GCSE level chemistry exam revision notes

Rates of reaction Part 3a. Reaction rate and the effect of changing concentration on the rate/speed of a chemical reaction

3.(a) The Factors affecting the Rate of Chemical Reactions

Varying the CONCENTRATION of a reactant

Introduction to this section looking at the effect of changing concentration on the speed of a reaction

3.(b) An experimental method for investigating the effect of reactant concentration on the rate (speed) of a chemical reaction and the analysing and interpretation of the data obtained

• A typical gas syringe experiment set-up is illustrated above.
  • In this example a gas is generated in the reaction and the volume measured and monitored with time using a gas syringe collection system.
  • The independent variable is the concentration of the reactant solution, the variation of which is your choice.
  • You keep all things constant e.g. the controlled variables like temperature and both mass and particle size of a solid reactant or catalyst and so you only vary the concentration of the reactant solution.  It must be the same apparatus, same volume of reacting solution.
  • The dependent variable is the volume of gas or its rate of evolution
  • It is really important to stir the mixture by swirling the contents at as constant rate as possible throughout an experiment and for each experiment, not easy, require good technique and concentration unless you can use a magnetic stirrer system to control this variable!

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3.(c) Investigation 1. Limestone and acid reaction

• 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.
    • Ideally you can use a magnetic stirrer and keeping the rotation rate the same for every experiment.
  • 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 gas formed and collected with a gas syringe system (diagram above).  V_CO2 is the dependent variable
  • 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 to rates of reaction.
  • You can use this method to investigate the rate of reaction of dilute hydrochloric acid and magnesium, but I think it would be difficult to get accurate results despite a good stirring technique, the magnesium ribbon tends to float a bit when surrounded by hydrogen bubbles!

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3.(d) Investigation 2. The catalytic decomposition of hydrogen peroxide to evolve oxygen

• The same gas syringe apparatus can be used to investigate the how the rate of the catalytic decomposition of hydrogen peroxide varies with different concentrations of hydrogen peroxide 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 the oxygen gas formed.
  • Gentle constant 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 and collected with a gas syringe system (diagram above).  V_O2 is the dependent variable
  • 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.

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3.(e) Data interpretation - graphical analysis of results from investigations 1. and 2.

• 
• The volume of gas evolved is plotted versus time.
• 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 relative rate is proportional to the gradient of the graph.
• 
• 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.

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.
• e.g. doubling the concentration of a reactant often results in a doubling in the rate/speed of the reaction.

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

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3.(f) Investigation 3. The reaction between hydrochloric acid and sodium thiosulfate solution to precipitate sulfur

• 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).
  • When hydrochloric acid is added to sodium thiosulfate solution the ensuing chemical reaction of the mixture produces a sulfur precipitate.
  • The speed of the reaction is measured in terms of the time it takes for enough sulfur precipitate to form to obscure from view a black cross on white paper underneath the conical flask of reactants.
  • Controlled variables: All the solutions must be at the same temperature and the same conical flask used.
    • The total volume should be constant, so only the concentration of one of the reactants should vary.
    • Use the same X on the same paper.
  • The dependent variable is the concentration of the hydrochloric acid.
  • The time taken to obscure the X is the dependent variable.

mix => ongoing => watch stopped =>

• You must keep the volumes of reactants constant, the temperature of ALL the reactants constant, the same person making all the observations with 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 due to the formation of the sulfur precipitate.
• You repeat the experiment with different concentrations of sodium thiosulfate or different concentrations of 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).

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3.(g) 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!)

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!

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

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3.(h) Example of student data from the limestone and acid reaction

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

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

The results tabulated in Microsoft Excel software for four steadily increasing concentrations of the hydrochloric acid, in this case four different molarities of hydrochloric acid from 0.50 mol/dm³ to 2.0 mol/dm³.

Experiments should 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 hydrochloric 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 average rate gradients (speed) from the graph in cm³/min as a measure of the speed of the reaction

e.g. consider the volume of gas formed during the first two minutes of the experiment

Series 1: for the 0.5 molar acid, initial average gradient = rate = 5/2 = 2.5 cm³/min

Series 2: for the 1.0 molar acid, initial average gradient = rate = 19/2 = 9.5 cm³/min

Series 3: for the 1.5 molar acid, initial average gradient = rate = 30/2 = 15 cm³/min

Series 4: for the 2.0 molar acid, initial average gradient = rate = 44/2 = 22 cm³/min

You observe a steady increase in the rate of the reaction with increase in the concentration (molarity) of the acid, but these are real results and you do not observe a perfect linear relationship between concentration of the reactant and the speed of the reaction.

You can plot the rate of reaction data of 'speed' in cm³ CO₂/min versus the molarity of the acid concentration. Since the first point is theoretically 0,0 it suggest the slowest value is rather inaccurate but the three other points are close to forming a linear graph, which is what you expect from collision theory i.e. double the concentration of a reactant, you double the probability of a fruitful collision.

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

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3.(i) Practise multiple choice exam questions on the effect of concentration change on the speed of a chemical reaction

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3.(j) Learning objectives for investigating the effect of changing concentration on the rate/speed of a chemical reaction

Effect of concentration - investigation 1. Be able to describe how to set up a gas syringe system to follow the speed of a chemical reaction in which a gas is evolved e.g. the reaction between limestone chips and dilute hydrochloric acid and varying the concentration of the hydrochloric acid.

Know that the reaction is followed by measuring the volume of carbon dioxide formed and describe how to obtain the results carefully and accurately.

Be able to explain how to keep, and why, all variables constant, except to change the solution concentration of one of the reactants, in this case the dilute hydrochloric acid solution.

Be able to plot the volume of carbon dioxide versus time and determine from the graph a gradient to represent the relative rate of the chemical reaction and the effect of changing the concentration on the speed of the reaction.

Be able to explain, with the aid of a particle model diagram, the results in terms of the probability of hydrochloric acid particles colliding with limestone with increased chance of a fruitful collision forming carbon dioxide molecules with increase in concentration of the hydrochloric acid.

Effect of concentration - investigation 2. Be able to describe how to set up a gas syringe system to follow the speed of a chemical reaction in which a gas is evolved e.g. the decomposition of hydrogen peroxide and the effect on the speed by varying the hydrogen peroxide solution concentration.

Know that the reaction is followed by measuring the volume of oxygen formed by the decomposition of the hydrogen peroxide and describe how to obtain the results carefully and accurately.

Be able to plot the volume of oxygen versus time and determine from the graph a gradient to represent the relative rate of the chemical reaction and the effect of changing the concentration on the speed of the reaction.

Be able to explain how to keep, and why, all variables constant, except to change the solution concentration of the reactant hydrogen peroxide.

Be able to, with the aid of a particle model diagram, explain the results in terms of the probability of hydrogen peroxide particles colliding with the catalyst with increased chance of a fruitful collision forming oxygen molecules with increase in concentration of the hydrogen peroxide.

Effect of concentration - investigation 3. Be able to describe how to measure the speed of the reaction between sodium thiosulfate solution and dilute hydrochloric acid by measuring the time it takes for sufficient sulfur to form to obscure from view a black cross marked on white paper and varying the concentration of the dilute hydrochloric acid.

Be able to, with the aid of a particle model picture, explain how to keep all variables constant except to change the solution concentration of one of the reactants, in this case the dilute hydrochloric acid solution.

Be able to describe how to obtain the results carefully and accurately for how long it takes for the sulfur precipitate to obscure the view of the cross.

Be able to plot the reaction time versus concentration to determine the general effect of increasing the concentration of the dilute hydrochloric acid.

Be able to do a 2nd graph of the reciprocal of the reaction time as a measure of the rate of formation of sulfur versus the dilute hydrochloric concentration.

Be able to explain the results in terms of the probability of hydrochloric acid particles colliding with sodium thiosulfate particles with increased chance of a fruitful collision forming sulfur with increase in concentration of hydrochloric acid.

3.(k) Exam board key revision points about the effect changing the concentration of reactant on the rate of a chemical reaction

Key revision notes tailored to the major UK GCSE boards (AQA, Edexcel, OCR Gateway, OCR 21st Century, WJEC, CCEA) and CIE Cambridge IGCSE.

They are aligned with the content of syllabus expectations, focusing on rates of reaction, concentration effects, experimental methods, and collision theory, with exam tips and misconceptions highlighted.

Revision Notes: Rates of Reaction – Concentration Effects

1. Core Concept

• Rate of reaction = how quickly reactants are converted into products.
• Measured by:
  • Disappearance of reactants (e.g., mass loss, colour change).
  • Appearance of products (e.g., gas volume, precipitate formation).

2. Experiments to Investigate Concentration Effects

Common GCSE/IGCSE Practical Examples:

• Magnesium + hydrochloric acid → measure volume of hydrogen gas produced.
• Sodium thiosulfate + hydrochloric acid → measure time for sulfur precipitate to obscure a cross.
• Calcium carbonate + hydrochloric acid → measure mass loss (CO₂ escapes).
• Typically the concentration of acid is varied, everything else must be constant.

Method:

1. Keep all variables constant (temperature, surface area, volume of acid).
2. Change only concentration of reactant.
3. Record reaction time or rate (gas volume per unit time, time to obscure cross, etc.).
4. Plot graphs:
   • Concentration versus rate → typically a positive correlation.
   • Steeper gradient = faster reaction.

3. Collision Theory Explanation

• Particles must collide with sufficient energy (activation energy) to react.
• Increasing concentration:
  • More particles per unit volume.
  • Higher collision frequency.
  • Greater chance of successful collisions.
• Therefore, rate increases with concentration.

4. Analysis of Results

• Graphs show:
  • Higher concentration → faster initial rate.
  • Curves flatten as reactants are used up.
• Rate can be calculated from:
  • Gradient of graph (steeper = faster).
  • 1/time method (for “disappearing cross” experiments).

5. Typical Exam Board Specification Content

• Required practical – effect of concentration on rate (thiosulfate + HCl), graphical analysis.
• Importance of practical skills, rate graphs, particle collision theory.
• Experimental design, evaluation of rate experiments and interpreting data, linking to theory.

6. Student Exam Tips

 Always state controlled variables (temperature, volume, surface area).
 Use precise terms: “frequency of collisions increases” rather than “more collisions happen.”
 Quote data when analysing graphs (e.g., “At 2 mol/dm³, reaction finished in 20s compared to 40s at 1 mol/dm³”).
 Draw and interpret graphs carefully – label axes, units, and trends.
 Link practical observations to theory (e.g., faster disappearance of cross = higher collision frequency).
 Use ‘rate of reaction’ not ‘speed of reaction’ in formal answers.

7. Typical Misconceptions

 “Higher concentration gives particles more energy.”

• Correction: Energy of particles is unchanged; only collision frequency increases.

 “Reaction rate stays constant throughout.”

• Correction: Rate decreases as reactants are used up.

 “All collisions lead to reaction.”

• Correction: Only collisions with sufficient energy (≥ activation energy) are successful.

 “Graphs always straight lines.”

• Correction: Rate graphs often curve as reactants are consumed.

8. Quick Summary Table