(c) doc bDoc Brown's Chemistry KS4 science GCSE/IGCSE/O level Revision Notes

REACTION RATE and CONCENTRATION

Factors affecting the Speed-Rates of Chemical Reactions

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

How can we investigate the effect of concentration on the rate of a reaction? How does changing concentration affect the rate of a reaction? Why does changing concentration change the speed of a reaction? How can we investigate the effect of concentration on the rate of a chemical reaction? What apparatus do we need to investigate the effect of concentration on the speed of a reaction? How do we process the results from concentration experiments? These revision notes are suitable for GCSE IGCSE O Level KS4 science chemistry students studying how the rate of a reaction varies with the concentration of a reactant in solution. The descriptions of experiments to do with rate concentration experiments and theoretical explanations should help with homework, coursework assignments, laboratory experiment investigations 'labs' on how the speed of a reaction depends on concentration. These notes on the effect of changing concentration on reaction rate, and the experiment methods to follow the speed of the reaction as it changes with change in concentration of reactant and the experimental methods and investigations involved 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 the effect of changing reactant concentration on 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 INDEX: 1. What do we mean by rate/speed of reaction? and how can we measure the speed?  2. Collision theory of chemical reactions  3. Factors affecting the speed of a chemical reaction: 3a Effect of changing concentration  3b Effect of changing pressure  3c Effect of changing particle size/surface area & stirring of a solid reactant  3d Effect of changing temperature  3e Effect of using a catalyst  3f Light initiated reactions 4. More examples of graphs and their interpretation  *  Advanced A Level pages on rates - chemical kinetics


3. The Factors affecting the Rate of Chemical Reactions

Varying the CONCENTRATION of a reactant

(c) doc b3a The effect of Concentration(c) doc b (see also graphs 4.6, 4.7 and 4.8)

When dealing with concentration and its effect on rates of reactions, you are usually concerned with solutions, BUT, all the arguments, graphs, explanations etc. described here, could equally apply to a mixture of gases.


Experimental methods for investigating the effect of reactant concentration 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 solution concentration 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
  • (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
    • 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 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 speed of the decomposition of hydrogen peroxide varies with different concentrations in the presence 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 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 (see left diagram) 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).
  • (c) doc bThe set of 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.
    • (c) doc bQuite 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 => Factors affecting the rates of Reaction - theory and methods of measuring the speed of a reaction (c) Doc Brown ongoing =>Factors affecting the rates of Reaction - theory and methods of measuring the speed of a reaction (c) Doc Brown watch stopped =>Factors affecting the rates of Reaction - theory and methods of measuring the speed of a reaction (c) Doc Brown

  • 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.
  • (c) doc bThe 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.

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 SPEED 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 because there are more of them in the same volume and so this increases the chance of a fruitful collision forming products.

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

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 

  • 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).

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 in the right-hand diagrams!

  • Both diagrams illustrate a change from a low concentration to a higher concentration (for solutions) of reactant to illustrate the effect of increasing concentration.

  • Increasing the concentration of reactant A or B will increase the chance or frequency of collision between them and increase the speed of product formation (slower => faster)

  • Quite often a simple proportionality rule applies to the effect of changing the concentration of a reactant.

    • e.g. If you double the concentration of a reactant, the rate of reaction doubles.

    • This can be explained using particle collision theory by envisaging twice as many particles in the same volume will collide twice as frequently.

    • This will then double the chance of a fruitful collision producing the products of the reaction.

    • You can apply the same idea to ANY factor, quadruple the concentration of a reactant concentration and the reaction goes 4 times faster, half the concentration halves the rate etc. etc.

  • For more details on concentration see Advanced Level Chemistry Theory pages on "CHEMICAL KINETICS"

 


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

A more detailed example of 'effect of concentration' results using the Excel software package.

e.g. from the limestone - hydrochloric acid reaction using a gas syringe system

The results tabulated in Microsoft Excel software for four steadily increasing concentrations of the hydrochloric acid

(four different molarities of hydrochloric acid, 0.50 mol/dm3 to 2.0 mol/dm3)

These experiments would be done with a constant volume of acid, constant mass of limestone and constant temperature.

The Excel graph of the results for four concentrations of the acid

Series 1 = 0.50 mol/dm3, series 2 = 1.0 mol/dm3, series 3 = 1.5 mol/dm3, series 4 = 2.0 mol/dm3

From the plots you get four initial rate gradients from the graph

eg series 2, for the 1.0 molar acid, initial gradient = rate = 19/2 = 8.5 cm3/min

Reminder: You should measure the gradient by drawing a line from 0,0 on the axis over the first few minutes, where the graph is reasonably linear, because the rate is decreasing as the reactants are being used up. Beyond the initial few minutes the graph becomes quite curved and inaccurate.

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

 


RATES of REACTION INDEX: 1. What do we mean by rate/speed of reaction? and how can we measure the speed?  2. Collision theory of chemical reactions  3. Factors affecting the speed of a chemical reaction: 3a Effect of changing concentration  3b Effect of changing pressure  3c Effect of changing particle size/surface area & stirring of a solid reactant  3d Effect of changing temperature  3e Effect of using a catalyst  3f Light initiated reactions 4. More examples of graphs and their interpretation  *  Advanced A  Level pages on rates - chemical kinetics



GCSE chemistry IGCSE chemistry revision notes on reaction rates effect of concentration KS4 GCSE Science revision notes on reaction rates effect of concentration GCSE chemistry guide notes on reaction rates effect of concentration for schools colleges academies science course tutors images pictures diagrams for reaction rates effect of concentration science chemistry revision notes on reaction rates effect of concentration for revising chemistry module topics notes to help on understanding of reaction rates effect of concentration university courses in science careers in science jobs in the industry laboratory assistant apprenticeships technical internships USA US grade 8 grade 9 grade10 AQA chemistry science GCSE notes on reaction rates effect of concentration Edexcel chemistry science notes on reaction rates effect of concentration for OCR 21st century chemistry science notes on reaction rates effect of concentration OCR GCSE Gateway science chemistry notes on reaction rates effect of concentration WJEC gcse science chemistry notes on reaction rates effect of concentration CCEA/CEA gcse chemistry notes science O level chemistry notes for reaction rates effect of concentration IGCSE chemistry revision notes on reaction rates effect of concentration O level chemistry notes should prove useful for the new AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (91, 9-5 & 5-1) science courses Using the kinetic particle model, how do you explain the effect of changing concentration on the speed or rate of a chemical reaction? what apparatus do you need to investigate the effect of changing the concentration on the speed of a reaction? describe an experimental procedure to investigate the effect of changing concentration on the rate of a chemical reaction, how do you draw the graphs of data from an investigation into the effect of changing concentration on the speed/rate of a chemical reaction? investigating the effect of changing the concentration of hydrochloric acid reacting with limestone (calcium carbonate), experiment to investigate the effect of changing concentration on the rate of decomposition of hydrogen peroxide solution using a manganese dioxide catalyst, how to investigate the effect of changing concentration on the rate of reaction between sodium thiosulfate and hydrochloric acid solutions?, particle model diagrams to show the effect of increasing or decreasing concentration on the rate/speed of a reaction gcse chemistry revision free detailed notes on reaction speed effect of changing concentration to help revise igcse chemistry igcse chemistry revision notes on reaction speed effect of changing concentration O level chemistry revision free detailed notes on reaction speed effect of changing concentration to help revise gcse chemistry free detailed notes on reaction speed effect of changing concentration to help revise O level chemistry free online website to help revise reaction speed effect of changing concentration for gcse chemistry  free online website to help revise reaction speed effect of changing concentration for igcse chemistry free online website to help revise O level reaction speed effect of changing concentration chemistry how to succeed in questions on reaction speed effect of changing concentration for gcse chemistry how to succeed at igcse chemistry how to succeed at O level chemistry a good website for free questions on reaction speed effect of changing concentration to help to pass gcse chemistry questions on reaction speed effect of changing concentration a good website for free help to pass igcse chemistry with revision notes on reaction speed effect of changing concentration a good website for free help to pass O level chemistry GCSE (91, 9-5 & 5-1) science courses


ALPHABETICAL SITE INDEX for chemistry     

 For latest updates see https://twitter.com/docbrownchem