Practical Tasks:(I) Elephant Toothpaste
(II) Investigation ‐ Catalysts for the decompositionof hydrogen peroxide
(III) Reaction of Phenolphthalein in Sodium hydroxide Solution
Topic IX Rate of ReactionTopic XIII Industrial Chemistry
(I) Elephant Toothpaste
ObjectivesTo follow the change in rate of the catalyseddecomposition of H2O2
(I) Elephant Toothpaste50 cm3 of 6%H2O2(aq) 0.5 g MnO2Soap Solution
Reaction:H2O2(aq) H2O(l) + O2(g)
The experiment
Adding soap solution to H2O2(aq)
Adding MnO2(s) to 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 ‐ Stopwatch• Photo‐taking ‐ Digital Camera / Mobile devices
Measurement: photo‐taking
Time after adding MnO2 into H2O2 solution (seconds)10 40 70 100 130 160 190 220Height of foam rise as indicated on measuring cylinder (cm3)80 210 280 330 380 410 440 460
Height of foam rise vs Time
0
50
100
150
200
250
300
350
400
450
500
0 40 80 120 160 200 240
Measurin
g cylin
der reading
(cm
3 )
Time (seconds)
Calculation of Instantaneous rate / average rate
Following the progress of a chemical reactionAny Advantages / Disadvantages / Limitations / Drawbacks?
(II) Investigation ‐ Catalysts for the decomposition of hydrogen peroxide
ObjectivesTo 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
to data loggerOR
to gas syringePlant extract (potato)
One of the following catalysts:Powdered manganese(IV) oxide (MnO2)Fresh potato extractFresh celery extractA small piece of fresh pig liver1 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)
C
CO
O
HO OH
2 OH -
- H2OC
COO
O
O
(fast)
(Colourless)H2Ph Ph2-
(Pink)(pH = 0 - 8.2) (pH = 8.2 - 12)
C
O O
OH
COO
OH -
(slow)
POH3-
(Colourless)(pH > 12)PhOH3-PhOH3-
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?
Complementary Colour
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 range (0.05 – 0.70)
Experiment ProcedureStep 1Step 1
• Use 10 mL pipette transfer 5 ml 1.0 M NaOHinto cuvette
Step 2Step 2• Start a timer to record for 30 seconds
Step 3Step 3• Use pipette 1 mL of 0.01% phenolphthalein into the cuvette immediately
Step 4
• Sharply when 30 seconds countdown ends, start the data‐logger to measure for absorbance (Abs).
Step 5
• Obtain absorbance at time = 0s and 60s from the Abs–time plot.
• Use “slope tool” to measure the rate at t = 0s
Time
Absorbance (Green)Computer
(Data logging Software)
Data logger
Light SourceLight Detector + Filter
Cuvette5 mL NaOH(aq) +
1 mL 0.01% Phenolphthalein
The experiment
Using “Slope Tool” in dataloggersoftware
m is the slope of the curve at time = 0 s
Sample Absorbance‐Time Curve (NaOH and Phenolphthalein)
Experiment result
Concentration of sodium hydroxide
solution
Slope of the Absorbance‐
time curve at 0th 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