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RATES OF REACTION
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Rates of Reaction
• Reactions can be very fast, like fireworks or
explosives, but they can also be very slow –
such as an apple turning Brown.
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Rates of Reaction
• Chemical reactions occur when particles of reactant
collide with enough energy to react.
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Speeding up Reactions
• Anything that increases the chance of effective
collision increases the rate (speed) of reaction.
Factors include:
• Increased surface Area
• Increased concentration
• Increased temperature
• Use of a catalyst
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Surface Area
• The reactions of solids can clearly only take place at the
surface of the solid.
• If we break a solid into smaller pieces we get more area and a
faster reaction.
Molecules collide with the
surface of the solid
Extra surface for molecules to
collide with.
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Surface Area
• If we grind up a solid to a powder we massively increase
the surface area.
• We therefore massively increase the rate of any reaction
Slow
Very fast
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Activity
• An indigestion tablet fizzes in water – but fizzes
much faster if it is crushed.
• Which glass has the crushed tablet?
A
B
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Concentration
• Reactions in solution involve dissolved paticles that must
collide before reaction is possible.
• The more crowded (concentrated) the solution, the faster
the reaction.
Collisions infrequent
Collisions frequent
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Activity
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Pressure
• Reactions involving gases are affected by the
pressure of the gases present.
• If we cover one end of a bicycle pump and push
in the plunger we increase the pressure.
Low
pressure
High pressure
• What we are doing is squeezing the gas
molecules closer together or making them
more concentrated.
• And so - pressure speeds up gas reactions
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Activity
• The Haber Process, in which nitrogen reacts
with hydrogen to form ammonia, is carried out at
200 atmospheres pressure.
• How and why will this affect the rate of reaction?
• The particles will be 200 times
closer together and so will
collide much more often.
• The reaction will be much
faster.
compress
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Temperature
• At higher temperatures molecules move faster. As a
result there are more collisions per second and so a
faster reaction occurs. Slow molecules are also less
likely to lead to a reaction than fast ones.
Fewer collisions per second
More collisions per second
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Activity
• Food spoils because
of chemical
reactions that occur.
• Why does food
remain usable for so
much longer if it is
kept in a freezer?
The reactions that cause the food to go off will be slower
because there will be fewer and “softer” collisions between
molecules at a reduced temperature.
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Activity
• Before microwave ovens were
common many people used
pressure cookers.
• It was a pan that stopped the
water boiling until it reached
about 115oC.
• How would this help cooking?
The molecules move faster and collide more often and with
more energy. Cooking times were greatly reduced.
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Activation Energy
• Chemical reactions involve the formation of
Energy is
Activation
bonds between atoms
often before
new
given but
out as
energy
bonds can be formed
ones have to
be
new old
bonds
needed to
broken.
form
break existing
• This means that there has to be enough
energy
bonds
(activation energy)to start breaking the old
bonds before a reaction can occur.
Reactants
Old bonds
start to break
New bonds
form
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Activation Energy
• Most reactions are exothermic (give out heat)
overall but there is still a need for energy to
get the reaction started.
Activation energy
Break old bonds
Energy taken in
Form new bonds
Energy given out
Reaction
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Activity
1. Why doesn’t petrol catch fire when it is
poured through air?
2. Why is just one spark enough to create a
major explosion?
1. Energy is needed to break the bonds in petrol before new
bonds can be formed by a reaction with oxygen.
2. Once some of the bonds in one petrol molecule have been
broken the subsequent reaction with oxygen gives out
enough energy to break the bonds in several other petrol
molecules - and so on.
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Activity
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Catalysts
• For chemical reactions to occur:
– Existing bonds have to begin breaking so that new
ones can be formed.
– The molecules have to collide in such a way that the
reacting parts of the molecules are brought together.
• Catalysts can help with either or both of these
processes.
• A catalyst is a substance that speeds up a reaction
without getting used up in the process.
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Catalysts
• In the presence of a nickel catalyst vegetable oil
and hydrogen react to form margarine.
• Nickel adsorbs hydrogen gas onto its surface in
such a way that the bond holding the hydrogen
molecule together becomes stretched.
• This partial breaking of the bond lowers the
activation energy making hydrogen more reactive.
H H
H H
H HH
H HHH
H
Ni Ni Ni Ni Ni Ni
catalyst
The stretching of the H-H
bond lowers the
activation energy and
helps hydrogen react
with the oil
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Catalysts
• Other catalysts, especially enzymes, absorb molecules in a
way that not only stretches bonds but also brings the
reacting parts of reactants right next to each other.
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Catalysts
• Catalysts are used in the manufacture or application
of a huge number of products.
Biological soap
powder uses
biological catalysts
(enzymes)
Manufacture of
Fertiliser via the
Haber Process
involves use of an
iron catalyst.
Enzymes in
pineapple help
cooked ham
cook to be more
tender.
Plastics are
made using
catalysts
Synthetic
materials like
polyester are
made using a
catalyst
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Activity
Inside car engines some of the
nitrogen and oxygen from the
air combine to form poisonous
nitrogen oxide. Inside the
exhaust system a catalyst
encourages decomposition
back into nitrogen and oxygen.
1. Copy the energy profile for the
uncatalysed reaction and draw
in new lines showing how the
presence of a catalyst will alter
the profile.
2NO2
N2 + 2O2
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Measuring reaction rates.
• Rate implies we are measuring how things
change over a period of time.
• To measure the rate of a reaction we have to
track the manner in which the amount of product
(or reactant) changes over time.
Rate of gas formation can be
measured using a syringe
For a reaction
in which
sulphur is
precipitated we
can time how
long the
solution takes
to go cloudy
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Slower and slower
• Reactions do not proceed at a steady rate.
• They start fast and get slower and slower.
• This is not surprising because the reactant
concentration (and the chance of collision) gets
lower and lower as time progresses.
slower
fast
0
25
Very slow
75
stopped
100
Percentage completion of reaction
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Rates and Graphs
Shallow gradient
Slow reaction
Steep gradient
Fast reaction
Time
Amount of reactant
Amount of product
• These show the increasing amount of product
or the decreasing amount of reactant.
Steep gradient
Fast reaction
Shallow gradient
Slow reaction
Time
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Rate Graphs and Reactant
Concentrations
Amount of product
All product
Mix of reactant
And product
Reactant Concentration falls
Rate of Reaction falls
Gradient of graph decreases
All reactant
Time
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Activity
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Activity
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Some Reaction Rates
Experiments
The following slides describe the four
chemical reactions that are commonly used
as examples.
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Acid and Marble
• Marble chips are calcium carbonate.
• They react with acid to evolve a gas.
calcium
carbonate
+ hydrochloric
acid
 calcium
chloride
+ water
+ carbon
dioxide
CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g)
The gas given off can
be collected in a
syringe and readings
taken every 30
seconds or so.
Glass tube
Gas syringe
Hydrochloric acid
Marble chips
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Activity
Acid and Marble
1. Measure the agreed
mass of marble chips
2. Set up the syringe, flask
and connector
3. Measure the acid /
water.
4. Add the marble chips
and quickly insert the
bung and start stop
clock.
5. Take syringe readings
at 30 second intervals.
Time
0s
Reading
0 cm3
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Activity
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Acid and Metal
• Reactive metals (eg. Magnesium) react with
acid to evolve hydrogen gas.
magnesium + hydrochloric
acid
Mg(s)
+ 2HCl(aq)
 magnesium
chloride
+ hydrogen
 MgCl2(aq)
+ H2(g)
As the gas given off leaves the flask
the total mass of the flask and its
contents decreases slightly.
HCl
Readings of the mass(g) can be
taken. Typically at 1 minute intervals.
Mg
11.73
11.71
11.80
11.72
11.74
11.77
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Activity
1. Measure the agreed volume
of acid / water into the
conical flask.
2. Have a loose plug of cotton
wool to prevent “spitting” of
droplets of liquid.
3. Have a piece of magnesium
of known mass ready.
4. Add the magnesium, place
the cotton wool in the neck
and start taking mass
readings immediately.
Cotton
wool
HCl
Mg
11.73
11.71
11.80
11.72
11.74
11.77
Time
0s
Reading
0 cm3
60
120
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Activity
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Decomposition of Hydrogen Peroxide
• Hydrogen peroxide decomposes into water and
oxygen.
Hydrogen peroxide
2H2O2(aq)

water
 2H2O(l)
+ oxygen
+ O2(g)
Oxygen gas is given off and can be measured using
a gas syringe or a balance.
The reaction is catalysed by a wide range of solids.
Remember the catalyst NEVER produces more
product - just quicker
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Activity
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Acid and Sodium Thiosulphate
• In this reaction sulphur is precipitated which
makes the solution turn cloudy.
+ hydrochloric  sodium + water + sulphur + sulphur
Sodium
acid
thiosulphate
chloride
dioxide
Na2S2O3(aq)
+ 2HCl(aq)
 2NaCl(aq) + H2O(l) + SO2(g) + S(s)
The effect of changing conditions such as
temperature or concentrations can be studied
by measuring how long it takes to produce
enough sulphur to make the solution opaque
(non see-through).
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Activity
1.
2.
3.
4.
5.
Measure the agreed volume
of thiosulphate / water into
the conical flask.
Prepare a piece of paper
with a cross drawn on it.
Measure the required
volume of acid in a
measuring cylinder.
Add the acid to the flask,
start the clock, swirl the
flask.
Look down through the flask
until the cross disappears.
Note the time.
Look down here
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Activity
2.
Time(s) vs Temp(oC)
120
100
Temp
(oC)
Time taken
(s)
80
25
100
30
60
35
40
40
25
Time
1.
Imran studied the effect
of temperature upon the
time it took for the flask
to go cloudy.
Sketch a graph of the
results.
Using the reactions at
25oC and 40oC, explain
how the time taken lets
you work out the relative
rate of reaction.
60
40
20
45
0
20
5030
Temp
40
15
10
50
60
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Some Rates Questions
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Activity
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Activity
A pupil performed an investigation into the rate of reaction between a
metal and an acid. The results below where obtained.
Time
/seconds
0
10
20
30
40
Volume of
gas (cm3)
0
25
80
179 245
50
60
70
80
273
282
282
282
i) Plot a graph of gas volume (y-axis) against time (x-axis)
ii) When was the rate of reaction fastest?
iii) Use the graph to find the volume of gas produced after 35
seconds.
iv) Use the graph to tell after how long the reaction stopped.
v) On the graph sketch a line showing the experiment repeated
at a higher temperature.
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Answer
Volume / cm3
Experimental Results
300
250
200
150
100
50
0
0
10
20
30
40
50
60
70
80
90
Time / seconds
ii) The reaction was fastest at about 25 seconds as the gradient of
the line is highest at this point.
iii) About 175 cm3
iv) About 55 seconds.
v) Higher temperature reaction is in red.
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Activity
A flask was connected to a gas syringe by a glass delivery
tube. 30cm3 of water and 0.5g of manganese dioxide were
added to the flask. Then 5cm3 of hydrogen peroxide was
added and the stopper quickly fitted. Readings of the volume
of gas produced were taken every 10 seconds.
Time
/seconds
Volume
/cm3
0
10
20 30 40 50 60 70 80
0
18
30 40 48 53 57 58 58
i) Plot a graph of volume of gas (y-axis) against time (x-axis).
Label this curve A.
ii) Without emptying the flask another 10cm3 of water and a
further 5cm3 of hydrogen peroxide were added. Sketch the
shape of the second experiment and label it B.
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Answer
Volume of gas / cm3
70
60
A
50
40
B
30
20
10
Time (s)
0
0
10 20 30 40 50 60 70 80 90 100 110 120
Notes: Curve B is an experiment with half the
concentration of hydrogen peroxide. This should
produce about half the rate as shown by a line with half the
gradient of A. However, the same amount was added so
58cm3 of gas will still be produced.
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Activity
Time /seconds
0 15
30
45
60
75
90
120
Mass loss /g
0 0.21 0.45 0.67 0.85 1.01 1.13 1.31
Time /seconds 150 180 210 240 300 360 420
Mass loss /g
1.41 1.48 1.51 1.54 1.56 1.58 1.59
i) The results above were obtained from an experiment
where the loss in mass was recorded as lumps of
zinc reacted with hydrochloric acid. Plot a graph of
mass loss (y-axis) against time (x-axis).
ii) On the graph sketch the lines you would expect if
a) the concentration of acid was reduced,
b) the temperature was increased.
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Answer
loss in mass /g
2
(b)
(a)
1.5
1
0.5
0
0
200
400
600
Time / seconds
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Which of these would speed up the rate at
which magnesium dissolves in acid?
A.
B.
C.
D.
Cool the acid
Cut up the magnesium
Add water
Coat the magnesium in oil.
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Why does breaking up solids increase the rate
of reaction?
A.
B.
C.
D.
Makes more solid
Creates more energy
Increases surface area
Increases the concentration.
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Why does temperature increase the rate of
reaction?
A.
B.
C.
D.
Acts as a catalyst
Increases the concentration.
Increases number of molecules
Makes collisions more frequent and harder
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Why does a catalyst increase the rate of reaction?
A. Provides a route with a lower activation energy
B. Helps provide energy for the reaction.
C.Increases the speed of reactant molecules
D. Reduces the number of molecular collisions
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Why do most reactions start fast and get slower
and slower?
A. They run out of energy
B. They run out of catalyst.
C. The concentration of reactant molecules gets
less and less.
D. The surface area increases.
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