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3b. What is the effect of changing pressure on the rate of a reaction involving gaseous reactants?

GCSE level Chemistry Revision Notes: The effect of pressure on reaction rate (speed)

Doc Brown's 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 - Doc Brown's Chemistry KS4 science GCSE/IGCSE/O level Revision Notes - Factors affecting the Speed-Rates of Chemical Reactions

Rates of reaction notes INDEX

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3. Factors affecting the Rate of Chemical Reactions


Varying the PRESSURE of a reactant gas

3b The effect of Pressure

(important 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!)



  • 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 section 3a.).

    • 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 frequently.

    • The particle diagrams below could represent lower to higher pressure situations, resulting in lesser to greater concentration and so a slower to faster reaction.

    • This all because of the increased chance of a 'fruitful' collision, on increasing the total pressure of the reaction system.

    • 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 the 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.

    • The graph on right illustrates both points.

    • It shows the results of a series of experiments at a constant temperature and constant initial composition for the reaction:
      • N2(g) + 3H2(g) (c) doc b 2NH3(g

    • (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 the INTRODUCTION.


Applying particle models

The first diagram gives an idea of how to think about the probability of fruitful collisions.

Pictures of a gaseous particles (molecules) undergoing changes in a gaseous chemical reaction

Factors affecting the rates of Reaction - particle collision theory model (c) Doc Brown == increase pressure ==> Factors affecting the rates of Reaction - particle collision theory model (c) Doc Brown 

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 right-hand diagram!

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 concentrations', ignition temperatures and activation energies are all important knowledge to help design systems of operation to minimise risks.

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Rates of reaction notes INDEX