Chapter 36
Rate of chemical reaction
36.1 Introducing rate of reaction
36.2 Describing rate of reaction — concentrationtime graph, average rate and instantaneous
rate
36.3 Methods of following the progress of a
chemical reaction
Key terms
Progress check
Summary
Concept map
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36.1 Introducing rate of reaction
Different chemical reactions proceed with different
rates
(a)
(b)
(c)
Figure 36.1 (a) Reaction between hydrogen and oxygen in the ‘pop’ sound
test is extremely fast. (b) Reaction between magnesium and dilute sulphuric
acid occurs at a moderate rate. (c) Rusting of iron is a slow process.
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Key point
Chemical reactions proceed with a wide range of
rates.
Class practice 36.1
36.1 Introducing rate of reaction
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Why do we study rate of chemical reactions?
It helps to determine the best conditions for
producing the largest amount of products in a
given time.
Faster chemical reactions will lead to higher
productivity.
Activity 36.1
36.1 Introducing rate of reaction
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Rate of reaction
The rate of reaction tells how quickly a chemical
reaction occurs.
It can be defined as
↑ concentration of a product per unit time, or
↓ concentration of a reactant per unit time
Key point
Rate of reaction
change
in
concentration
of
a
reactant
or
a
product
_________________________________________________________________
=
time
36.1 Introducing rate of reaction
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Rate of reaction =
∆[P] ……
________
(1)
∆t
∆[R] ……
________
Rate of reaction = −
(2)
∆t
[P] and [R] represent the concentrations of a product and
a reactant (usually in mol dm–3) respectively
∆ represents the change (final state – initial state)
t represents the time taken for the change
Learning tip
The negative sign in expression (2) indicates that the
concentration of the reactant is decreasing with time.
But, by convention, rate is expressed as a positive value.
Example 36.1
Class practice 36.2
36.1 Introducing rate of reaction
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Concentration of product
Concentration of reactant
36.2 Describing rate of reaction —
concentration-time graph, average rate
and instantaneous rate
Concentration-time graph
Time
Time
(a)
(b)
Figure 36.3 A plot of concentration of (a) reactant or (b) product against time.
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Key point
Concentration-time graph is a plot of the
concentration of a reactant (or a product) against
time. It shows the progress of a chemical reaction.
36.2 Describing rate of reaction — concentration-time graph, average rate
and instantaneous rate
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Interpreting the concentration-time graph
Concentration of product
final concentration
of product
end of reaction
F
∆[P]
∆t
total time of
reaction
Time
Figure 36.4 A typical graph for the change in concentration of product
with time.
36.2 Describing rate of reaction — concentration-time graph, average rate
and instantaneous rate
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1. The curve starts from the origin when it is a plot
of the concentration of product against time.
∵ no products are present at time zero
2. The curve is steep at first but becomes less
steep with time.
3. The curve becomes horizontal finally. This shows
that the concentration of product remains constant.
The reaction stops and no more products form.
The point F indicates the end of the reaction.
4. The curve shows the final concentration of product
and the total time of the reaction.
36.2 Describing rate of reaction — concentration-time graph, average rate
and instantaneous rate
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Average rate of reaction
The average rate of a reaction is the change in
concentration of a reactant, –∆[R] (or a product,
∆[P]) over a certain interval of time, ∆t.
Average rate of reaction = −
∆[R]
_______
∆t
or
∆[P]
_______
∆t
Example 36.2
36.2 Describing rate of reaction — concentration-time graph, average rate
and instantaneous rate
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Instantaneous rate of reaction
(a)
(b)
Figure 36.5 A Shanghai Maglev train. (a) The instantaneous speed when the
train is at rest and (b) the instantaneous speed when the train runs fastest.
36.2 Describing rate of reaction — concentration-time graph, average rate
and instantaneous rate
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Key point
The rate at a particular instant of the reaction is
called the instantaneous rate.
In a concentration-time graph, the instantaneous
rate can be determined by finding the gradient or
slope of the tangent to the curve at that instant.
36.2 Describing rate of reaction — concentration-time graph, average rate
and instantaneous rate
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Concentration of product
0
t1
t2
t3
Time
Figure 36.6 A concentration-time graph with tangents drawn at different points.
36.2 Describing rate of reaction — concentration-time graph, average rate
and instantaneous rate
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• The tangent drawn at time zero is the steepest
and its slope is the largest. This shows that the
rate of reaction is the highest at the beginning.
This is called the initial rate (i.e. the
instantaneous rate at time t = 0) of reaction.
• As the reaction proceeds, the slope of the
tangents decreases (from t1 to t3) and finally
becomes zero. This indicates that the reaction
becomes slower and slower and finally stops.
Example 36.3
Class practice 36.3
Example 36.4
Class practice 36.4
36.2 Describing rate of reaction — concentration-time graph, average rate
and instantaneous rate
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36.3 Methods of following the progress of a
chemical reaction
1. Methods that measure different kinds of physical
properties.
For example, methods that measure
(a) the change in volume of a gaseous product,
(b) the change in pressure of a reaction system,
(c) the change in mass of a reaction mixture,
(d) the change in colour intensity of a reaction
mixture
2. Method based on titration, known as titrimetric
analysis.
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Following the progress of a chemical reaction by
measuring the change in volume of a gaseous product
This method is used for the reactions involving the
production of a gas.
Example
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
The rate of the reaction can be investigated by
measuring the volume of hydrogen gas produced
at regular time intervals.
36.3 Methods of following the progress of a chemical reaction
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after a certain time
at start
plunger
cotton
thread
magnesium
ribbon
gas syringe
gas collected
dilute HCl(aq)
(usually an excess
is used)
magnesium
ribbon
Figure 36.7 Investigating reaction rate by measuring volume of hydrogen gas
produced from the reaction between magnesium and dilute hydrochloric acid
at regular time intervals.
Learning tip
The hydrogen gas produced pushes
the plunger out of the barrel.
36.3 Methods of following the progress of a chemical reaction
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Steps for following the progress of the reaction
1. Drop the magnesium ribbon into dilute HCl(aq).
2. Start the stopwatch.
3. Record the volume of gas in the syringe at regular
time intervals until the end of the reaction.
SBA note
To ensure that the plunger is not stuck in the barrel, it
is necessary to rotate it before taking readings. It also
ensures that the pressures inside and outside the gas
syringe are the same.
Example 36.5
Class practice 36.5
Experiment 36.1
36.3 Methods of following the progress of a chemical reaction
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Experiment 36.1
Following the progress of a chemical reaction by
measuring the change in pressure of a reaction system
This method is used for the reactions involving a
change in the number of moles of gas.
The reaction system has to be a closed system
which has a constant volume.
36.3 Methods of following the progress of a chemical reaction
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Example
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Since hydrogen is the only gas among the
reactants and products, the number of moles of
gas in the reaction increases.
If the reaction occurs in a closed container, the
pressure inside the container will increase.
The more the gas produced, the higher is the
pressure measured.
The pressure inside the container can be
measured with a pressure sensor connected to a
data-logger that interfaces with a computer.
36.3 Methods of following the progress of a chemical reaction
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computer
pressure
sensor
data-logger
interface
suction
flask
dilute
HCl(aq)
magnesium
ribbon
Figure 36.8 Investigating reaction rate by measuring the pressure inside the
reaction container when magnesium reacts with dilute hydrochloric acid at
regular time intervals.
SBA note
The reaction flask must be air-tight during the
experiment. This enables a more accurate
measurement of the pressure.
36.3 Methods of following the progress of a chemical reaction
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Steps for following the progress of the reaction
1. Run the data-logging software on the computer.
2. Tilt the bottle containing dilute HCl(aq) so that the
acid reacts with the magnesium ribbon.
3. Immediately start recording the pressure inside the
suction flask until the magnesium has completely
reacted.
Example 36.6
Class practice 36.6
36.3 Methods of following the progress of a chemical reaction
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Experiment 36.2
Following the progress of a chemical reaction by
measuring the change in mass of a reaction mixture
This method is used for the reactions involving a
change in mass of the reaction mixture.
If a gas is given off from a reaction and is allowed
to escape from the reaction mixture, the mass of
the reaction mixture decreases.
36.3 Methods of following the progress of a chemical reaction
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Example
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
The rate of the reaction can be investigated by
measuring the decrease in mass of the reaction
mixture at regular time intervals.
SBA note
The decrease in mass of the flask is not very fast
initially. This is because the reaction mixture has to
be saturated with the CO2 formed first.
36.3 Methods of following the progress of a chemical reaction
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after a certain time
start of reaction
cotton wool
plug
limestone pieces
of known mass
measured volume
of standard HCl(aq)
limestone pieces
of known mass
Figure 36.9 Investigating reaction rate by measuring the mass of reaction
mixture at regular time intervals.
Think about
36.3 Methods of following the progress of a chemical reaction
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Steps for following the progress of the reaction
1. Put a conical flask that contains dilute HCl(aq) on
an electronic balance.
2. Start the reaction by pouring the limestone pieces
into dilute HCl(aq).
3. Start the stopwatch. Replace the cotton wool
immediately.
4. Record the mass of the flask at regular time
intervals until the end of the reaction.
36.3 Methods of following the progress of a chemical reaction
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In the experiment, the carbon dioxide gas
produced escapes from the flask through the
cotton wool plug.
∴ ↓ total mass of the flask and its content
= the mass of carbon dioxide produced
Example 36.7
Experiment 36.3
Class practice 36.7
Experiment 36.3
36.3 Methods of following the progress of a chemical reaction
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Following the progress of a chemical reaction by
measuring the change in colour intensity of a
reaction mixture
This method is used for the reactions involving
one of the reactants or products is coloured.
The colour intensity of the mixture will gradually
increase if the product is coloured.
The reaction mixture will become paler and paler if
the reactant is coloured.
The colour intensity of a solution depends on the
concentration of the coloured species.
36.3 Methods of following the progress of a chemical reaction
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Example
2MnO4–(aq) + 5C2O42–(aq) + 16H+(aq) → 2Mn2+(aq) + 8H2O(l) + 10CO2(g)
purple
colourless
very pale pink colourless
As the reaction proceeds, the concentration of the
purple MnO4–(aq) ions decreases.
The solution becomes paler and paler.
Figure 36.10 The colour intensity of the reaction mixture, MnO4–(aq)
and C2O42–(aq), decreases with time.
36.3 Methods of following the progress of a chemical reaction
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A colorimeter can be used to measure the change
in colour intensity.
It can be used alone or connected to a data-logger
that interfaces with a computer.
Figure 36.11 A colorimeter with
some cuvettes for holding the
solutions to be analysed.
36.3 Methods of following the progress of a chemical reaction
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Steps for following the progress of the reaction
1. Mix acidified potassium permanganate solution
with sodium oxalate solution in a test tube (or a
cuvette).
2. Put the test tube into the colorimeter immediately.
3. Record the fraction of light absorbed (called the
absorbance) by the reaction mixture at regular time
intervals. The higher the concentration of the
coloured species, the higher is the absorbance
recorded.
Example 36.8
Experiment 36.4
Experiment 36.5
Experiment 36.4
Experiment 36.5
36.3 Methods of following the progress of a chemical reaction
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Following the progress of a chemical reaction by
titrimetric analysis
For some reactions, titration is used to determine
the concentrations of either a reactant or a product
at regular time intervals.
36.3 Methods of following the progress of a chemical reaction
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Titrimetric analysis involves withdrawing and
analysing small portions of the reaction mixture
at regular time intervals.
The reaction in these small portions has to be
slowed down or stopped (i.e. quenched) before
doing the analysis.
This prevents further changes in the
concentration of the reactant or product to be
analysed.
Learning tip
Using titrimetric analysis, the original reaction mixture
is disturbed when the rate of reaction is measured.
36.3 Methods of following the progress of a chemical reaction
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Quenching can be done by the following methods:
1. Cooling the reaction mixture rapidly in ice.
2. Diluting the reaction mixture with a sufficient
amount of cold water or an appropriate solvent.
Think about
36.3 Methods of following the progress of a chemical reaction
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Titrimetric analysis for the alkaline hydrolysis of
ethyl ethanoate
CH3COOCH2CH3(l) + NaOH(aq) → CH3COONa(aq) +
CH3CH2OH(aq)
Steps
1. Mix ethyl ethanoate and dilute sodium hydroxide
measuring cylinder
solution.
ethyl ethanoate
dilute NaOH(aq)
magnetic stirring bar
(1)
magnetic stirrer-hotplate
Figure 36.12 Investigating reaction rate of alkaline hydrolysis of
ethyl ethanoate by titrimetric analysis.
36.3 Methods of following the progress of a chemical reaction
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2. At regular time intervals, withdraw small portions
of the reaction mixture using a pipette.
3. Quench the reaction in the small portions by
pouring them into ice-cold distilled water.
a small portion (e.g.
10.0 cm3) of reaction
mixture withdrawn at
2-minute intervals
ice
distilled water
(2)
(3)
36.3 Methods of following the progress of a chemical reaction
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4. Titrate the quenched portions against standard
HCl(aq), using phenolphthalein as indicator.
Example 36.9
standard HCl(aq)
Class practice 36.8
reaction mixture with
phenolphthalein indicator
(4)
Experiment 36.6
Experiment 36.6
white tile
From the titration results, the concentrations of
sodium hydroxide solution at regular time intervals
can be determined.
36.3 Methods of following the progress of a chemical reaction
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Property
followed
Example of reaction
Method to
follow/study the
progress
Reaction is
‘on-going’ or
‘quenched’?
Mg(s) + 2HCl(aq)
→ MgCl2(aq) + H2(g)
Measuring change in
volume of H2(g) with
gas syringe
On-going
Gas pressure
Mg(s) + 2HCl(aq)
→ MgCl2(aq) + H2(g)
Measuring change in
pressure of reaction
system with pressure
sensor
On-going
Mass
CaCO3(s) + 2HCl(aq)
→ CaCl2(aq) + CO2(g) +
H2O(l)
Measuring change in
mass of reaction
mixture with electronic
balance
On-going
Gas volume
Table 36.1 Different methods of following/studying the progress of a reaction.
36.3 Methods of following the progress of a chemical reaction
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Property
followed
Method to
follow/study the
progress
Example of reaction
Reaction is
‘on-going’ or
‘quenched’?
Colour
intensity
2MnO4–(aq) + 5C2O42–(aq)
+ 16H+(aq)
→ 2Mn2+(aq) + 8H2O(l) +
10CO2(g)
Measuring change in
colour intensity of
reaction mixture with
colorimeter
On-going
Concentration
CH3COOCH2CH3(l) +
NaOH(aq)
→ CH3COONa(aq) +
CH3CH2OH(aq)
Measuring change in
concentration of
NaOH(aq) by
titrimetric analysis
Quenched
Table 36.1 Different methods of following/studying the progress of a reaction.
Example 36.10
STSE connections 36.1
Class practice 36.9
STSE connections 36.2
Activity 36.2
36.3 Methods of following the progress of a chemical reaction
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Key terms
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
absorbance 吸光度
alkaline hydrolysis 加鹼水解
average rate 平均速率
colorimeter 比色計
colour intensity 顏色深度
concentration-time graph 濃度對時間的坐標圖
data-logger 數據記錄器
initial rate 初速
instantaneous rate 瞬間速率
pressure sensor 壓強傳感器
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11.
12.
13.
quenching 驟冷
rate of reaction 反應速率
titrimetric analysis 滴定分析
Key terms
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Progress check
1. How do we define ‘rate of reaction’?
2. What is a concentration-time graph? What
information can we get from the graph?
3. How is the average rate of a reaction expressed
mathematically?
4. What is the instantaneous rate of a reaction? How
can it be determined from a concentration-time
graph?
5. What is the initial rate of a reaction? How can it
be determined from a concentration-time graph?
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6. How do we follow the progress of a reaction by
measuring the following changes in the reaction?
(a) Volume of a gaseous product
(b) Pressure of a reaction system
(c) Mass of a reaction mixture
(d) Colour intensity of a reaction mixture
7. What is the meaning of ‘quenching a reaction
mixture’?
8. What common methods can be used to quench a
reaction?
9. How do we follow the progress of a reaction by
titrimetric analysis?
Progress check
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Summary
36.1 Introducing rate of reaction
1.
Chemical reactions proceed with a wide range
of rates.
2.
The rate of reaction can be defined as the
change in concentration of a reactant (or a
product) per unit time.
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36.2 Describing rate of reaction – concentration-time
graph, average rate and instantaneous rate
3.
4.
Concentration-time graph is a plot of the
concentration of a product (or a reactant)
against time. It shows the progress of a
chemical reaction.
Average rate of a reaction is the change in
concentration of a reactant (or a product) over
a certain interval of time.
Summary
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5.
Instantaneous rate of a reaction is the rate at
any particular instant during the reaction. It is
found by calculating the slope of the tangent to
the curve in the concentration-time graph at
that particular instant.
36.3 Methods of following the progress of a chemical
reaction
6.
For reactions involving the production of a gas,
the progress of the reaction can be followed by
measuring the change of volume of gas formed,
the pressure of the reaction system (a closed
system) or the decrease in mass of the reaction
mixture with time.
Summary
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7.
8.
For reactions involving a change in the
concentration of a coloured reactant or product,
the progress of reaction can be followed by
measuring its colour intensity with time using a
colorimeter.
The progress of a chemical reaction can be
followed by titrimetric analysis of small portions
of a quenched reaction mixture at regular time
intervals.
Summary
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Concept map
Instantaneous rate
_____________
= the rate at a
particular instant
Average rate = the
__________
change in concentration
of reactant or product over
a certain interval of time
Concentration-time
graph
______________________
is a graph plotting the
concentration of reactant
or product against time
The rate at which a
reactant/product
_______________
is used or produced
described as
RATE OF CHEMICAL REACTION
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RATE OF CHEMICAL REACTION
followed by
quenching
is needed
Titrimetric analysis
no quenching
is needed
Measuring the
Measuring the
Measuring the
colour
volume/ change in _____
mass
change in ________
change in ________
intensity of a reaction
pressure of gas
_________
of a reaction
________
formed with time
mixture with time
mixture with time
Concept map
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