THE SPEED OF REACTIONS
There have been several massive explosions in flour mills when flour dust has caught fire. These
occur because the dust burns very quickly. An explosion is a very fast chemical reaction. Other
reactions are much slower. For example, copper domes on the roofs of buildings will react with the
air and turn green, but it can take many years for this to happen.
The speed of a reaction is very important. In industry, chemical engineers need to know how long it
will take for substances to react to make a product. Chemists have done a lot of work studying
what makes a reaction fast or slow. All chemical reactions produce new substances. The speed of a
reaction can be found by measuring how fast a new substance is produced.
The reaction between acid and
marble chips is a useful one to
investigate. When marble chips
are added to dilute acid in a
flask, carbon dioxide is
produced. As the reaction goes
on, the flask becomes lighter
because the carbon dioxide
escapes. The speed of the
reaction can be measured by
putting the flask on the balance
and measuring the loss of mass
at regular intervals. Once the
results have been obtained, they
can be plotted on a graph. If you put time on the horizontal axis, the slope of the graph tells you
how fast the reaction is at different times.
Part A: this is the steepest part of the curve, which means that the reaction is fastest here. Lots
of carbon dioxide is given off each second.
Part B: the slope is less steep here; the reaction is slowing down.
Part C: here the grapf has levelled off. This means that the reaction has stopped. No more carbon
dioxide is being produced.
Time/ seconds
0
10
20
30
40
50
Mass of carbon dioxide
produced / grams
0.0
1.0
1.8
2.4
2.8
3.0
SPEEDING UP REACTIONS
Imagine you are an athlete and you want to
be able to run faster. You decide to change
your diet, buy new running shoes and try out
a weight-training programme. After doing
all this, you find that there has been an
improvement and you do run faster.
However, because you changed all three
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things at the same time, you do not know which of them actually helped you to run faster. The only
way to find out is to change one thing at a time and measure any increase in your speed.
The same problems can be found in chemistry. If you want to find out what makes a reaction go
faster then you must change only one thing at a time. The things that can be changed in a chemical
reaction are called variables.
slow reactions
large particles
low concentrations
low temperatures
fast reactions
small particles
high concentrations
high temperatures
CATALYSTS
There is a way to speed up a reaction without increasing the
temperature or the concentration of the reactants. That is,
by adding a substance called a catalyst. The picture shows
the effects of adding manganese dioxide to hydrogen
peroxide solution. Hydrogen peroxide solution is a liquid
which decomposes very slowly to give oxygen and water, as
shown in the equation:
hydrogen peroxide → oxygen + water
decomposition of hydrogen peroxide.
Normally this reaction is so slow that you cannot see any
signs of it taking place. But when manganese dioxide is
added, bubbles of oxygen are produced very quickly. We can
say that manganese dioxide is a catalyst for the
Chemist have discovered many different catalysts. However, all catalysts
 speed up some reactions
 are not used up during a reactiom.
catalysts
used in industry
for making
iron
platinum
vanadium oxide
ammonia
nitric acid
sulphuric acid
Catalysts are used in industry because they can speed up
reactions. In a factory, costs are reduced by making
products more quickly. Catalysts also let some reactions take
place at lower temperatures.
CATALYSTS AND THE ENVIRONMENT
Catalytic converter is a device that fits onto the exhausts of cars and helps to break down
harmful oxides of nitrogen into substances that do less damage to the environment. The catalyst
contains metals such as platinum.
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