Aristo Book 5 experiment answer

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HKDSE CHEMISTRY – A Modern View (Chemistry)
Experiment Workbook 5
Suggested answers

Chapter 52 Importance of industrial processes
Chapter 53 Rate equation
Experiment 53.1 Determining the rate equation of a reaction using method of initial rate (A microscale experiment) 1
Chapter 54 Activation energy
Experiment 54.1 Determining the activation energy of a chemical reaction 3
Chapter 55 Catalysis and industrial processes
Experiment 55.1 Investigating the action of a catalyst
6
Experiment 55.2 Investigating homogeneous catalysis
8
Experiment 55.3Investigating ways to change the rate of a reaction with a suitable catalyst 9
Experiment 55.3 Sample laboratory report
13
Experiment 55.4Preparing ethanol by fermentation
16
Chapter 56 Industrial processes
Chapter 57 Green chemistry for industrial processes

Chapter 53Rate equation

Experiment 53.1Determining the rate equation of a reaction using method of initial rate (A microscale experiment)

7. and 11.(a)
Well number
1
2
3
4
5
6
7
8
Number of drops of 0.5 M Na2S2O3(aq)
10
9
8
7
6
5
4
3
Time, t (s)
14.2
15.8
17.8
20.4
23.8
28.6
35.7
47.6
(s1)
0.070
0.063
0.056
0.049
0.042
0.035
0.028
0.021

10. and 12.(a)
Well number
1
2
3
4
5
6
7
8
Number of drops of 1.0 M H2SO4(aq)
10
9
8
7
6
5
4
3
Time, t (s)
59.4
59.7
60.0
60.7
59.9
60.0
61.0
60.5
(s1)
0.017
0.017
0.017
0.016
0.017
0.017
0.016
0.017

11.(a)inversely
(b)

12.(b)0
(c)From the results in Table 53.2, the readings of time are close, indicating that the reaction is of zeroth order with respect to H+(aq).

13.Rate = k[S2O32(aq)]

14.S2O32(aq) + 2H+(aq)  S(s) + SO2(g) + H2O(l)

15.In this experiment, the time for the formation of a fixed, but small amount of insoluble sulphur precipitate is measured. The shorter the time, the faster is the reaction. It is assumed that the extent of reaction is still small when the time is recorded, so that the time recorded can be used as a measurement of initial rate of the reaction.

Chapter 54Activation Energy

Experiment 54.1Determining the activation energy of a chemical reaction

5.
Temperature of the reaction mixture (°C)
15
25
35
45
55
Time for the appearance of dark blue colour (s)
679
(at 11°C)
232
(at 27°C)
112
(at 37°C)
80
(at 43°C)
33
(at 56°C)

6.(a)rate constant; activation energy; Universal gas constant; temperature;
(b)
log ()
2.83
2.37
2.05
1.90
1.52

3.52
3.33
3.23
3.16
3.04

(c)

(d)2750
(e)slope = 2750 =
Ea = 2750 × 2.3 × 8.314 J mol1
= 52 586 J mol1
= 52.6 kJ mol1
7.Arrhenius equation; log k = log A
8.straight line;

9.S2O82(aq) + 2I(aq)  2SO42(aq) + I2(aq)

10.To monitor the formation of iodine from the reaction of S2O82(aq) ions and I(aq) ions.

11.When all S2O82(aq) ions have reacted, any iodine formed will turn the starch solution dark blue. The time for this colour change is a measure of the rate of reaction shown in question 9. (Note: The reaction rate is inversely proportional to the time taken for the starch solution to turn dark blue.)

12.The amount of reactants used in each experiment may not be exactly the same.
There may be an error in measuring or reading the temperatures from the thermometers.
As the colour change of the solution mixture is not a sudden one, especially at low temperatures, there may be an error in recording the time of colour change.

Chapter 55Catalysis and industrial processes

Experiment 55.1Investigating the action of catalyst

1.(b)No.

5.(b)
Time (s)
10
20
30
40
50
60
Volume of O2(g) released (cm3), with the addition of 0.5 g MnO2(s) 30
60
85
95
96
96
Time (s)
70
80
90
100
110
120
Volume of O2(g) released (cm3), with the addition of 0.5 g MnO2(s) 96

6.(b)
Time (s)
10
20
30
40
50
60
Volume of O2(g) released (cm3), with the addition of 1.5 g MnO2(s) 70
90
95
96
96

Time (s)
70
80
90
100
110
120
Volume of O2(g) released (cm3), with the addition of 1.5 g MnO2(s)

9.Manganese(IV) oxide
MnO2(s)
10.2H2O2(aq)  2H2O(l) + O2(g)

11.(a)The addition of manganese(IV) oxide greatly increases the rate of decomposition of hydrogen peroxide.
(b)(i)The initial rate of reaction is higher.
(ii)The total time of reaction is shorter. (Note: increasing the amount of catalyst would increase the reaction rate.)
(c)No.

12.Add more H2O2(aq) to the reaction mixture, rapid effervescence shows that manganese(IV) oxide has not been used up in the reaction. The catalytic property of manganese(IV) oxide is still present. Experiment 55.2Investigating homogeneous catalysis

5.Mixture ‘y’.
It has a characteristic sweet smell like certain glues or nail polish removers.

6.ethyl ethanoate; concentrated sulphuric acid

7.CH3COOH(l) + CH3CH2OH(l) ⇌ CH3COOCH2CH3(l) + H2O(l)

8.Homogeneous catalyst. This is because all species are in the same phase in the reaction, i.e. the liquid phase.

9.Sodium carbonate solution reacts with any unreacted ethanoic acid left in the reaction mixture. The strong vinegar smell of ethanoic acid is thus removed. The salt sodium ethanoate formed has no smell. Besides, the ester is insoluble in water and floats on the water surface. This makes us easier to detect the smell of ester. Experiment 55.3Investigating ways to change the rate of a reaction with a suitable catalyst

1.Apparatus:
Safety spectacles
Protective gloves
Conical flask (100 cm3)
5 measuring cylinders (10 cm3)
Dropper
Stopwatch
Boiling tube
White tile
Chemicals:
Ammonium peroxodisulphate solution (0.020 M)
Potassium iodide solution (0.50 M)
Sodium thiosulphate solution (0.010 M)
0.2% starch solution
Iron(II) chloride solution (~0.010 M)
Distilled water
2.
What you will keep constant
(Controlled variable)
What you will change
(Independent variable)
What you will measure
(Dependent variable)
volume of ammonium peroxodisulphate solution
volume of potassium iodide solution
volume of sodium thiosulphate solution
amount of 0.2% starch solution
with or without using iron(II) solution
the time for the appearance of the dark blue colour

Figure 1
4.
(1)Using a measuring cylinder, add 10 cm3 of ammonium peroxodisulphate solution to a conical flask. (2)Using different measuring cylinders, add 5 cm3 of potassium iodide solution, 5 cm3 of sodium thiosulphate solution, 1
cm3 of iron(II) chloride solution and 2.5 cm3 of starch solution to a boiling tube. (3)Pour the contents in the boiling tube into the conical flask. (4)Immediately start the stopwatch.

(5)When a dark blue colour of the starch-iodine complex appears in the solution, stop the stopwatch. (6)Record the time for the appearance of the dark blue colour in Table 1. (7)Repeat steps (1) to (6), but replace iron(II) chloride solution with 1 cm3 of distilled water.

5.
Risk assessment form

Time for the appearance of the dark blue colour
With Fe2+(aq) ions (as a catalyst) added
59 s
Without any catalyst added
3 mins and 52 s
Table 1

7.The reaction involves the collision of two negatively charged ions, S2O82(aq) ions and I(aq) ions, which actually repel each other.

8.S2O82(aq) + 2Fe2+(aq)  2SO42(aq) + 2Fe3+(aq)
2Fe3+(aq) + 2I(aq)  2Fe2+(aq) + I2(aq)

9.Referring to the two equations in question 8, the S2O82(aq) ions oxidize the Fe2+(aq) ions to Fe3+(aq) ions. At the same time, the S2O82(aq) ions are reduced to SO42(aq) ions. The Fe3+(aq) ions are strong oxidizing agents that oxidize I(aq) ions to I2(aq). At the same time, Fe3+(aq) ions are reduced back to Fe2+(aq) ions (i.e. the catalyst is regenerated).

Both the equations shown in question 8 involve the collision between positive and negative ions. This will be much more likely to be successful
than the collision between two negative ions in the uncatalysed reaction. Thus, the activation energy of this pathway will be lower and the reaction rate will also be higher.

10.The chemical reaction can be speeded up by the addition of iron(II) ions, which act as a homogeneous catalyst of this reaction.

11.It can be regenerated after the reaction. OR It is specific in action. OR A small amount of catalyst is usually enough for the catalytic action.

12.Homogeneous catalyst is one which has the same phase as the reactants and products. Sample laboratory report

Title: Investigating ways to change the rate of a reaction with a suitable catalyst

Objective
To design and carry out an experiment to investigate ways to change the rate of a reaction – by the use of a suitable catalyst.

Apparatus and materials
Safety spectacles
Protective gloves
Conical flask (100 cm3)
5 measuring cylinders (10 cm3)
Dropper
Stopwatch
Boiling tube
White tile
Ammonium peroxodisulphate solution (0.020 M)
Potassium iodide solution (0.50 M)
Sodium thiosulphate solution (0.010 M)
0.2% starch solution
Iron(II) chloride solution (~0.010 M)
Distilled water

Chemical reactions involved
S2O82(aq) + 2Fe2+(aq)  2SO42(aq) + 2Fe3+(aq)
2Fe3+(aq) + 2I(aq)  2Fe2+(aq) + I2(aq)

Procedure
1.Using a measuring cylinder, 10 cm3 of ammonium peroxodisulphate solution was added to a conical flask. 2.Using different measuring cylinders, 5 cm3 of potassium iodide solution, 5 cm3 of sodium thiosulphate solution, 1 cm3 of iron(II) chloride solution and 2.5 cm3 of starch solution were added to a boiling tube. 3.The contents in the boiling tube were poured into the conical flask. 4.The stopwatch was started immediately.

5.When a dark blue colour of the starch-iodine complex appeared in the solution, the stopwatch was stopped. 6.The time for the appearance of the dark blue colour was recorded in Table 1. 7.Steps (1) to (6) were repeated, but iron(II) chloride solution was replaced with 1 cm3 of distilled water.

Results

Time for the appearance of the dark blue colour
With Fe2+(aq) ions (as a catalyst) added
59 s
Without any catalyst added
3 mins and 52 s
Table 1

After mixing all the chemicals in the conical flask, the reaction mixture with Fe2+(aq) ions will take a shorter time for the dark blue colour to appear.

Analysis
1. In the absence of Fe2+(aq) ions, the reaction between S2O82(aq) ions and I(aq) ions is slow. As both reactant ions are negatively charged, they tend to repel each other. However, when Fe2+(aq) ions are added, the
reaction becomes faster. Fe2+(aq) ions have the same phase (i.e. aqueous phase) as the reactants and products, so they are homogeneous catalyst of this reaction. 2. Fe2+(aq) ions is a reducing agent which can reduce S2O82(aq) ions to SO42(aq) ions. The Fe3+(aq) ions formed act as an oxidizing agent, which oxidize I(aq) ions to I2(aq) ions and regenerate Fe2+(aq) ions again. Being a catalyst, Fe2+(aq) ions are not consumed in the catalytic process.

Discussion
1.Either Fe2+(aq) ions are Fe3+(aq) ions is a good choice of catalyst for this reaction because the interconversion between Fe2+ and Fe3+ facilitates the reaction between S2O82(aq) ions and I(aq) ions to occur. 2.The catalytic property of Fe2+(aq) ions may be due to the fact that it is easier for the negatively charged S2O82(aq) ions to approach the positively charged Fe2+(aq) ions. The same is true when the positively charged Fe3+(aq) ions formed can approach the negatively charged I(aq) ions easier. 3.The experiment is just a simple test tube experiment but the result (colour change) is quite obvious and easy to detect.

Conclusion
The chemical reaction can be speeded up by the addition of Fe2+(aq) ions, which act as a homogeneous catalyst of this reaction.

Answers to questions for further thought
11.It can be regenerated after the reaction. OR It is specific in action. OR A small amount of catalyst is usually enough for the catalytic action.

12.Homogeneous catalyst is one which has the same phase as the reactants and products. Experiment 55.4Preparing ethanol by fermentation

3.(b)

Glucose solution
with yeast
Glucose solution
without yeast
Appearance of the glucose solution
a pale brown suspension
a clear solution
Observations in the limewater
clear and colourless
clear and colourless

6.(d)Acidified potassium dichromate solution changes colour from orange to green.
(f)No colour change for the acidified potassium dichromate solution.

Glucose solution
with yeast
Glucose solution
without yeast
Appearance of the glucose solution
cloudy; a pale brown suspension
clear, no visible change
Observations in the limewater
milky
remains clear and colourless
Smell of the glucose solution
a smell of alcohol
no characteristic smell

8.catalyst

9.ethanol; carbon dioxide

10.The solution turns milky. It indicates that carbon dioxide is produced during fermentation.

11.In the presence of yeast, glucose is converted to ethanol. The presence of ethanol is indicated by the colour change of the reaction with acidified potassium dichromate solution. Ethanol is a reducing agent. It reduces dichromate ions to chromium(III) ions.