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GCSE Chemistry Notes: The effect of surface area on reaction rate (speed)

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(c) doc b3c. What is the effect of particle size / surface area on the rate of a reaction involving a solid reactant?

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3. The Factors affecting the Rate of Chemical Reactions

REACTION RATE and SURFACE AREA and a SOLID REACTANT PARTICLE SIZE

Varying the SOLID PARTICLE SIZE - varying SURFACE AREA

(c) doc b3c The effect of Surface Area - particle size of a solid reactant


Experimental methods for investigating the effect of particle size (surface area) on the rate of a chemical reaction

Parts of the sections of 1. Introduction and 2. collision theory are repeated here, but with extra experimental methods and theoretical details applied to experiments and theories linked to the effect of changing the particle size/surface area of a solid reactant on the rate of a chemical reaction

  • Factors affecting the rates of Reaction - theory and methods of measuring the speed of a reaction (c) Doc Brown

  • The above diagram illustrates how you can investigate how varying the particle size/surface area of limestone affects the rate at which it reacts with a given concentration of hydrochloric acid.

    • The flask and gas syringe system for measuring the rate of a chemical reaction.

 

  • (i) The above diagram illustrates how you can investigate how varying the size of the limestone particles affects the rate at which it reacts with a given quantity of limestone granules.
    • calcium carbonate (marble chips)  + hydrochloric acid ==> calcium chloride + water + carbon dioxide
    • CaCO3(s) + 2HCl(aq) ==> CaCl2(aq) + H2O(l) + CO2(g)
    • In the diagram above, the white 'blobs' represent carbon dioxide gas being evolved and the grey lumps the limestone chips, granules or powder.
    • You must keep the following variables constant - the volume of hydrochloric acid, the concentration of the hydrochloric acid, the temperature of ALL the reactants, the mass of limestone, and TRY to keep a gentle constant stirring rate as you are noting down the time and volume of carbon dioxide gas formed.
    • Gentle stirring (swirling action) is important, if you don't, the bottom layers of acid become depleted in acid giving a falsely slow rate of reaction.
    • You follow the reaction by measuring the volume of carbon dioxide formed using the gas syringe system.
    • You repeat the experiment with different particle sizes to vary the surface area and its effect on the rate-speed of the reaction between hydrochloric acid and limestone/marble chips-powder.
    • You need a variety of limestone particle sizes e.g. marble chips - lumps, granules and fine limestone powder.

 

  • (ii) The same apparatus can be used to investigate the how the speed of the decomposition of hydrogen peroxide varies with different particle size of a fixed amount of catalyst.
    • hydrogen peroxide ==> water + oxygen
    • 2H2O2(aq) ==> 2H2O(l) + O2(g)
    • You must keep the following variables constant - the volume of hydrogen peroxide solution, the concentration of the hydrogen peroxide solution, the temperature of ALL the reactants, the mass of catalyst and TRY to keep a gentle constant stirring rate as you are noting down the time and volume of carbon dioxide gas formed.
    • Gentle stirring is important, if you don't, the bottom layers of hydrogen peroxide become depleted in acid giving a falsely slow rate of reaction.
    • You follow the reaction by measuring the volume of oxygen gas formed.
    • You can vary the particle size of the catalyst by grinding it down with a pestle and mortar.
    • You repeat the experiment with different particle sizes of the same catalyst to see its effect on the rate-speed of the catalysed decomposition of hydrogen peroxide.
    • More details of laboratory investigations ('labs') involving 'rates of reaction' i.e. experimental methods for observing the speed of a reaction and including the effect of particle size/surface area/stirring are given in the INTRODUCTION.
    • In both these cases, measuring the initial rate of gas formation (see left and below diagrams) gives a reasonably accurate measure of how fast the reaction is for that concentration.
    • The initial gradient, giving the initial rate of reaction, is the best method i.e. the best straight line covering several results at the start of the reaction by drawing the gradient line using the slope of the tangent from time = 0, where the graph is nearly linear.
    • Examples of graph data for two experiments where one of the reactants is completely used up - all reacted.
    • The two graph lines represent two typical sets of results to explain how the rate of reaction data can be processed.
    • Graph A (for a faster reaction) could represent a where the solid reactant was ground into smaller pieces to increase the surface compared to Graph B (a slower reaction).
    • (c) doc bThe set of graphs (left) shows you some typical results.
    • The rate of reaction order is X > E > Y > Z, and could represent four decreasing particle sizes of (i) limestone or (ii) solid catalyst in that order as the surface area is increased.
    • For example the graph lines might represent, in terms of particle size - X a fine powder, Y granules and Z larger lumps.
      • The smaller the solid particles, the greater the surface area exposed to the reactant in solution, the steeper the initial gradient, the faster the reaction.
      • The more surface area of the solid reactant, the more chance of a successful 'fruitful' collision with the reactant in solution.
    • For the effect of surface area (particle size) on the rate of reaction, under some circumstances graph W could represent the result of taking twice the mass of solid reactant (e.g. double amount of marble chips) or twice the concentration (same volume) of a soluble reactant, BUT it does depend on which reactant is in excess, so take care in this particular graph interpretation.
  • PLEASE note the discussion on stirring further down the page, this also affects the speed of the reaction of hydrochloric acid, whether its powder, granules of lumps!

 


Theoretical interpretation of the results of the effect of particle size/surface area on the rate a chemical reaction

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 particle size/surface area of a solid reactant (some 'work' better than others!)

A picture of a particles (ions or molecules) undergoing changes in a chemical reaction occurring on the surface of a solid reactant OR a sold catalyst surface

  • WHAT HAPPENS TO THE SPEED OF A REACTION IF WE CHANGE THE PARTICLE SIZE OF A REACTING SOLID?

  • WHAT DOES BREAKING UP A SOLID REACTANT INTO FINER PIECES DO TO IT IN TERMS OF HOW IT REACTS?

  • If a solid reactant or a solid catalyst is broken down into smaller pieces the rate of reaction increases.

  • The speed increase happens because smaller pieces of the same mass of solid have a greater surface area compared to larger pieces of the solid.

    • The smaller pieces have an increased surface to volume ratio.

  • Therefore, there is more chance that a reactant particle will hit the solid surface and react in a given time.

    • We are talking about an increased frequency of fruitful collisions on the surface leading to product formation.

    • Increased frequency means an increase in 'rate of reaction'.

  • The diagrams below illustrate the acid–marble chip reaction (slower => faster, but they could also represent a solid catalyst mixed with a solution of reactants (e.g. the catalysed decomposition of hydrogen peroxide).

  • See also graphs 4.1 and 4.8(iii) for a numerical-quantitative data interpretation.

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

  • The product molecules are not shown, but just imagine how more collisions will occur on the surface of the solid particles in the right-hand diagram!

  • Industrial Note to do with the effect surface area/particle size on the rate of a reaction:

    • Flammable fine dust powders can be easily ignited e.g. coal dust in mines, flour in mills, custard powder production lines!

    • Fine powders have a large surface area which greatly increases the reaction rate causing an explosion.

      • Any spark from friction is enough to initiate the reaction!

 

3c continued. The effect of Stirring

  • CAN STIRRING AFFECT THE RATE OF A REACTION?

  • DOES STIRRING AFFECT THE SPEED OF THE REACTION BETWEEN A SOLID AND A SOLUTION?

  • Why does stirring speed up a reaction between a solid and a solution?

  • In doing rate experiments with a solid and solution reactant e.g. marble chips-acid solution or a solid catalyst like manganese(IV) oxide catalysing the decomposition of hydrogen peroxide solution, it is sometimes forgotten that stirring the mixture is an important rate factor.

  • If the reacting mixture is not stirred ‘evenly’, the reactant concentration in solution becomes much less near the solid, which tends to settle out at the bottom of the flask.

  • Therefore, at the bottom of the flask the reaction prematurely slows down distorting the overall rate measurement and making the results uneven and therefore inaccurate. The 'unevenness' of the results is even more evident by giving the reaction mixture the 'odd stir'! You get jumps in the graph!!!

  • Stirring cannot affect a completely mixed up solution at the particle level i.e. two solutions of soluble substance that react together are unaffected by stirring.

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

  • More details of laboratory investigations ('labs') involving 'rates of reaction' i.e. experimental methods for observing the speed of a reaction and including the effect of particle size/surface area/stirring are given in the INTRODUCTION

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