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Doc Brown's Chemistry KS4 science GCSE/IGCSE/O level Revision Notes

REACTION RATE and GAS PRESSURE

Factors affecting the Speed-Rates of Chemical Reactions

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

 Can changing the pressure affect the speed of a reaction? Why does changing pressure only affect a reaction involving reactant gases? These revision notes are suitable for GCSE IGCSE O Level KS4 science chemistry students studying the effect of pressure on the rate of a reaction involving gaseous reactants. The descriptions of such reactions and theoretical explanations should help with homework, coursework assignments etc. These notes on the effect of pressure on reaction rate and its importance in the chemical industry are designed to meet the highest standards of knowledge and understanding required for students/pupils doing GCSE chemistry, IGCSE chemistry, O Level chemistry, KS4 science courses and can be useful primer for A Level chemistry courses. These revision notes on how gas pressure affects the rate of a chemical reaction speed, should prove useful for the new AQA GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (91), (9-5) & (5-1) science courses.


Rates of reaction notes INDEX


3. The Factors affecting the Rate of Chemical Reactions

Varying the PRESSURE of a reactant gas

3b The effect of Pressure

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 EFFECT?

  • 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.

  • 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

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= inc. P =>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 product molecules are not shown, but just imagine how more collisions will occur in the right-hand diagrams!

The diagram at the top of the page gives an idea of how to think about fruitful and unfruitful collisions.

 

Pictures of a gaseous particles (molecules) undergoing chemical changes on the surface of a catalyst

Factors affecting the rates of Reaction - particle collision theory model (c) Doc Brown== inc. P =>Factors affecting the rates of Reaction - particle collision theory model (c) Doc Brown 

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.

 


Rates of reaction notes INDEX

GCSE/IGCSE MULTIPLE CHOICE QUIZ on RATES of reaction


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