SPEEDIN
G UP and SLOWING DOWN
rates of reaction.
Reactions can be speeded up by:
1.
Using reagents that are either: more CONCENTRATED (for LIQUIDS) or
 at a higher PRESSURE (for GASES)
(THIS IS NOT THE SAME AS USING MORE REAGENTS)
2. Increasing the TEMPERATURE at which a reaction occurs – generally for every
10oC rise in temperature the reaction rate doubles.
3. Using reagents that are finely divided – POWDERS generally work faster than
LUMPS. They have a BIGGER SURFACE AREA.
4. Using a CATALYST – this can generally speed up or slow down the rate at
which a chemical reaction occurs.
Reactions can be slowed down by:
1.


Using reagents that are either:more DILUTE i.e add lots of water (for LIQUIDS) or
at a lower PRESSURE i.e use a large container (for GASES).
(THIS IS NOT THE SAME AS USING LESS REAGENTS)
2. Reducing the TEMPERATURE at which a reaction occurs i.e COOL things
down.
3. Using reagents that have a SMALL SURFACE AREA i.e the substance is in
large lumps.
4. Using a CATALYST – the right catalyst can slow down the rate at which a
chemical reaction occurs.
EXPLANATION: THE COLLISION THEORY
If the particles making up the substances are to react with one another they must hit
each other
 hard enough
 in the right place.
If both things occur the collision is said to be effective.
So NO reaction will occur if:
 the particles are moving slowly.
 they don't hit each other in the right place.
No reaction will be seen to occur if not enough particles react.
Reactions generally slow down as the reaction proceeds because:
 the number of reacting particles available become smaller and smaller
 it takes longer for these reacting particles to meet one another and react.
1.
THE EFFECT OF CONCENTRATION / PRESSURE.
Generally the greater the concentration /pressure of the reagents the faster the
reaction.
Think about drinking
5 glasses of vodka (40% proof)
5 glasses of wine (12% proof)
Which will have the bigger effect – the obvious answer is the vodka because there is
more alcohol in the vodka than there is in the wine.
If we make solid polyethene we squeeze ethene gas particles together . The greater the
pressure used the more likely the reaction is to occur.
Look at the diagrams below
In the diagram opposite are 3
acidic solutions A, B and C
Which acidic solution is the:
(a) most concentrated ?
(b) least concentrated.
If 2g of magnesium ribbon
were put into each acidic
solution which solution would
react the fastest ? Explain
giving your reasons.
WITH GASES
In the diagrams opposite we
have reacting gas particles in a
metal tube. The pressure of the
gases is recorded by the use of
a pressure gauge.
In which metal tube (A, B or C)
is the pressure highest.
Explain your answer.
In which metal tube will the
reaction be fastest.
Explain your answer.
If an inert gas is put into metal
tube A what will happen to
(i) the pressure ?
(ii) the speed of the reaction
2.
THE EFFECT OF TEMPERATURE.
Think of a piece of smelly cheese. On a hot day you can really smell the cheese – its
as if all the smelly cheese particles are moving out and about – we say they are
EXCITED. If the day is really cold – you don't really smell the cheese unless you get
close up.
When we prepare food we
 put it in the fridge to stop it from going bad.
 cook it in the oven to make it more edible. If we use a low oven temperature it
takes hours for the food to cook. If we use a high oven temperature the food not
only cooks but burns.
So in general if we want :
 the particles to do something we give them energy i.e we raise their temperature –
they become excited – they rush around madly hitting one another.
 the particles to do nothing we remove their energy i.e we lower their temperature
– we cool them down – they move around slowly hitting one another.
Again the diagrams below illustrate the point.
NOTE
If we talk about biological chemicals – the biological catalysts and proteins that work
in our body – they work best at 40oC – at higher temperatures the shape of the
biological catalysts and proteins change – we say that they are DENATURED and
chemical reactions tend to stop.
At lower temperatures the biological catalysts and proteins hardly have enough
energy to react - we say that the reactants are DEACTIVATED – they just can't be
bothered to react.
3. THE EFFECT OF SURFACE AREA
Imagine a block of cheese . 20 mice want to eat the cheese. Is it easier for each mouse
to take a lump of cheese and then pass it on or will it be quicker for the cheese to be
first of all cut into 20 pieces and then passed on ?
In general the more finely divided a substance is the quicker it will react. POWDERS
react faster than LUMPS. Look at the diagram below.
In the first diagram we
have a lump of rock
containing 36 particles
(6 x 6). The lump can only
be attacked at its edges –
there are 20 contact points.
In the second diagram the
same lump (18 x 2) is now
more spread out and the
number of contact points
has almost doubled to 36.
So the more spread out –
the bigger the surface areathe more contact points –
the faster the reaction.
4. THE EFFECT OF A CATALYST.
A catalyst is a substance that speeds up (or slows down) a reaction without itself
being changed. Only small amounts of catalyst are needed. When substances react
they need to have a certain amount of energy – i.e they need to be EXCITED.
Catalysts work by decreasing the amount of energy needed.
Another way of looking at the problem is MOUNTAIN WALKING – most people
will be able to tackle low lying hills – they have enough energy to get up these hills –
as these hills get bigger fewer people climb up them – not everyone has enough
energy. Look at the diagrams below.
It is obviously so much easier to climb over the small hill than the large hill
Catalysts reduce the amount of energy required for the reactants to react.
If a graph of energy against progress of
reaction is drawn you should see that
for a catalysed reaction
less energy is needed to go from reactants
to products
and for an uncatalysed reaction
more energy is needed to go from
reactants to products.
Hence catalysed reactions are more likely
to occur than uncatalysed reactions.
Catalysts are used a lot in industry – if
they make things happen faster then you
can make more in a given time and you
can improve your profit margins.
CATALYSTS MAKE MONEY
A LOOK at a catalytic reaction: a DEMONSTRATION REACTION
Hydrogen peroxide - a bleach used by hairdressers slowly decomposes with water
and oxygen being produced.
decomposes slowly
Hydrogen
peroxide
water
+
oxygen
The reaction can be speeded up using potassium iodide as the catalyst.
decomposes quickly
Hydrogen
peroxide
water
+
oxygen
If washing up liquid is added to the hydrogen peroxide and the whole experiment is
done in a large measuring cylinder (in a large glass trough) lots and lots of froth is
produced.
The rate of reaction can be determined by measuring the time taken for the froth
(produced by the evolution of oxygen) to get to the top of the measuring cylinder.
NOTE
For really SPECTACULAR results use 50 mls of 100 volume Hydrogen peroxide.
Also try out other potential catalysts such as:
e.g the biological
celery
blood
chopped liver
catalysts
carrot juice
chemical catalysts
manganese dioxide
copper oxide
lead oxide
potassium
potassium bromide
potassium chloride
dichromate
A LOOK at a reaction dependant on SURFACE AREA: a DEMONSTRATION
REACTION.
For this experiment we use aluminium metal lumps, aluminium foil and aluminium
powder. We see if the aluminium reacts with powdered iodine crystals.
a) aluminium metal lumps + powdered iodine crystals.
In a beaker
b) aluminium foil + powdered iodine crystals.
c) aluminium powder + powdered iodine crystals.