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CIE IGCSE Chemistry Your notes

6.1 Chemical Change & Rate of Reaction

Contents
6.1.1 Physical & Chemical Changes
6.1.2 Rates of Reaction Factors
6.1.3 Collision Theory
6.1.4 Explaining Rates Using Collision Theory
6.1.5 Investigating The Rate of a Reaction
6.1.6 Interpreting Data

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6.1.1 Physical & Chemical Changes

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Physical & Chemical Changes
Physical change
Physical changes (such as melting or evaporating) do not produce any new chemical substances
These changes are often easy to reverse
Making a mixture from 2 or more substances or dissolving a solute in a solvent are examples of physical
changes as no new substances are produced and are usually relatively easy to separate
Chemical change
During chemical changes (usually referred to as chemical reactions), new chemical substances are
formed that have very different properties to the reactants
There may be signs that a new substance has formed, such as:
A colour change
A precipitate being formed
Bubbles of gas being produced
Most chemical reactions are very difficult to reverse
Energy changes also accompany chemical changes and energy can be given out (exothermic) or
taken in (endothermic)
The majority of chemical reactions are exothermic with only a small number being endothermic

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6.1.2 Rates of Reaction Factors

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Rates of Reaction Factors
There are several factors that can affect the rate of a reaction. These are:
Concentration of the reactants in solution or the pressure of reacting gases
Temperature at which the reaction is carried out
Surface area of solid reactants
The use of a catalyst
Changes in these factors directly influence the rate of a reaction
It is of economic interest to have a higher rate of reaction as this implies a higher rate of production and
hence a more efficient and sustainable process
The effect of increased concentration or pressure

Explanation:

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Compared to a reaction with a reactant at a low concentration, the graph line for the same reaction but
at a higher concentration/pressure has a steeper gradient at the start and becomes horizontal sooner
This shows that with increased concentration of a solution or increased pressure of a gas, the rate of Your notes
reaction will increase
The effect of surface area

Graph showing the effect of increased surface area on the rate of reaction
Explanation:
Compared to a reaction with lumps of reactant, the graph line for the same reaction but with powdered
reactant has a steeper gradient at the start and becomes horizontal sooner
This shows that with increased surface area of the solid, the rate of reaction will increase
The effect of temperature

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Your notes

Graph showing the effect of temperature on the rate of reaction

Explanation:
Compared to a reaction at a low temperature, the graph line for the same reaction but at a higher
temperature has a steeper gradient at the start and becomes horizontal sooner
This shows that with increased temperature, the rate of reaction will increase
The effect of using a catalyst
Catalysts are substances which speed up the rate of a reaction without themselves
being altered or consumed in the reaction
The mass of a catalyst at the beginning and end of a reaction is the same and they do not form part of
the equation

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Graph showing the effect of using a catalyst on the rate of reaction

Explanation:
Compared to a reaction without a catalyst, the graph line for the same reaction but with a catalyst has a
steeper gradient at the start and becomes horizontal sooner
This shows that with a catalyst, the rate of reaction will increase

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6.1.3 Collision Theory

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Collision Theory
What is Collision Theory?
EXTENDED
Collision theory states that in order for a reaction to occur:
The particles must collide with each other
The collision must have sufficient energy to cause a reaction i.e. enough energy to break bonds
The minimum energy that colliding particles must have to react is known as the activation energy
Collisions which result in a reaction are known as successful collisions
If they have sufficient energy (i.e. energy greater than the activation energy), they will react, and
the collision will be successful
Not all collisions result in a chemical reaction:
Most collisions just result in the colliding particles bouncing off each other
Collisions which do not result in a reaction are known as unsuccessful collisions
Unsuccessful collisions happen when the colliding species do not have enough energy to break the
necessary bonds (i.e. they collide with energy less than the activation energy)

Diagram showing a successful and an unsuccessful collision

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Increasing the number of successful collisions means that a greater proportion of reactant particles
collide to form product molecules
The number of successful collisions depends on: Your notes
The number of particles per unit volume - more particles in a given volume will produce more
frequent successful collisions
The frequency of collisions - a greater number of collisions per second will give a greater number
of successful collisions per second
The kinetic energy of the particles - greater kinetic energy means a greater proportion of
collisions will have an energy that exceeds the activation energy and the more frequent the
collisions will be as the particles are moving quicker, therefore, more collisions will be successful
The activation energy - fewer collisions will have an energy that exceeds higher activation energy
and fewer collisions will be successful
These all have an impact on the rate of reaction which is dependent on the number of successful
collisions per unit of time

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6.1.4 Explaining Rates Using Collision Theory

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Explaining Rates Using Collision Theory
EXTENDED
Temperature
Increasing the temperature will increase the rate of reaction
This is because the particles will have more kinetic energy than the required activation energy
Therefore there will be more frequent collisions and a higher proportion of particles have energy
greater than the activation energy
This causes more successful collisions per second, increasing the rate of reaction
The effect of temperature on collisions is not so straightforward as concentration or surface area; a
small increase in temperature causes a large increase in rate
For aqueous and gaseous systems, a rough rule of thumb is that for every 10 oC increase in
temperature, the rate of reaction approximately doubles

Diagram showing the increased kinetic energy that particles have at higher temperatures

Concentration
Increasing the concentration of a solution will increase the rate of reaction
This is because there will be more reactant particles in a given volume, allowing more frequent and
successful collisions per second, increasing the rate of reaction

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For a gaseous reaction, increasing the pressure has the same effect as the same number of
particles will occupy a smaller space, increasing the concentration
If you double the number of particles you will double the number of collisions per second Your notes
The number of collisions is proportional to the number of particles present

Diagram showing the decrease in space between particles at higher concentrations

Surface Area
Increasing the surface area of a solid will increase the rate of reaction
This is because more surface area of the particles will be exposed to the other reactant, producing a
higher number of collisions per second
If you double the surface area you will double the number of collisions per second

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Your notes

An increase in surface area means more collisions per second

Catalysts
Catalysts are substances which speed up the rate of a reaction without themselves
being altered or consumed in the reaction
The mass of a catalyst at the beginning and end of a reaction is the same and they do not form part of
the equation
Different processes require different types of catalysts but they all work on the same principle of
providing a different pathway for the reaction to occur that has a lower activation energy
This means a higher proportion of the reactant particles have energy greater than the activation energy
and will result in more successful collisions per second
An important industrial example is iron, which is used to catalyse the Haber Process for the production
of ammonia
Iron beads are used to increase the surface area available for catalysis
Enzymes are biological catalysts, they work best at specific temperature and pH ranges
Normally only small amounts of catalysts are needed to have an effect on a reaction

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Your notes

A catalyst lowers the activation energy of a reaction by providing an alternative reaction pathway

Exam Tip
When answering questions on the effect of concentration on the rate of reaction, you should mention
that there are more particles per unit volume (usually cm3) and this causes an increase in the rate of
collisions.

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6.1.5 Investigating The Rate of a Reaction

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Investigating The Rate of a Reaction
To measure the rate of a reaction, we need to be able to measure either how quickly the reactants are
used up or how quickly the products are formed
The method used for measuring depends on the substances involved
There are a number of ways to measure a reaction rate in the lab; they all depend on some property that
changes during the course of the reaction
That property is taken to be proportional to the concentration of the reactant or product, e.g., colour,
mass, volume
faster reactions can be easier to measure when the reaction is over, by averaging a collected
measurement over the course of the reaction some reaction rates can be measured as the
reaction proceeds (this generates more data)
Three commonly used techniques are:
measuring mass loss on a balance
measuring the volume of a gas produced
measuring a reaction where there is a colour change at the end of the reaction
Investigating the effect of surface area on the rate of reaction

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Diagram showing the process of downwards displacement to investigate the effect of the surface area
of a solid on the rate of reaction
Method:
Add dilute hydrochloric acid into a conical flask
Use a delivery tube to connect this flask to a measuring cylinder upside down in a bucket of water
(downwards displacement)
Add calcium carbonate chips into the conical flask and quickly put the bung back into the flask
Measure the volume of gas produced in a fixed time using the measuring cylinder
Repeat with different sizes of calcium carbonate chips (lumps, crushed and powdered)
Result:
Smaller sizes of chips cause an increase in the surface area of the solid, so the rate of reaction will
increase
This is because more surface area of the particles will be exposed to the other reactant so there will be
more frequent and successful collisions, increasing the rate of reaction

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Effect of concentration of a solution on the rate of reaction:

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Diagram showing the apparatus needed to investigate the effect of concentration on the rate of
reaction
Method:
Measure 50 cm3 of sodium thiosulfate solution into a flask
Measure 5 cm3 of dilute hydrochloric acid into a measuring cylinder
Draw a cross on a piece of paper and put it underneath the flask
Add the acid into the flask and immediately start the stopwatch
Look down at the cross from above and stop the stopwatch when the cross can no longer be seen
Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium
thiosulfate solution with water to dilute it)
Result:
With an increase in the concentration of a solution, the rate of reaction will increase
This is because there will be more reactant particles in a given volume, allowing more frequent and
successful collisions, increasing the rate of reaction
Effect of temperature on the rate of reaction:

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Your notes

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Diagram showing the apparatus needed to investigate the effect of temperature on the rate of reaction
Method:
Dilute hydrochloric acid is heated to a set temperature using a water bath
Add the dilute hydrochloric acid into a conical flask
Add a strip of magnesium and start the stopwatch
Stop the time when the magnesium fully reacts and disappears
Repeat at different temperatures and compare results
Result:
With an increase in the temperature, the rate of reaction will increase
This is because the particles will have more kinetic energy than the required activation energy,
therefore more frequent and successful collisions will occur, increasing the rate of reaction
Effect of a catalyst on the rate of reaction:

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Your notes

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Diagram showing the apparatus needed to investigate the effect of a catalyst on the rate of reaction
Method:
Add hydrogen peroxide into a conical flask
Use a delivery tube to connect this flask to a measuring cylinder upside down in a tub of water
(downwards displacement)
Add the catalyst manganese(IV) oxide into the conical flask and quickly place the bung into the flask
Measure the volume of gas produced in a fixed time using the measuring cylinder
Repeat experiment without the catalyst of manganese(IV) oxide and compare results
Result:
Using a catalyst will increase the rate of reaction
The catalyst will provide an alternative pathway requiring lower activation energy so more colliding
particles will have the necessary activation energy to react
This will allow more frequent and successful collisions, increasing the rate of reaction
Monitoring changes in mass
Many reactions involve the production of a gas which will be released during the reaction
The gas can be collected and the volume of gas monitored as per some methods above
Alternatively, the reaction can be performed in an open flask on a balance to measure the loss in mass
of reactant
Cotton wool is usually placed in the mouth of the flask which allows gas out but prevents any materials
from being ejected from the flask (if the reaction is vigorous)

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Diagram showing the set-up for measuring the rate of reaction by loss in mass
This method is not suitable for hydrogen and other gases with a small relative formula mass, Mr as the
loss in mass may be too small to measure

Exam Tip
There are many different methods of investigating the rate of reaction. Another method of gas
collection you may see uses a gas syringe.
Those students taking the extended course may be required to devise and evaluate methods of
investigating rates of reaction.

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Evaluating Investigations of Rates of Reactions

EXTENDED Your notes
When investigating rates of reaction, there are a number of different methods that can be used to carry
out the same investigation
Evaluating what is the best method to use is part of good experimental planning and design
This means appreciating some of the advantages and disadvantages of the methods available
Table showing some Examples of Advantages and Disadvantages of Methods of Investigating Rates of
Reaction

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6.1.6 Interpreting Data

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Interpreting Data
Data recorded in rate studies is used to plot graphs to calculate the rate of a reaction
Plotting a graph until the completion of the reaction shows how the rate changes with time
Over time the rate of reaction slows as the reactants are being used up so the line becomes less steep
and eventually becomes horizontal, indicating the reaction has finished
You can plot more than one run of a variable on the same graph making it easier to see how the variable
influences the rate
For example, plotting the effect of concentration on a reaction between the acid and marble chips

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The steeper the curve, the faster the rate of the reaction
The curve is steepest initially so the rate is quickest at the beginning of the reaction
As the reaction progresses, the concentration of the reactants decreases and the rate decreases Your notes
shown by the curve becoming less steep
When one of the reactants is used up, the reaction stops, the rate becomes zero and the curve levels
off to a horizontal line
The amount of product formed in a reaction is determined by the limiting reactant:
If the amount of limiting reactant increases, the amount of product formed increases
If the amount of the reactant in excess increases, the amount of product remains the same
Drawing a tangent to the slope allows you to show the gradient at any point on the curve
The steeper the slope, the quicker the rate of reaction
The volume of a gaseous product would increase to a maximum over time, so the line levels out
indicating the reaction is over
Since the volume and mass would be proportional, this could also be a graph of the mass of product
versus time

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Worked example
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0.2 g of manganese(IV) oxide was added to 25 cm3 of 0.1 mol/dm3 hydrogen peroxide solution. The
volume of oxygen produced every minute was recorded and the results are shown on the graph.

The experiment was repeated using the same mass of manganese(IV) oxide and at the same
temperature but using 25 cm3 of 0.2 mol/dm3 of hydrogen peroxide solution.
Sketch the curve for the results of this experiment on the same grid.

Answer
Step 1 - Deduce how the initial gradient will be different from the original graph

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The hydrogen peroxide solution is twice as concentrated so the rate of reaction will be greater and
the initial gradient will be steeper
Your notes
Step 2 - Deduce how much product will be formed compared to the original experiment
The amount of hydrogen peroxide determines the amount of oxygen produced. In the 2nd
experiment, there are twice as many hydrogen peroxide molecules in the same volume so the
amount of oxygen gas produced will be doubled
Step 3 - Sketch the graph

Calculating the Rate of Reaction at a Particular Point

To do this you need to find the gradient of the curve at that point
To do this a tangent is drawn to the curve and then the gradient of the tangent calculated

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Worked example
Your notes
Iodine and methanoic acid react in aqueous solution.
I2 (aq) + HCOOH (aq) → 2I− (aq) + 2H+ (aq) + CO2 (g)
The rate of reaction can be found by measuring the volume of carbon dioxide produced per unit time
and plotting a graph as shown:

Calculate the rate of reaction at 20 seconds

Answer:

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Draw a tangent to the curve at 20 seconds:

Your notes

Complete the triangle and read off the values of x and y

Determine the gradient of the line using change in y / change in x
Rate of reaction = 24 ÷ 40 = 0.60 cm3/s

Exam Tip
If the amount of reactant used up is being monitored, then the graph will fall with the steepest gradient
at the start, becoming less steep until it levels off to a horizontal line.

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