MODULE 4
15
WORKSHEET
WORKSHEET
REACTION RATES
Syllabus reference 8.5.5
1
Complete the following to check your understanding.
a
slow
Combustion reactions occur at very different rates. The reaction will be _____________
when
large lumps of coal and a limited supply of air is used. This is because the burning only takes
surface
place on the _____________
of the coal. When the coal is ground into very fine particles the
fast
surface
reaction is _____________
because there is a large _____________
area of fuel exposed to the
oxygen. In diesel engines the liquid fuel reacts within a few milliseconds producing an
explosion
_____________,
which is an extremely rapid reaction.
b
Combustion is just one example of a chemical reaction which occurs at different rates. One
activation
factor that determines the speed or rate at which a reaction occurs is the _____________
high
energy. Usually slow reactions have _____________
values for this. Chemists alter the rate at
concentration
which a reaction occurs by changing one or more of the following factors: _______________,
temperature
particles
catalyst
_______________,
size of the _______________
or by using a _______________.
c
collide
For any reaction to occur the reacting particles must _______________.
Increasing the rate at
increase
which this occurs will _______________
the rate of the reaction.
d
particles
Concentration is a measure of the number of _______________
per unit volume.
more
Increasing the concentration means there are _______________
particles per unit volume
increases
so the chance of collisions between particles _______________
and this, in turn, increases
the reaction rate.
e
surface
Breaking solid substances into smaller pieces increases the _______________
area of the solid.
increases
The greater this is, the more collisions can occur so reaction rate _______________.
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CONQUERINGCHEMISTRY PRELIM
MODULE 4 WS 15
average kinetic energy of the particles. This means
As temperature increases so does the _______________
f
increase
that the rate of collisions will _______________
and so cause an increase in the reaction rate.
g
For the particles to successfully react they must not only collide but collide with enough
activation energy
energy to overcome an ____________________
barrier. If particles have insufficient energy
reactants
they bounce apart and stay as _____________.
An increase in temperature means the average
higher
kinetic energy of the reactant particles will be ____________.
Therefore more particles will
proceed
have enough energy to overcome the energy barrier and the reaction will _______________.
h
catalyst
The fourth factor which changes the rate of reaction is using a _______________.
This is a
consumed
substance which increases the rate of reaction without being _______________
in the reaction.
lower
Catalysts work by providing a reaction pathway with a _______________
activation energy.
i
lower
Catalysts play an important role in industry by allowing reactions to occur at _____________
reducing
temperatures thus saving energy and _______________
production costs. In oil refining
cracking
catalytic _______________
is used to break large molecules into smaller ones and so produce
catalytic
more petrol. In motor vehicles _______________
converters remove pollutants from exhaust
enzymes
gases. Biological catalysts are called _______________.
These are very specific in their activity
slowly
and without them biological reactions would proceed extremely _______________.
j
Catalysts generally operate by forming compounds where the bond breaking and rearrangement
less
to form new compounds requires _______________
energy. With solid catalysts the reactant
absorb/stick to the surface of the catalyst. In doing this bonds may be weakened
particles _______________
broken
or _______________
and other reactant particles collide with those on the catalyst allowing
surface
a reaction
_______________
to occur and the products to leave the _______________
of the catalyst.
2
A common method of preparing hydrogen for industrial purposes is to heat methane and steam
with a catalyst at high temperature. The endothermic reaction is:
CH4(g)  H2O(g) → 3H2(g)  CO(g)
When 1 mole of methane was reacted with excess steam at 1100°C in a vessel of fixed volume,
curve A in the following diagram was obtained. When the experiment was repeated at a different
temperature, curve B was obtained.
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CONQUERINGCHEMISTRY PRELIM
MODULE 4 WS 15
Moles of CO formed
1.0
B
A
0.5
0
a
10
Time (min)
20
What do the initial slopes of the curves tell you about the initial rates at the two temperatures?
What is the reason for this difference?
Reaction B has a steeper slope so this reaction occurs faster. Temperature affects the rate at
which a reaction occurs.
b
Was the temperature for curve B greater or less than that for curve A? Explain.
Temperature for B was greater because the reaction occurred at a faster rate and there were
more moles of CO (product) formed in the same time as reaction A.
3
Explain why food spoils less quickly when stored in the refrigerator.
Refrigerator lowers the temperature so the reaction which causes the food to spoil occurs at a
slower rate.
4
The following observations were made in simple experiments.
i If a lump of iron is heated with sulfur, a slow reaction starts and continues slowly.
ii If a mixture of iron filings and sulfur is heated for only a few seconds a vigorous reaction
occurs, which becomes self-sustaining.
iii When iron filings are mixed with sulfur no apparent reaction occurs.
Explain the reasons for the difference in reaction rates between:
a the first and second reaction
In the first reaction a lump of iron is used while in the second reaction iron filings are used.
The filings have a greater surface area so the reaction proceeds more rapidly.
b
the second and third reaction
In the second reaction the mixture is heated so this provides the energy to overcome the
activation energy barrier and allows the reaction to proceed, whereas in the third reaction no
energy is provided so reaction does not occur.
5
When a match is put to a plate of flour it goes out, but workers in a flour mill must be extremely
careful to avoid explosions. Explain why this could be so.
The flour particles are dispersed through a volume of air providing a large surface area for reaction.
Copyright © 2008 McGraw-Hill Australia
CONQUERINGCHEMISTRY PRELIM
MODULE 4 WS 15
6
In an experiment in which calcium carbonate reacted with hydrochloric acid, the carbon dioxide
gas (CO2) was collected in a tube and the volume of gas produced over a period of time was
recorded as shown below.
a
Time (s)
0
2
4
6
8
10
12
14
16
18
Volume of
CO2 (mL)
0
10
17
22
29
29
30
31
31.5
31.5
Plot a graph showing volume of CO2 produced vs time.
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From your graph state the way in which the rate of the reaction between calcium carbonate
and hydrochloric acid varies with time. How did you decide this? Explain why rate changes this
way.
Reaction rate decreases as the reaction proceeds which can be seen by the decrease in slope
of the graph. The rate decreases because the reactants are being used up and there are less
reactant particles to collide to produce the CO product.
c
Suggest one way in which the rate of this reaction can be increased and explain why the
increase occurs.
Reaction rate could be increased by:
•
grinding up the calcium carbonate creating a greater surface area
•
increasing the concentration of hydrochloric acid so there are more particles available for
collision
•
increasing the temperature causing more successful collisions to occur between particles.
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CONQUERINGCHEMISTRY PRELIM
MODULE 4 WS 15
7
Describe one example of the role of a catalyst used in industry.
•
Synthesis of ammonia for fertiliser from N2 and H2 using an iron catalyst
•
Sulfuric acid synthesis using a vanadium pentoxide catalyst for the SO2  O2 step
•
Hydrogenation of liquid vegetable oils into semi-solid fats for use in margarine using a nickel
catalyst
•
Catalytic cracking (using zeolites) of hydrocarbons in oil refining to convert higher boiling
point fractions into petrol
•
Catalytic reforming of gasoline using a platinum catalyst to improve octane rating and
eliminate the need for lead compounds
Copyright © 2008 McGraw-Hill Australia
CONQUERINGCHEMISTRY PRELIM
MODULE 4 WS 15