GCSE level chemistry rates 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)

3. Factors affecting the Rate of Chemical Reactions

REACTION RATE and GAS PRESSURE of REACTANTS

Varying the PRESSURE of a reactant gas

• WHAT IS THE EFFECT OF CHANGING PRESSURE ON THE SPEED OF A REACTION?

• DOES INCREASING THE PRESSURE ALWAYS HAVE AN EFFECT?

• 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:

    • N_2(g) + 3H_2(g) 2NH_3(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.

THEORETICAL INTERPRETATION of CHANGING THE PRESSURE of a REACTING GAS MIXTURE

Red + green represent reactants, blue + purple represent products, white represents reactants gone!

Fruitful collisions = chemical change, if only it were that simple!, lots of factors to consider!

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.

Key revision points about the effect of changing gaseous reactant pressure on reaction rates

Tailored for WJEC, CCEA, CIE IGCSE, AQA, Edexcel, OCR Gateway, and OCR 21st Century GCSE Chemistry specifications.

Revision Notes: Effect of Pressure on Rates of Gaseous Reactions

2. Collision Theory Explanation

• Collision theory states: particles must collide with sufficient energy (≥ activation energy) to react.

• At higher pressure:

  • Gas particles are closer together.

  • Collisions occur more often.

  • More successful collisions per second.

• At lower pressure:

  • Particles are further apart.

  • Collisions are less frequent.

  • Reaction rate decreases.

3. Experimental Context

4. Graphical Representation

Typical Exam Board Requirements

6. Student Exam Tips

7. Typical Misconceptions

8. Quick Summary Table