Practical Tasks in Teaching, Learning and Assessment 2

Topic IX
Topic XIII
Rate of Reaction
Industrial Chemistry
Practical Tasks:
(I) Elephant Toothpaste
(II) Investigation ‐ Catalysts for the decomposition
of hydrogen peroxide
(III) Reaction of Phenolphthalein in Sodium hydroxide Solution
(I) Elephant Toothpaste
Objectives
To follow the change in rate of the catalysed
decomposition of H2O2
(I) Elephant Toothpaste
50 cm3 of 6% H2O2(aq) 0.5 g MnO2
Soap Solution
Reaction:
H2O2(aq) Æ H2O(l) + O2(g)
The experiment
Adding soap Adding MnO2(s) to solution to H2O2(aq) the mixture
Oxygen generated create bubbles in soap to produce foam Enhancing visual impact of the experiment
Link with the curriculum
‐
More can be done besides “visual impact” from the reaction
you will need
• Time keeping ‐
• Photo‐taking ‐
Stopwatch
Digital Camera / Mobile devices Measurement: photo‐taking
Time after adding MnO2 into H2O2 solution (seconds)
10
40
70
100
130
160
190
220
Height of foam rise as indicated on measuring cylinder (cm3)
80
210
280
330
380
410
440
460
Height of foam rise vs Time
Measuring cylinder reading (cm3)
500
450
400
350
300
Calculation of Instantaneous rate / average rate
250
200
150
100
50
0
0
40
80
120
160
Time (seconds)
200
240
Following the progress of a chemical reaction
Any Advantages / Disadvantages / Limitations / Drawbacks?
(II) Investigation ‐ Catalysts for the decomposition of hydrogen peroxide
Objectives
To investigate the effect of the use of catalyst on reaction rate
Catalysts for the decomposition of hydrogen peroxide
• Possible catalysts: KI(aq) / MnO2(aq) / Celery / Pig Liver ….
The experiment
Ö to data logger
OR
Ö to gas syringe
Plant extract (potato)
One of the following catalysts:
Powdered manganese(IV) oxide (MnO2)
Fresh potato extract
Fresh celery extract
A small piece of fresh pig liver
1 M Fe(NO3)3(aq) / FeCl3(aq)
1 M KI(aq)
Adding catalyst into 100 cm3
H2O2(aq)
Experiment Result
Biological Catalyst
‐
‐
Testing of oxygen gas produced in the reaction
Cooked pig liver loses its catalytic power
Related Question: HKDSE 2014 Paper 2 Q.1 (a) (ii)
(III) Reaction of Phenolphthalein in Sodium hydroxide solution
Objectives:
To investigate the relationship between the concentration of sodium hydroxide solution and the rate of decolourisation
Determining the order of NaOH(aq) in the reaction
• Colorimetry (Colorimeter + Data‐logger)
• Phenolphthalein: 0.01 % (1 mL)
• NaOH: 0.25 M / 0.50 M / 1.00 M (5 mL)
HO
O
OH
O
O
O
2 OH -
C
O
C
- H2O
(fast)
OH C
(slow)
COO
OH
COO
O
H2Ph (pH = 0 - 8.2)
(Colourless)
C
Ph2- (pH = 8.2 - 12)
(Pink)
POH3-3-3- (pH > 12)
PhOH
PhOH
(Colourless)
Data‐logger System Setup
• PASCO colorimeter + USB Link + Desktop Computer with Data‐studio installed
• Measurement: Absorbance (565 nm) vs time (s)
• Calibration using blank solution
Colorimetry? Absorbance?
Colorimetry: Beer‐Lambert Law
‐ The concentration of solution is directly proportional to the light absorbance
‐ The instrument responses linearly in a certain range only
‐ Control the absorbance within the Complementary Colour
range (0.05 – 0.70)
Experiment Procedure
• Use 10 mL pipette transfer 5 ml 1.0 M NaOH
into cuvette
Step 1
Step 2
• Start a timer to record for 30 seconds
• Use pipette 1 mL of 0.01% phenolphthalein into the cuvette immediately
Step 3
• Sharply when 30 seconds countdown ends, start the data‐logger to measure for Step 4
absorbance (Abs).
• Obtain absorbance at time = 0s and 60s from the Abs–time plot.
Step 5 • Use “slope tool” to measure the rate at t = 0s
The experiment
Computer
(Data logging Software)
Absorbance (Green)
Data logger
Time
Light Source
Light Detector + Filter
Cuvette
5 mL NaOH(aq) + 1 mL 0.01% Phenolphthalein
Using “Slope Tool” in datalogger
software
m is the slope of the curve at time = 0 s
Sample Absorbance‐Time Curve (NaOH and Phenolphthalein)
Experiment result
Slope of the Concentration of Absorbance‐
sodium hydroxide time curve at 0th solution
second
Absorbance at 0th second
Absorbance at 60th second
0.25 M
‐0.003 s‐1
0.979
0.843
0.50 M
‐0.005 s‐1
0.940
0.683
1.00 M
‐0.010 s‐1
0.767
0.305
• Beware of the non‐linearity
• NaOH – 1st Order w.r.t. the reaction

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