Topic 6:
Chemical Kinetics
6.1 Collision Theory and Rates of Reaction
● What makes particles react?
● How can this be used to explain how we change
the rate of a reaction?
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What does rate of reaction mean?
The speed of different chemical reactions varies hugely.
Some reactions are very fast and others are very slow.
The speed of a reaction is called the rate of the reaction.
What is the rate of these reactions?
rusting
Very slow
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baking
Slow
------
explosion
Very fast
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Why study rates? What is the real-world context?
Time is money in industry, the faster the reaction can be done, the more economic it is.
Which reactions do you know?
Health and Safety Issues:
The faster a reaction, the more likely there might be health and safety issues to contend
with.
The physical state or concentration of a material can be an important factor.
Mixtures of flammable gases in air present an explosion hazard (gas reactions like this are
amongst the fastest reactions known).
If the flammable/explosive gas is in low concentration, there may be no risk, but you need
to know the safe limits!
e.g. Methane gas in mines, petrol vapour etc. are all potentially dangerous situations
so knowledge of 'explosion/ignition threshold concentrations', ignition temperatures
and activation energies are all important knowledge to help design systems of 4
operation to minimise risks.
Collision Theory
Reactions take place when particles collide with a
certain amount of energy.
The minimum amount of energy needed for the particles to
react is called the activation energy, and is different for each
reaction.
The rate of a reaction depends on two things:
● the frequency of collisions between particles
● the energy with which particles collide.
If particles collide with less energy than the activation energy,
they will not react. The particles will just bounce off each
other.
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In this video learn about Collision Theory and find out
what is necessary for reactions to take place.
https://www.youtube.com/watch?v=eSInI1xHvh4
Am booklet page 76
Study very well !!!!
For a reaction to take place; successful collisions must occur
1- Particles must collide
2- with sufficient energy
3- corect orientation
It is the minimum amount of energy needed to start a reaction
MCQ Am booklet
Page 78
Reactants could be of different states and they can react!
Page 83
Based on the definition of rate:
Concentration unit is mol.dm-3
Time unit is seconds (s)
Unit of rate=
mol.dm-3
s
Defining the Rate of Reaction
The rate of reaction is expressed as the change in concentration of a particular
reactant/product per unit time.
Amount of products
increasing with time
Being produced.
◎ Where:
○ ∆[R] is change in concentration of reactants
○ ∆[P] is change in concentration of products
○ ∆t is change in time
Amount of reactants are decreasing
With time, getting used up
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◎ Official Units: mol dm-3 s-1
(‘moles per cubic decimetre per second’)
Slower and slower!
Reactions do not proceed at a steady rate. They start off at
a certain speed, then get slower and slower until they stop.
As the reaction progresses, the concentration of reactants
decreases.
This reduces the frequency of collisions between particles
and so the reaction slows down.
0%
initially
High
concentration of
reactants.
no products
formed yet
10 of
25%
50%
75%
As the reaction progresses, the amount of reactants decreases and amount of
products increases
reactants
product
100%
At the end, all reactants are
Used up and maximum number of
products are formed
percentage completion of reaction
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Graphing rates of reaction
With time the rate of reaction
decreases with time,
(Reaction slows) until it stopes.
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Reactant–product mix
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The reactant/product mix
E
A
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D
B
C
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Am booklet page 93
Reaction stops when the the curve
Becomes a horizontal (flat) line
Maximum yield; maximum amount of products
formed
Graph line starts to flatten
Slope gets less steep
thus rate deceases
Initillay the rate is high
Steep slope
Rate of reaction decreases with time, since the concentration of the reactants decreases;
Thus less frequent collisions
Refresher
Am booklet pages 102-103
Change in the concentration of reactants or products with respect to time.
Or write the equation.
1- particles must collides
2- with the sufficient energy
3- correct orientation
Refresher
Am booklet page 104
The minimum amount of energy needed to start a reaction.
Instantaneous and Initial Rate of a reaction
How to draw tangents to a
curve
◎ Determining the rate at a particular point in time (instantaneous
rate and initial rate) requires us to draw tangents to the graph line
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You need to know how to get the rate of the
reaction using the graph:
When drawing the tangent to the
curve at a specific point
Be careful to hold the
ruler in the correct
side
Touching the curve at
only the specific point
Am booklet page 106. Write and solve in the space provided
Calculating rate - at various points in a reaction
1.
Calculate the average rate for the reaction from start to end. Where is the end?
The reaction ends here.
(120, 58)
Average rate =
58 - 0
= 0.483 cm3/s
120-0
Is the average rate for the reaction
The reaction starts here
(0,0)
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The slope of the tangent to the line joining
the starting point and the end point
Am booklet page 106. Write and solve in the space provided
Calculating rate - at various points in a reaction
2. Calculate the average rate for the first 50 seconds of each reaction.
Average rate=
53 - 0
(50, 53)
= 1.06 cm3/s
50 - 0
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Slope of the line
Is the average rate
0,0
Am booklet page 106. Write and solve in the space provided
Calculating rate - at various points in a reaction
3. Calculate the average rate between 10 - 90 seconds in each reaction.
(90, 57)
Average rate
= 57 - 18
90 - 10
= 0.4875 cm3/s
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Slope of the line
Is the average rate
(10, 18)
Am booklet page 106. Write and solve in the space provided
Calculating rate - at various points in a reaction
4. Calculate the instantaneous rate at 25 seconds
Instantaneous rate is the rate
of the reaction at a specific
instance in time
Draw a tangent to the graph at
the point.
(Make sure the line touches the
cur ve at only the specific point
asked for.
(25, 38)
get the slope of the tangent.
(It is recommended to extend the
line until the y-axis), choose t wo
points on the tangent and get
the slope.
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the slope of the tangent is the
rate
0, 10
Slope = 38 - 10 = 1.12 cm3/s
25 - 0
Am booklet page 106. Write and solve in the space provided
Calculating rate - at various points in a reaction
5. Calculate the initial rate of reaction (i.e. t = 0)
Initial rate is the instantaneous
rate at time t= 0
(Start)
Draw a tangent to the cur ve and
t=0
Calculate the slope of the
tangent
37-0
= 17 cm3/s
20-0
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(20, 37)
(0,0)
What happens to rate as the reaction proceeds?
◎
Look at the calculated rates for each stage of the previous
reaction. What do you see? The highest rate was for the initial rate
◎
Why do you think this happens?
Rate decreases, since the curve gets flattened (less steep)
Anything? Wonder
The rate decreases and slope of the tangents to the cur ve of increased
Times decreases; since the concentration of the reactants are
decreasing,
less frequent collosions, thus reaction slows down with time until it
stops when reactants are used up.
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◎
Analysis of graphical and numerical data from rate experiments.
Average rate bet ween time t=0 and t= 20
Get the slope
For this line
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Calculation of reaction rates from tangents of graphs of
concentration, volume or mass vs time should be covered.
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Make sure you know how to get these from the graph!!!!!
Review
What is the best definition of rate of reaction?
The time it takes to use up all the reactants
The rate at which all the reactants are used up
The time it takes for one of the reactants to be used up
The increase in concentration of a product per unit time
Increase or decrease is another word for a change in an amount
The rate must be defined with respect to time!!!!
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A.
B.
C.
D.
Am booklet page 74
HW
decrease
is the change in the concentration (or amount) of reactants or products per unit time.
(Or write the mathematical equation)
Rate of reaction decreases with time since the concentration of reactants is decreasing, less frequent collisions.
initially, the rate is highest (fastest/ steep line) with a high concentration of reactants. With time the reactants are getting used up, thus
slower reaction, the line gets less steep, until it flattens.
flat line indicates rate is zero , where the reaction stops, reactants are used up.
Changing the rate of reactions
Anything that increases the number of successful collisions
between reactant particles will speed up a reaction.
What factors affect the rate of reactions?
● increased temperature
● increased concentration of
dissolved reactants, and increased
pressure of gaseous reactants
● increased surface area of solid
reactants
● use of a catalyst.
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MUST BE STUDIED!! V. IMP
For aqueous solutions
Particle size
Collisions and reactions: summary
Complete the sentences about the factors affecting rate of reactions:
1. Changing the
collisions.
Temperature
affects both the frequency and energy of
Concentration
2. Changing the
affects the frequency of collisions of
dissolved reactants. (aqueous solutions)
3. Changing the
solid
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Particle size (surface area)
affects the frequency of collisions involving a
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Effect of temperature on rate
The higher the temperature, the faster the rate of a reaction.
In many reactions, a rise in temperature of 10 °C causes the
rate of reaction to approximately double.
Why does increased temperature
increase the rate of reaction?
At a higher temperature, particles
have more energy. This means
they move faster and are more
likely to collide with other particles.
When the particles collide, they
do so with more energy, and so
the number of successful
collisions increases.
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Temperature and particle collisions
Higher temperature will have more frequent collisions
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Temperature and batteries
Why are batteries more likely to rundown more quickly in
cold weather?
At low temperatures the
reaction that generates the
electric current proceeds
more slowly than at higher
temperatures.
This means batteries are
less likely to deliver enough
current to meet demand.
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Effect of concentration on rate of reaction
The higher the concentration of a dissolved reactant, the
faster the rate of a reaction.
Why does increased concentration increase the rate of
reaction?
At a higher concentration, there are more particles in the
same amount of space. This means that the particles are
more likely to collide and therefore more likely to react.
Shorter distance
bet ween the
particles
;
Less time to collide
lower concentration
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higher concentration
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Concentration and particle collisions
more particles in a specific volume.
Particles travel less distance to collide. More frequent
collisions; thus higher rate
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The Maxwell-Boltzmann Energy Distribution
Describes how energy is distributed in a collection of particles:
You need to know how to:
1- Label the y and x axis.
2- label activation energy
3- draw the curve
4- label t wo areas under the
curve
green and purple)
Number of particles
That do not have
enough energy to collide
successfully. (these particles
can not react)
The curve must not touch the
x-axis (this means there are
no particles)
Number to particles with enough energy
to collide successfully and react. They
have energy that is equal to or greater
than the activation energy.
Effect of Changing the temperature on Maxwell Boltzmann
Watch the video on the powerpoint.
Page 77
Area under the curve
Shows the number of with insufficient energy
to collide successfully and react
Area under the curve
Shows the number of particles with
sufficient energy that is greater or equal
to activation energy, and are able to
collide successfully and react.
E
Ea
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Increase in the temperature results in:
1. increase in the average kinetic energy of the particles
2. more particles have sufficient energy to react
3. more frequent collisions
Area under the curve increases;
More particles have sufficient energy (Ea> E)
to react
T
T1 > T
Ea
Activation energy:
This is the minimum amount
of energy needed to start the
reaction
Energy
Page 97
For higher temperature:
Make sure that the peak is switched
to the right, and flattens a bit.
for colder temperature
make sure that the peak is tending
towards the y-axis and the peak is
sharp
Increase in the temperature results in:
1. increase in the average kinetic energy of the particles
2. more particles have sufficient energy to react (shaded area under the
cur ve gets bigger)
3. more frequent collisions
Repeated practice
Make sure to draw in pencil and label
the axis; activation energy, and the
t wo curves
Page 101
Effect of pressure on rate of reaction
For reactants that are gasses ONLY
Why does increasing the pressure of gaseous reactants
increase the rate of reaction?
As the pressure increases, the space in which the gas
particles are moving becomes smaller.
The gas particles become closer together, increasing the
frequency of collisions. This means that the particles are more
Note: Pressure is inversely proportional to volume;
likely to react.
Decreasing the Volume means increasing the pressure
lower pressure
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higher pressure
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Effect of surface area on rate of reaction
OR particle size OR state of division for Solid reactants ONLY
Any reaction involving a solid can only take place at the
surface of the solid.
If the solid is split into several pieces, the surface area
increases. What effect will this have on rate of reaction?
low surface area
Larger particle size
high surface area
Smaller particle size
This means that there is an increased area for the
reactant particles to collide with.
The smaller the pieces, the larger the surface area. This
means more collisions and a greater chance of reaction.
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Surface area and particle collisions
Smaller surface area means less
particles are exposed.
Only the outer particles will react.
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Greater surface area means more particles are
exposed to the other reactants
Thus more frequent collisions.
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Pages 74-75
State:
Increasing the temperature; increases the rate of the reaction, faster reaction.
Explain:
increasing the temperature; increases the average KE of the particles; more particles will have enough energy to
react; thus more frequent collisions
State: Increasing the concentration of the aqueous solution; increases the rate of the reaction.
Explain: more particles in a specific volume; less space bet ween them, thus more frequent collisions
State: Increasing the particle size; means decreasing the surface area; thus the rate of the reaction decreases.
Explain: decreasing the surface area, less particles are exposed the the other reactant, thus less frequent collisions
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Changes that could increase the rate:
1- for SOLID Mg: increase the surface area; by using smaller pieces (or particle size)
2- for AQUEOUS Solution Hal: use a higher concentration
3- increase the temperature
Changes that could decrease the rate:
1- for SOLID Mg: decrease the surface area; by using larger pieces (or particle size)
2- for AQUEOUS Solution Hal: use a lower concentration
3- decrease the temperature
What are catalysts?
Catalysts are substances that change the rate of a reaction
without being used up in the reaction.
Catalysts never produce more product – they just
produce the same amount more quickly.
energy (kJ)
Ea without
catalyst
Different catalysts work in
different ways, but most
lower the reaction’s
activation energy (Ea).
Ea with
catalyst
reaction (time)
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Effect of a catalyst on Maxwel Botlzmann
Glossary
Catalysts: true or false?
Catalysts in industry
Why are catalysts so important for industry?
● Products can be made more
quickly, saving time and money.
● Catalysts reduce the need for
high temperatures, saving fuel
and reducing pollution.
Catalysts are also essential for living cells. Biological
catalysts are special types of protein called enzymes.
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The minimum amount of energy needed to start a reaction.
Enthalpy level diagram
Exothermic
Endothermic
Reactants
products
Heat
energy
Heat
energy
(KJ)
(KJ)
Products
reactants
Time
time
Pages 96-97
Reactants
Products
Exothermic
Page 103
A catalyst increases the rate of a reaction by :
1- lowering the activation energy
2- providing an alternative pathway
3- providing the correct orientation
4- are not involved in the reaction.
Page 105
The number
of particles
More particles having
enough energy to react
Ea with
A catalyst
Energy
Ea without a
catalyst
Page 107
Number of particles
These particles
Have unsuccessful
collisions
Ea with
Catalyst
Area increases
Ea without a catalyst
When activation energy is lower; area under the cur ve increases
Thus; MORE particles now have enough sufficient energy to react.
Energy
Measuring Reaction Rates
Concentration changes in a reaction can be followed indirectly by monitoring
changes in mass, volume and colour.
◎ Why is rate difficult to measure directly?
◎ Measured INDIRECTLY. We need to monitor something changing over time
such as:
Technique
Apparatus/Notes
Mass loss
Using a gas syringe or inverted measuring cylinder filled with water
When a gas
is produced
Reaction conducted on a balance…if it produces a gas the mass will decrease
Colour change
Measured using a colorimeter
Only when we know if a reactant or product is colored
pH
Monitored using a pH probe
Only when we know if there is an acid or a base
Temperature change
Not ideal as difficult to prevent heat loss (think calorimetry)
We can’t use change in
temperature
Because some reactions and
endothermic and others are
exothermic
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Collecting gas
Common techniques for measurements
What equipment is needed to investigate the rate of hydrogen production?
Stop watch
Or
timer
Stop watch
The cotton allows the gas to escape
But not any other matter to enter
or leave
Record the increase in the volume of the gas with time
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Record the decrease in the mass of the contents with
time
Setting up rate experiments
What equipment is needed to investigate the rate of
hydrogen production?
glass tube
conical
flask
rubber connecter
gas syringe
rubber bung
hydrochloric
acid
magnesium
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How can rate of reaction be measured?
Measuring the rate of a reaction means measuring the
change in the amount of a reactant or the amount of a
product.
What can be measured to calculate the rate of reaction
between magnesium and hydrochloric acid?
magnesium
+
hydrochloric ¨ magnesium
acid
chloride
+ hydrogen
● The amount of hydrochloric acid used up (cm3/min).
● The amount of magnesium chloride produced (g/min).
● The amount of hydrogen product (cm3/min).
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Steeper graph
Faster reaction
Both will end with the same
Amount of products
How does temperature affect rate?
The reaction between sodium thiosulfate and hydrochloric
acid produces sulfur.
sodium
thiosulfate
Na2S2O3
(aq)
sodium
+
hydrochloric
acid
¨ chloride
+
2HCl
(aq)
2NaCl
¨
(aq)
+
sulfur
dioxide
+
SO2
(g)
+
sulfur
+
S
(s)
+ water
+
H2O
(l)
Sulfur is solid and so it turns the solution cloudy.
How can this fact be used to measure the effect of
temperature on rate of reaction?
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How does temperature affect rate?
The reaction between sodium thiosulfate and hydrochloric acid
produces sulfur.
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The effect of temperature on rate
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Reaction between a carbonate and acid
Marble chips are made of calcium carbonate. They react
with hydrochloric acid to produce carbon dioxide.
calcium
carbonate
CaCO3
(aq)
+
+
hydrochloric
¨
acid
2HCl
(aq)
¨
calcium
chloride
CaCl2
(aq)
+
water
+
H2O
(aq)
+
carbon
dioxide
+
CO2
(g)
The effect of increasing surface area on the rate of reaction
can be measured by comparing how quickly the mass of the
reactants decreases using marble chips of different sizes.
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The effect of surface area on rate
Faster reaction
Slower reaction
Interpreting Rate Graphs
What does it mean if it is steeper?
A bigger change is occurring per unit time therefore
rate is increasing.
Fastest reaction will have a steeper
cur ve; tending more towards the y-axis
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Lowest concentration will have a lower rate (slowest) ; least steep;
Highest pressure for gas; fastest rate
Rates of reaction: summary
Higher
Higher
faster
Higher
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Refer back to Maxwell Boltzman
distribution
True
True
True
The only factor that affects the activation energy is the catalyst
Flase
True