REACTION RATE and CONCENTRATION

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

3a. What is the effect of changing concentration on the rate of a chemical reaction?

Rates of reaction notes INDEX

3. The Factors affecting the Rate of Chemical Reactions

Varying the CONCENTRATION of a reactant

Experimental methods for investigating the effect of reactant concentration on the rate of a chemical reaction.

• 
• A typical experiment set-up is illustrated above.

• (i) The above diagram illustrates how you can investigate how varying the concentration of hydrochloric acid affects the rate at which it reacts with a given quantity of limestone granules.
  • The flask and gas syringe system for measuring the rate of a chemical reaction.
  • calcium carbonate (marble chips)  + hydrochloric acid ===> calcium chloride + water + carbon dioxide
  • CaCO_3(s) + 2HCl_(aq) ===> CaCl_2(aq) + H₂O_(l) + CO_2(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 temperature of ALL the reactants, the mass of limestone AND its particle size, 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 (an often operational neglected factor), if you don't, the bottom layers of acid become depleted in acid giving a falsely slow rate of reaction (see section on stirring, bottom of page 3c).
  • You follow the reaction by measuring the volume of carbon dioxide formed with a gas syringe system (diagram above).
  • You repeat the experiment with different concentrations of hydrochloric acid to see its effect on the rate-speed of the reaction between hydrochloric acid and limestone/marble chips-powder.
  • More details of laboratory investigations ('labs') involving 'rates of reaction' i.e. experimental methods for observing the speed of a reaction including the effect of reactant concentration are given in the INTRODUCTION

• (ii) The same gas syringe apparatus can be used to investigate the how the rate of the decomposition of hydrogen peroxide varies with different concentrations in the presence of a fixed amount of catalyst.
  • hydrogen peroxide ===> water + oxygen
  • 2H₂O_2(aq) ===> 2H₂O_(l) + O_2(g)
  • You must keep the following variables constant - the volume of hydrogen peroxide solution, the temperature of ALL the reactants, the mass of catalyst AND its particle size, 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 repeat the experiment with different concentrations of hydrogen peroxide to see its effect on the rate-speed of the catalysed decomposition of hydrogen peroxide.

• In both these cases, measuring the initial rate of gas formation 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 greater concentration than in Graph B (a slower reaction).
• The graphs (left) shows you some typical results.

• volume of gas cm3	0.0	7.0	10.5	13.5	15.5	17.5	19.0	19.5	20.0	20.0	20.0	20.0	20.0
  time mins (for run E)	0.0	0.5	1.0	1.5	2.0	2.5	3.0	3.5	4.0	4.5	5.0	5.5	6.0

• The rate of reaction order is X > E > Y > Z, and could represent four increasing concentrations of (i) hydrochloric acid or (ii) hydrogen peroxide in that order.
  • The greater the concentration, the steeper the initial gradient, the faster the reaction.
  • The more concentrated the reactants, the more chance of a successful 'fruitful' collision.
• For the effect of concentration 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.
• You can then plot initial rate of reaction versus concentration to establish or sort of 'rate equation' that enables you to predicts how fast the reaction will go for a particular concentration.
  • Quite often, BUT not always, there is a linear relationship between how fast a reaction goes and the concentration of one of the reactants.
• More details of laboratory investigations ('labs') involving 'rates of reaction' i.e. experimental methods for observing the speed of a reaction including the effect of reactant concentration are given in the INTRODUCTION

• (iii) You can investigate how the varying the concentration of either sodium thiosulfate or hydrochloric acid affects the rate at which they react together to give a precipitate of sulfur (diagram below).

  mix => ongoing => watch stopped =>

• You must keep the volumes of reactants constant, the temperature of ALL the reactants, the same person making all the observations the same size cross on white paper.
• It is important you take the same total volume of solution to give the same depth of liquid you are viewing the cross through.
• Everything should be mixed quickly and the clock started, but there is no need to stir the mixture once it is fully mixed.
• You note the time when the cross first disappears.
• You repeat the experiment with different concentrations of sodium thiosulfate or hydrochloric acid to see their effect on the rate-speed of the acid promoted decomposition of sodium thiosulfate to form a sulfur precipitate.
• The graph on the left shows how the reaction time varies to obscure the X with increase in concentration of the hydrochloric acid or sodium thiosulfate.
  • You can then take the reciprocal of the time to give a measure of the rate of reaction and plot the rate versus concentration of hydrochloric acid OR sodium thiosulfate (graph on the right).

Theoretical interpretation of the results of the effect of concentration on the rate of 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 reactant concentration (some 'work' better than others!)

A picture of a particles (ions or molecules) undergoing changes in a chemical reaction

• WHAT WAS THE EFFECT OF CHANGING THE CONCENTRATION OF A REACTANT?

• AND WHY IS THE REACTION RATE CHANGED?

• Why does increase in concentration speed up a reaction?

• If the concentration of any reactant in a solution is increased, the rate of reaction is increased

  • Increasing the concentration, increases the probability of a collision between reactant particles in a given time because there are more of them in the same volume and so this increases the chance of a fruitful collision forming products.

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

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

  • e.g. Increasing the concentration of acid molecules increases the frequency or chance (in a given time) at which they hit the surface of marble chips to dissolve them (slower => faster, illustrated below)

• In general, increasing the concentration of reactant A or B will increase the chance or frequency of a successful collision between them and increase the speed of product formation (slower => faster, illustrated below).

• 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

• See also graphs 4.6, 4.7 and 4.8 for a numerical-quantitative data interpretation AND the introduction page

APPENDIX 3a (Data for a student's exercise in May 2000, at the 'birth' of my website!)

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GCSE/IGCSE MULTIPLE CHOICE QUIZ on RATES of reaction

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Rates of reaction notes INDEX

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