anderson prelim trend qn with answers
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Anderson Sec Trend Question
1
Bromine is an important chemical use for making flame retardants. Bromine is
extracted from sea water, which is naturally rich in bromide ions. Chlorine can be
bubbled through sea water to convert bromide ions into bromine.
A student carried out an experiment in a laboratory to investigate the reaction
between bromide ions and chlorine gas
The student bubbled chlorine gas through dilute aqueous potassium bromide for 6
minutes. She took samples of the reaction mixture every 30 s and measured the
color intensity of each sample using a colorimeter.
A colorimeter measures light absorbed when the light passes through a coloured
solution.
The darker the color of the solution, the more is light absorbed and the higher the
absorbance reading on the colorimeter.
Aqueous bromine is reddish brown in color.
(a) (i) Give an ionic equation, with state symbols, for the reaction between
chlorine gas and aqueous potassium bromide solution. [1]
Cl2 (g) + Br- (aq) ���� Cl- (aq) + Br2 (aq)
(ii) What is the relationship between the absorbance and time during the first
minute of the experiment? Explain your answer with reference to the reaction
that occurs.
The absorbance reading increases linearly with time because more
and more aqueous bromine is formed when chlorine displaces
bromine from aqueous potassium bromide.
As more aqueous bromine is formed, the solution becomes darker and
therefore the absorbance reading increases.
b) From the graph, describe and explain, with the help of Collision Theory, the
rate of reaction in the experiment. [3]
Take note:
it is the gradient of the graph which will give you the rate or speed of reaction.
If you look carefully, every rate of reaction with time graph can be divided
into 3 portions.
The first portion is where the reaction is faster and the gradient is straight.
The second portion is where the gradient keeps on changing from steep to
less steep.
The third portion is where the gradient is flat or no gradient at all.
The rate of reaction is the fastest during the first minute as the
concentration of bromide ions is the most at the start of the reaction.
Hence there are more frequent collisions between the reactant
particles .[1]
Anderson Sec Trend Question
2
As the reaction progress from first to 5th minute, the rate of reaction
decreases (or rate of reaction decreases with time) .
This is because there are less bromide ions available as bromide ions
become bromine gas in the reaction, resulting in decrease in
frequency of effective collisions between reactant particles. [1]
The rate eventually becomes zero after 5th minute as all the bromide
ions (limiting reagent) are used up and the reaction stops. There is no
more collisions between reactant particles.
(c ) The student carried out four more experiments to determine the time taken
for each reaction to finish. She used the same volume of potassium bromide
solution each time. She recorded the time taken and the absorbance reading
at the end of each reaction in a table.
Experiment Time taken for reaction to
complete / mins
Absorbance reading at the
end of the reaction
I 5.00 0.8
II 6.00 0.4
III 2.50 0.8
IV 2.50 0.9
(i) Which experiment uses a lower concentration of potassium bromide
solution than experiment I?
Take note: lower concentration of bromide ions means less number of
moles of limiting reagent. Hence, less aqueous bromine formed as a
product.
Experiment II
(ii) Compare the result of experiment I with experiment III. Suggest an
explanation for the results.
In experiment I and experiment III, the absorbance reading at the
end of the experiments are the same.
This implies that in both experiments, the same number of moles
of potassium bromide solution used is the same.
The time taken for reaction to finish in experiment III is half the
time taken in experiment I.
This may be due to chlorine gas being bubbled into potassium
bromide solution at a faster rate.
(iii) State what is observed when chlorine is bubbled into aqueous potassium
bromide solution. [4]
When yellowish-green gas is bubbled into colourless solution, the
solution turns reddish brown.