Higher and foundation tier
Rates of reaction and catalysts
Catalysts are amazing! Catalysts are
substances that speed up a chemical reaction without being used up.
Catalysts as usually transition metals or
transition metal compounds. As an example of a catalyst at work consider
the following decomposition reaction of hydrogen peroxide:
hydrogen peroxide(l) → water(l) + oxygen(g)
2H2O2(l) → 2H2O(g) + O2(g)
Hydrogen peroxide is a corrosive, colourless liquid. Its main uses
are as bleach in the paper and textile industries
and also as a disinfectant, although it can be used as an oxidising agent and also as a propellant e.g. in submarine torpedoes.
According to the equation
above hydrogen peroxide decomposes to form water and also
oxygen gas. However under normal conditions hydrogen peroxide
is stable and you will be waiting a long long time to collect any oxygen gas or water from this reaction!
If the science lab technician had to prepare a class set of test-tubes full of oxygen gas for say a year 8 science lesson, they
would use the decomposition reaction of hydrogen peroxide to prepare the
test-tubes, so how would they do it? The lab
technician would set-up the apparatus below:
You may notice that in the conical flask is a dilute solution of hydrogen peroxide, it has been diluted in water,
and one spatulas of a solid compound called manganese dioxide has been added. Manganese dioxide
is a catalyst
for the decomposition of hydrogen peroxide. The hydrogen peroxide
will decompose violently and rapidly in the presence
of manganese dioxide.
Once the lab technician has filled a class set full of test-tubes for use by students, it would
be possible to
filter out the manganese dioxide catalyst, remember catalyst do NOT get used up. It would also be possible
the catalyst. It is worth mentioning that since the catalyst is not used up in the reaction only small amounts
of it are needed.
Hydrogen peroxide is produced in the body as a by-product of cell activity.
Since it is a corrosive substance it
could kill the cells if its concentration rose to high levels. In the body there are enzymes, one of
catalase will decompose hydrogen peroxide and quickly remove it from the cell. Enzyme are often called biological
How do catalyst work?
Before particles can react with each other they need to collide with enough energy to break the bonds in the reactants,
this is called the activation energy. Catalyst are surface active agents, what this
means is that reactions
involving them take place on the surface of the metal catalyst. The reactants absorb onto the surface of the
and are altered or changed in some way or perhaps aligned in a fashion that allows them to react in a way that requires
less activation energy from the reaction without a catalyst.
The catalyst basically
gets from reactants to products via
a route that requires less activation energy. More particles in the reactants are
likely to have this new lower activation
energy and therefore can react successful, so the rate of reaction will increase.
The energy profile diagram below is for an exothermic reaction. It shows how a catalyst speeds up a reaction by lowering
the activation energy
- Catalysts are used to speed up chemical reactions.
- Catalysts work by providing an alternative route from the reactants to the products which has a low activation energy.
- Catalysts are not used up in chemical reactions so only a small amount of them are needed.