration may
have increased as it appears that more sodium hydroxide has been added
whereas there is similar amounts used in each titration however there
is additional NaOH on the top of the volumetric flask which has not
come into contact with the Cyclohexane solution.

Overall I feel that my results were fairly good as my groups titre was
similar to the groups average and therefore my experimental work must
have been carried out well, with very few errors. Although I feel this
the above information suggests that this particular titre was which is
why we got such a variation in results. I feel that in order to
improve my results, that I need to find more accurate measuring
cylinders in order to measure the amount of Cyclohexane as the one I
used had a relatively high percentage error flask whilst the NaOH is
being added.

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