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.