Chemistry
Sensors:
Loggers:
Drop / Bubble counter
Any EASYSENSE
Logging time: EasyLog
45 Rate of reaction: Which catalyst is best?
Read
A chemical that speeds up the rate of a reaction, but does not become chemically changed by the
reaction is called a catalyst. You are going to be studying the catalytic decomposition of hydrogen
peroxide.
Hydrogen peroxide decomposes to form water and oxygen according to the following reaction
2H2O2(aq) Æ 2H2O(l) + O2(g)
The reaction is normally very slow and like all reactions temperature dependant. The decomposition of
the peroxide is also affected by light which is why it is sold in light-proof bottles.
Hydrogen peroxide solution should always be kept in a dark place with a low temperature (a fridge
would seem ideal but there is always the possibility someone would mistake it for a drink of some kind).
It should not be stored in a warm bathroom on a light window ledge (hydrogen peroxide is used in hair
colouring products to lighten the hair colour before replacing it with the colour of the dye in the
product).
Measuring the rate of oxygen production is useful method of studying the rate of this reaction. The
equation for the reaction shows us that for every hydrogen peroxide particle decomposed an oxygen
atom is produced. To account for oxygen gas being molecular the equation is balanced by showing that
2 hydrogen peroxides need to decompose to produce an oxygen gas molecule.
If the oxygen evolved is passed over water, the bubbles produced as the oxygen passes through the
water can be counted electronically with a Bubble counter.
In this experiment you are going to add metal oxides to hydrogen peroxide to see which, if any, speed
up the rate of the decomposition reaction.
What you need
1.
An EASYSENSE logger.
2. A Smart Q Drop / Bubble counter with the alignment adapter fitted.
3. Retort stand and clamps.
4. Reagent reservoir filled with water.
5. Measuring cylinder.
6. 2 x 200 ml conical flask.
7. Bung to fit conical flask, with a single hole fitted with a glass or hard plastic delivery tube.
8. Stopcock fitted to the tube from the conical flask’s bung.
9. Plastic delivery tubing (to fit glass or hard plastic tube).
10. Metal oxides (manganese, copper, iron, zinc).
11. 10 volume Hydrogen peroxide.
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12. Balance.
13. Syringe to test your set up.
Water in bubble chamber
Line up bottom of chamber
with light path
DATA HARVEST
Keep liquid in reaction
chamber below gas
delivery tube
Hazard information
Hydrogen peroxide: Use gloves and eye protection, wash spills immediately. Powerful bleaching agent.
Corrosive
Oxidising
Irritant
Manganese (iv) oxide
Follow advice from your teacher on safe disposal and handling of all metal oxides provided.
What you need to do
1.
Connect the Bubble counter to an input of the logger.
2. Set up the apparatus as shown. The reagent reservoir should be clamped between the receiver
and emitter of the Bubble counter so the reservoir touches the emitter moulding. Adjust the
reservoir vertically so the moulding ring at the bottom of the reservoir barrel is level with the
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lower edge of the emitter moulding (see diagram). Adjust the reservoir laterally so the
reservoir is just touching the moulding of the large hole in the alignment plate.
3. Test your set up before beginning the experiment to check that as bubbles rise they will be
detected by the Bubble Counter e.g. pour some water into the reservoir, attach a syringe full
of air to the plastic tube and gently push the syringe plunger in so the air produces a stream of
bubbles. If the LED does not respond adjust the reagent reservoir slightly and try again.
4. From the EasySense software’s Home screen select EasyLog.
5. Place 20 cm3 of 10 Vol hydrogen peroxide into a conical flask.
6. Weigh out 1.0 g of manganese (iv) oxide onto a filter paper.
7. Add the manganese oxide to the hydrogen peroxide, swirl the flask to mix the contents.
Quickly place the bung into the neck of the conical flask, open the stopcock, press the button
on the Bubble counter to set the counter back to zero and click on the Start icon. The LED on
the Drop counter will blink as a bubble passes through the infrared beam.
8. Click on the Stop icon to finish logging. Save the data.
9. Empty the conical flask. Select Overlay and repeat the experiment from step 5 using a
different metal oxide. Make sure you replace the hydrogen peroxide with a fresh solution and
reset the Bubble counter to zero each time.
Results and analysis
The results should be displayed as a series of lines, one line for each metal oxide. They should show an
increase in the number of bubbles with time.
Use Gradient to find the rate of bubble production for each metal oxide: Move the vertical cursor line
along the graph line until it is about halfway across the data collected. The gradient as ‘xx’
bubbles/sec will be shown in the data box above the graph. Make a note of this value in a table similar
to the one shown below.
Metal oxide used
Questions
1.
Rate (from gradient)
?
Which oxide produced the fastest rate of peroxide decomposition?
2. If you have enough data, does the position of the metal in the periodic table have any
relationship to its catalytic ability?
3. What does it mean when the peroxide is described as 18 Vol?
4. How would you test the gas evolved to show it was oxygen being produced?
5. The mass of each metal oxide used should be corrected to give the same number of moles. You
will need to have access to a data book or a scientific supplies catalogue to find the molecular
weights of the oxides used. Does the moles/rate value give any more information?
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