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:
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:
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 has divisions 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