Catalysts
The effect of various catalysts on the rate of
decomposition of hydrogen peroxide (H2O2)
This content is
aimed at KS3/4
students and is a
two lesson
worksheet (including
a practical
demonstration) to
introduce and study
catalysts.
Learning Objectives:
•
To understand that a catalyst can be used to increase a rate of reaction.
•
To learn that different catalysts work with different efficiencies.
•
To understand that catalysts are neither reactants nor products and
remain unchanged after the reaction is complete.
•
To practice designing a fair, accurate and reliable test.
•
To understand that catalysts work by offering an alternative reaction
pathway with a lower activation energy.
The primary program of study for this worksheet is chemistry although it also
includes elements of mathematics, physics and biology.
Key Vocabulary:
Reactant
Product
Catalyst
Rate of reaction
Reaction pathway
Activation energy
Endothermic / Exothermic reaction
Qualitative / Quantitative experiment
Lesson 1
Starter activity:
Find the correct answer from the 3 options given
Question
Answer
What is present at the start of every
chemical reaction?
reactant
oxygen
catalyst
H2O2 is better known as
water
hydrogen peroxide
hydrogenated oxygen
Repeating an experiment many times
would make the result more
reliable
safe
accurate
Oxygen atoms in the air are mostly found
in the form of
O
CO2
O2
In chemistry, a product is a chemical which can be sold
is present at the end of a reaction
is not present naturally and has
to be artificially produced
Practical
Resources:
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6 x 250ml measuring cylinders
6ml washing up liquid
90ml (6 x 15ml) hydrogen peroxide 15% (50 vol)
Catalyst 1: 0.5g manganese(IV) oxide (MnO2)
Catalyst 2: 0.5g lead(IV) oxide (PbO2)
Catalyst 3: 0.5g copper(II) oxide (CuO)
Catalyst 4: 1cm3 liver
Catalyst 5: 1cm3 boiled liver
Safety: Wear eye protection and gloves when performing the experiment. It is
the responsibility of teachers to perform an appropriate risk assessment.
Method:
Place the 6 250ml measuring cylinders on a tray and add each of the 5
catalysts to 5 of the cylinders. Leave 1 cylinder empty as this will be the
control.
Add 1 ml of washing up liquid to each measuring cylinder.
Simultaneously add 15ml of 15% hydrogen peroxide solution to each
measuring cylinder.
Introductory questions:
In BLOODHOUND SSC one method of propulsion is a rocket engine which uses
hydrogen peroxide as a fuel.
1. Write a balanced chemical equation for the decomposition of hydrogen
peroxide (H2O2).
2H2O2
O2 + 2H2O
2. What effect do you predict the presence of a catalyst will have on the
rate of reaction?
The rate of reaction will increase
3. What is the independent variable?
The type of catalyst used
4. What is the dependant variable?
The rate of reaction (or the amount of O2 / H2O produced in a specific
time)
5. What do you expect to see? Remember; the washing up liquid is not
reactant, it is only there to help visualise the reaction.
The oxygen produced will form foam/bubbles in the washing up liquid
Post-demonstration questions
6. What did you observe?
Bubbles of oxygen were produced at differing rates depending on the
catalyst present. The reaction catalysed by lead oxide was the fastest;
followed by manganese, copper and then liver (which contains a peroxide
enzyme). Boiled liver (where the peroxidase enzyme has become
denatured) and the reaction with no catalyst produced almost no
bubbles.
7. Was the result as you predicted? If not, why not?
8. Was the test fair? Consider any factors that may have affected the
results but were not well controlled.
9. Suggest any improvements to make the experiment fairer.
Answers to questions 7-9 should all involve ways is which the experiment
was unfair. Possible answers include mention of:
•
Surface area of catalysts.
•
Amount of each catalyst (extra credit for discussion of moles of
each).
•
Time of the start of the reaction, was it simultaneous?
•
Possible contamination of catalysts.
•
Accuracy of the volumes of hydrogen peroxide and/or washing up
liquid added to each catalyst.
•
Any other reasonable suggestion of a source of inaccuracy,
unreliability or unfairness.
Plenary
Write a short paragraph summarising the results of the experiment.
Lesson 2
Starter Activity
In relation to the experiment you did last lesson, which of the following
statements are true and which are false.
a) Hydrogen peroxide and water reacted to produce oxygen. False
b) The chemical equation for the reaction was
2H2O2 + Catalyst
False
2H2O + O2
c) The chemical reaction for the reaction was
2H2O2
Catalyst
2H2O + O2
True
d) Washing up liquid was used in the experiment but was not a reactant, it
was only there to visualise the gas production. True
e) In the reaction without a catalyst the reaction still happened, just at a
much slower rate. True
f) The independent variable in the experiment was the type of catalyst
present. True
g) All the catalysts sped up the rate of reaction by the same amount. False
h) O2 and H2O were the products in each reaction, just different amounts
were produced in the specific time. True
i) The liver contained a biological catalyst (enzyme) which was almost
completely denatured (the functionality was lost) by boiling. True
Activation energy
Draw your own reaction coordinate diagram for the decomposition of
hydrogen peroxide both in the presence and absence of a catalyst.
Include the specific reactants and products and be sure to label your
axis. Mark on your graph which parts represent the activation energy
(Ea) and total energy change (ΔE) for the reaction.
- No catalyst
Ea
Ea
H2O2
Energy
1.
ΔE
Reaction Path
- With a catalyst
O2 + H2O
Energy distribution
2.
On the energy distribution diagram above draw how the graph would
change in each of the following situations:
The reaction is performed at a higher temperature
Number of Particles
a)
Room temperature
Higher temperature
Energy
The reaction is performed in the presence of a catalyst
Number of Particles
b)
No catalyst
With a catalyst
Energy
3.
In this experiment hydrogen peroxide was used at 15% concentration.
Another way of expressing this concentration is 50 volume. The 'volume'
method of measuring concentration is defined by the volume of oxygen
produced when compared to the volume of the reactants (at standard
room temperate and pressure) i.e. hydrogen peroxide at 15%
decomposes to produce a volume of O2 which takes up 50x the volume of
the original hydrogen peroxide.
a)
What percentage hydrogen peroxide would represent 1 volume i.e.
the O2 produced would take up the same volume as the reactant.
15%/50 = 0.3%
In BLOODHOUND SSC the hydrogen peroxide used it at 86% (at this
percentage hydrogen peroxide is called high-test peroxide or HTP).
b)
Calculate the relative volume of the O2 produced compared to the
volume of the reactant when the HTP used in the BLOODHOUND
SSC rocket engine decomposes.
1% = 50/15
(50/15) x 86 = 860/3 rounded to 287 volume
4.
The decomposition of hydrogen peroxide releases a large amount of heat
into the surrounding environment. Is the reaction endothermic or
exothermic?
Exothermic
In fact, in BLOODHOUND SSC the decomposition of hydrogen peroxide releases
so much energy that the water produced is in the form of steam.
5.
The rapid decomposition of liquid hydrogen peroxide within the rocket
engine into two gasses propels the car forward.
a)
How does this happen? Consider physics (especially Newtons 3rd
Law of motion)
The gasses are pushed out of the exhaust of the rocket at high
speed and by Newtons 3rd Law (every action has an equal and
opposite reaction) this in turn propels the car forward.
b)
Draw a diagram to illustrate the forces acting on the rocket when it
fires
Normal
Air
resistance
Rocket
pushes on air
Gravity
Air pushes
on rocket
(thrust)
Friction
Extension activity
The experiment performed gave a qualitative result. Design an experiment to
measure the rate of reaction of the decomposition of hydrogen peroxide
quantitatively. Predict the results of your experiment and draw any relevant
graphs.
The experiment performed as above but without any washing up liquid.
Instead, the top of the measuring cylinder would be covered and there would
be an escape for any gasses into a syringe (set to 0ml at the beginning of the
experiment). As the reaction progresses gas would be produced and collected
in the syringe. The volume of gas produced can then be recorded at various
time intervals and these results can be plotted on a graph e.g.
Volume of gas produced (ml)
A graph to show the rate of gas production from the
deomposition of hydrogen peroxide with various catalysts
50
45
40
35
30
25
20
15
10
5
0
MnO2
PbO2
CuO
Liver
Boiled liver
No catalyst
0
10
20
30
40
50
60
Time (seconds)
The average rate of reaction in the presence of each catalyst can then be
determined by calculating the slope of the line for each catalyst e.g
Average rate of reaction for PbO2 = (Y2 – Y1) / (X2 – X1)
= (47 – 0) / (60 – 0)
= 0.78 ml/sec