3b. What is the effect
of changing pressure
on the rate of a reaction involving gaseous reactants?
Chemistry Revision Notes:
The effect of pressure on reaction rate (speed)
chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE chemistry, O level
& ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
science students for national examinations in chemistry -
Chemistry KS4 science GCSE/IGCSE/O level Revision Notes - Factors
affecting the Speed-Rates of
reaction notes INDEX
Factors affecting the Rate of Chemical Reactions
REACTION RATE and GAS PRESSURE
Varying the PRESSURE of a reactant gas
The effect of Pressure
in the Haber process for manufacturing ammonia)
For each factor I've presented
several particle diagrams to help you follow the text explaining how the
particle collision theory accounts for your observations of reaction rate
varying with the pressure of reactant gases (some 'work' better than others!)
WHAT IS THE EFFECT OF CHANGING PRESSURE ON THE
SPEED OF A REACTION?
DOES INCREASING THE PRESSURE ALWAYS HAVE AN
If there are no gaseous reactant
molecules, then pressure has no effect on the rate of reaction because
liquids and solids are almost impossible to compress to increase the
concentration - so no effect on the rate of collision determining the speed
of the reaction.
Why does an increase in pressure speed up a
reaction with a gaseous reactant?
If one or more of the reactants is a gas then
increasing pressure will effectively increase the concentration of the reactant molecules and
speed up the reaction (as described in
So, for gaseous reactants only,
pressure is essentially a concentration factor.
Increasing pressure has
virtually no effect on solids or solutions engaged in a chemical reaction.
The particles are, therefore on average,
closer together and collisions between the particles will occur more
The particle diagrams
below could represent lower to higher pressure situations, resulting in
greater concentration and so a slower to
This all because of the increased chance of a
'fruitful' collision, on increasing the total pressure of the
The arguments based on increased
reaction rate with increased pressure to gases reacting freely in the
gaseous state (gas phase),
OR, gaseous reactants impact on
a solid catalyst surface because the increase in pressure increases the
collision rate of the reactant molecules with the catalyst surface.
Increased pressure is used in
Haber Synthesis of Ammonia,
not only to increase the yield of ammonia, but to also increase the rate of
nitrogen combining with hydrogen to form ammonia.
- (e.g. starting with a 1:3 ratio of N2 : H2), and measure
the yield with time (until it settles out to the constant maximum equilibrium yield,
graph horizontal) for range of pressures, it shows that ...
- ... the greater the total pressure, the greater the
rate of reaction (steeper initial gradient) - due to effectively an
increase in the concentration of gas molecules, increasing the probability
of a fruitful collision leading to products - in this case ammonia.
Solid reactants and solutions
are NOT affected by change in pressure, their concentration is unchanged, so
no change in the rate of the reaction.
More details of laboratory investigations
('labs') involving 'rates of reaction' i.e. experimental methods for
observing the speed of a reaction are given in
THEORETICAL INTERPRETATION of CHANGING
THE PRESSURE of a REACTING GAS
Applying particle models
The first diagram gives an idea of how to think about the probability of fruitful
Pictures of a gaseous particles
(molecules) undergoing changes in a gaseous chemical reaction
increase pressure ==>
This illustrates a
mixture of gases A and B colliding and potentially reacting
The greater the
concentration (pressure) of the gas molecules, the greater the
probability of a fruitful collision producing the product
The product molecules are
not shown, but just imagine how more collisions will occur in the
Pictures of a gaseous particles
(molecules) undergoing chemical changes on the surface of a catalyst
inc. P =>
This illustrates a gas reacting
on the surface of a solid catalyst.
Again, the product
molecules are not shown, but just imagine how more collisions will occur
in the right-hand diagram on the catalyst surface.
As you increase the
pressure, you effectively increase the concentration of the reactants
and thereby increase the chance of a fruitful collision.
Industrial note on the effect
of gas pressure - or rather the concentration of potentially reactant gases
on the rate of reaction:
If the flammable/explosive
gas is in low concentration, there may be no risk, but you need to know
the safe limits!
e.g. Methane gas in
mines, petrol vapour etc. are all potentially dangerous situations
so knowledge of 'explosion/ignition threshold
ignition temperatures and activation energies are all
important knowledge to help design systems of operation to
reaction notes INDEX
Level (~US grade 8-10) School Chemistry Notes
(students age ~14-16)
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Advanced A Level KINETICS
GCSE level 'Rates of Reaction' multiple
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