This demonstration is a fun way of showing the effect of different
catalysts on the rate of
decomposition of hydrogen peroxide. Before moving on to the demonstration it
is important to establish for the students
that hydrogen peroxide is indeed decomposing all the time
otherwise the students may confuse reactants with catalysts.
One possible way of doing this is to show that the bottle
of hydrogen peroxide
is vented to allow oxygen to escape.
Another way is by pointing out that the hydrogen peroxide is stored in a refrigerator
and that it has a finite lifetime. Line up six large cylinders
and add catalyst to them as follows: half a gram of manganese dioxide,
half a gram of lead dioxide, half a gram of iron oxide,
a small amount of chopped potato, a small amount of chopped liver,
and a small amount of chopped boiled liver. It needs to be boiled in water for
about five minutes.
To each of these, add one centimeter cubed
of washing up liquid.
It doesn't need to be measured accurately
but you need to use the same amount each time to ensure that it's a fair test.
Avoid spilling the washing up liquid down the sides of the tube
and indeed the catalyst also
needs to be at the bottom of the tube, and not down the sides.
Then simultaneously add 25 centimetres cubed of 75 vol
hydrogen peroxide to each of the cylinders.
The foam will rise up the cylinders at widely differing rates.
The foam is formed from oxygen which is produced by the decomposition of the peroxide.
The rates at which they rise up the cylinders give a graphic comparison
of the differing effectiveness of the catalysts. The order is normally
lead dioxide, manganese dioxide,
fresh liver, potato, iron oxide and then boiled liver,
but this may be affected by the surface area of the powders.
Liver contains a peroxidise enzyme which is denatured by boiling it.
It is usually possible to light a glowing splint
in the foam, although sometimes this fails if there's too much water in the foam.
This demonstration can lead to plenty of discussion and possible further experimentation.
Is it in fact a fair test? Should we be measuring the number of moles of each of
the catalysts rather than the number of grams?
How much of the potato and liver
is actually enzyme? Are the surface areas of the powders comparable?